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Expert Guide
Color & Quality
Contents 1
Light and Color
1.1 1.2 1.3 1.4
Light is Color Color Perception Color Reproduction Color Systems
2 2.1 2.2 2.3 2.4 2.5
Color in Printing Ink Film Thickness Tonal Value Relative Print Contrast Color Balance/Composition Ink Trapping and Color Sequence Color Control Strips
2.6
2 4 5 7
10 10 17 18 21 22
3 3.1 3.2 3.3 3.4 3.5 3.6
Densitometry Reflection Densitometry Densitometer Filters Densitometric Values Measurement Evaluation The Limits of Densitometry
24 26 27 28 30 32
4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14
Colorimetry Measuring Colors Standard Color Values/ Reference White Standard Illuminants Standard Observers/ Spectral Value Functions Evaluation with a Spectrophotometer Color Distance (ΔE) Munsell Tristimulus Photometry Spectrophotometry Spectral Quality Control with Heidelberg Color Control Strips Color Control with Heidelberg Standardization of Printing Benefits of Colorimetry for Offset Printing
34 35 35 36 37 38 46 46 47 48 49 49 54 57
2 Expert Guide on Color & Quality Light and Color
1 Light and Color 1.1 Light Is Color We live in a world of color. We use colors to liven up our living space, so we feel good in it. The shapes and colors of things have a direct impact on our perceptions and feelings. Properly coordinated colors evoke a feeling of harmony, which puts us in a good mood. The printing industry also uses color to enhance its products; the aim is to consistently supply top quality to customers.
One of the prerequisites for this is established standards for measuring quality. And in order to assess colors, we have to be able to “see” them. This calls for light. The sun emits light – it is illuminated from within, driven by fusion processes that take place on a vast scale. In contrast, most of the objects surrounding us do not emit any light of their own. Consequently, we can only see them when they are illuminated by another light source.
Light and Color Expert Guide on Color & Quality 3
Light is radiation that travels at the extremely fast speed of 300,000 kilometers per second. More precisely, light consists of electromagnetic vibrations that propagate themselves through space like waves. Like ocean waves, each light wave has a crest and a trough.
Light wave crest
Light wave trough
A wave can be described either by giving its length or by indicating the number of vibrations per second. Wavelengths are measured in everyday units such as kilometers, meters, centimeters, millimeters, nanometers or picometers.
The number of vibrations per second – the frequency – is expressed in Hertz. Waves of different lengths have different properties and uses. X-rays, for example, are used for medical diagnostics, and many households are now equipped with microwave ovens. Other wavelengths are used to transmit telephone conversations and radio and television broadcasts. We only perceive a very small section of the overall electromagnetic spectrum as visible light. It extends from 380 nanometers (ultraviolet light) to 780 nanometers (infrared light). With the aid of a glass prism, light can be split into its color constituents. Because white light consists of a mix of colors across the whole visible spectrum, all of the colors of the rainbow can be seen (see figure on page 4). The adjacent figure shows how the wavelengths get steadily smaller as one moves from red across green to blue.
Red (700 nm)
Green (550 nm)
Blue (400 nm)
4 Expert Guide on Color & Quality Light and Color
X-rays
Gamma rays
1.2 Color Perception It is light that makes color visible – but why ?
Radio
IR
UV
Radar Microwaves
TV
Broadcast VHF
HF
MF
LF
Wavelength
Visible light
Color as such is not an attribute of an objective, such as its shape. But physical bodies do have the ability to absorb or ref lect light of certain frequencies. We only see those colors that correspond to the ref lected wavelengths. When white light strikes an object, one of the following cases occurs: • All of the light is absorbed. In this case, we see the object as black. • All of the light is ref lected. In this case, we see the object as white. • All of the light passes through the object. In this case, the object's color does not change. • Part of the light is absorbed while the rest is ref lected. We then see a color whose tone depends on which frequencies are ref lected and which are absorbed.
• Part of the light is absorbed, while the rest passes through the object. We see a color whose tone depends on which frequencies are absorbed and which pass through. • Part of the light is ref lected, while the rest passes through. The color of both the ref lected light and the light passing through changes accordingly. Which case occurs depends on the properties of the illuminated object. The light that an object ref lects or allows to pass through is captured by our eyes and converted into electrical signals that travel along nerve pathways to the brain, which interprets them as colors.
Light and Color Expert Guide on Color & Quality 5
The retina of the eye contains lightsensitive cells. There are two types of cells: rods and cones. The rods distinguish between light and dark, while the cones respond to color. There are three different kinds of cones, each of which is sensitive to a different range of wavelengths. One detects light from about 400 to 500 nanometers, or bluish colors. Other cones “see� only green light in the range from 500 to 600 nanometers. The third type is responsible for reddish colors in the spectrum between 600 and 700 nanometers. This design, with rods and different cones, makes the human eye so sensitive that we are able to perceive and distinguish several million different colors.
1.3 Color Reproduction 1.3.1 Additive Color Reproduction In the additive color reproduction process, light of different colors is combined. Blending all of the colors of the optical spectrum yields white light.
This process is used in color television and in the theater to generate all the colors of the visible spectrum.
The additive primary colors are red, green and blue light. Each of these represents one-third of the visible spectrum. Additive color reproduction can be demonstrated very well with three slide projectors, each of which casts a circle of light of one of the three additive primary colors onto a screen. Where the three circles of light overlap, the following secondary colors result:
Green
+
Red
=
Yellow
Green
+
Blue
=
Cyan
Blue
+
Red
=
Magenta
Blue
+
Red
=
White
=
Black
No light
Paper
+
Green
6 Expert Guide on Color & Quality Light and Color
1.3.2 Subtractive Color Reproduction
primary color out of white light (for example, using a filter) or by superimposing two additive primary colors. Printing inks are translucent substances that act like color filters. Which color do you get if you print a substance that absorbs blue light onto paper?
In the subtractive process, different color components are removed from the light ref lected by the white paper. Taking out all of them results in the color black. The subtractive primary colors are cyan, magenta and yellow. Each of them represents two-thirds of the visible spectrum. They can be created either by subtracting an additive
Blue is subtracted from the white light, while the other constituents (green and red) are ref lected. The additive combination of these two colorants results in yellow: this is the color we see. In other words, the printing ink removes one-third (blue) of the white light (consisting of red, green and blue). Suppose that two such translucent inks are printed one on top of the other, say yellow and cyan. The inks filter out first the blue and then the red part of the white light. What is left is green, which we perceive.
Paper
Paper
In subtractive color composition, overprinting cyan, magenta and yellow yields the following secondary colors:
Cyan
+
Yellow
=
Green
Yellow
+
Magenta
=
Red
Magenta
+
Cyan
=
Blue
Cyan
+
Magenta + Yellow
=
Black
=
White
No Color
Paper
Light and Color Expert Guide on Color & Quality 7
Together, the two inks subtract twothirds of the color components from the white light. If cyan, magenta and yellow are all overprinted, all of the light striking the surface is absorbed – so none is ref lected. As a result, we see black.
1.3.3 Autotypical Color Synthesis Color images are printed using a fourcolor process with cyan, magenta, yellow and black inks. The black improves the definition and contrast of images.
1.4 Color Systems
The black that is produced by subtractively combining cyan, magenta and yellow is, because of the nature of the pigments used in the inks, never completely pitch-black. In classical offset printing, the halftone dots are sized depending on the desired color tone (see section 2.2). When overprinted, some of the dots corresponding to the individual colors are adjacent to one another, while others partially or entirely overlap. If we look at the dots through a magnifying glass (see figure), we see colors that – with the exception of the paper's white – result from subtractive color mixing. Without a magnifying glass and when looking at an offset-printed item from the normal viewing distance, our eyes are unable to distinguish the individual dots. In this case, the colors are additively combined. A combination of additive and subtractive color reproduction is known as autotypical color synthesis.
Each individual perceives colors slightly differently. So if several people are asked to describe certain colors, the results can vary greatly. But printers need standardized yardsticks for identifying the colors they use in their work. To meet this need, various evaluation systems have been developed. Some ink manufacturers create sample books and give each color in them a unique name, such as Novavit 4F 434. Others use color gamuts, like HKS and Pantone. Color circles divided into 6, 12, 24 or more segments are also used.
8 Expert Guide on Color & Quality Light and Color
All these systems use samples or specimens to show the individual color tones and assign names to them. However, they are never exhaustive and are rarely suitable for making calculations. As we have seen, our color perceptions depend on how the red-, green- and blue-sensitive receptors in our eyes are stimulated. This indicates that three parameters are needed to unambiguously describe the set of all possible colors.
If we imagine that the primary colors are the axes of a three-dimensional system of coordinates, what we get is a so-called color space.
0.8
0.7
Many experts have tackled the problem of how to systematically organize colors, coming up with differing ideas on how a color space should be structured. All of the color spaces defined so far have advantages and disadvantages.
0.6
0.5
0.4
In such a system, green could be described as follows: Green = 0 × red + 1 × green + 0 × blue Or even more concisely: G=0×R+1×G+0×B
The most important color spaces have been standardized internationally. They are used in a wide range of industries: production of inks and coatings, textiles, food production and medicine, to name just a few. The CIE chromaticity diagram has prevailed as the most widely used standard (the acronym CIE stands for “Commision Internationale de l'Eclairage”).
0.3
0.2 Visually perceivable colors in a lightness plane of the CIE color space (the standard chromaticity diagram, which resembles a sail, tongue or sole of a shoe).
0.1
0.0 0.0
0.1
This system uses the letters X, Y and Z instead of R, G and B to designate the axes. For practical reasons, reference is usually made to the chromatic values x and y and the lightness value Y (used as a measure of brightness for body colors). A color's location within the space can be precisely defined using these three coordinates.
0.2
0.3
0.4
0.5
0.6
0.7
Colors with the same lightness value can be depicted two-dimensionally in a plane. If the CIE color space is sliced open along a lightness plane, what results is the CIE standard chromaticity diagram (the “tongue”, see figure above). The spectral colors are the ones with the greatest saturation reproducible in a given tone (wavelength). They are at the edges of the CIE standard color system. In the figure, their wavelengths are given in nanometers. The straight line connecting the wavelengths of 380 and
Light and Color Expert Guide on Color & Quality 9
0.8
0.7
The distribution of lightness values is very similar. All the colors located within the hexagon can be reproduced in the four-color process using the Euroscale. Colors outside this zone can only be reproduced by adding special colors.
0.6
0.5
0.4
The Euroscale specifies the following values for art paper under defined printing and measurement conditions:
0.3
0.2
0.1
0.0 0.0
The x, y and Y parameters are determined using a spectrophotometer or tristimulus colorimeter. These are available as handheld units and central stations with online color control (for instance, in Prinect® Axis Control® and Prinect® Image Control from Heidelberg®).
The standard color diagram illustrated here shows the color locations defined by DIN 16539 and the set of printable colors.
Colors reproducible with the Euroscale (DIN 16539).
0.1
0.2
0.3
0.4
0.5
780 nanometers is called the “purple line”. The area bounded by the spectral locus and the purple line contains all color valences that can be created by mixing spectral colors. The (white) center point has the coordinates x = 0.333 and y = 0.333. With primary light sources, it is indicated by an E (for “energy-equivalent spectrum”) and with body colors occasionally by an A (for “achromatic”).
0.6
0.7
The saturation of every color decreases from the center point toward the spectral locus.
Primary and secondary colors
The Euroscale (DIN 16539) defines the coordinates of the colors cyan, magenta and yellow for three- and four-color printing. Also defined are the locations of the subtractive secondary colors red, green and blue.
Yellow Magenta Cyan Yellow-Magenta Yellow-Cyan Magenta-Cyan
Proportions of standard colors x y 0.437 0.464 0.153 0.613 0.194 0.179
0.494 0.232 0.196 0.324 0.526 0.101
Lightness value Y 77.8 17.1 21.9 16.3 16.5 2.8
10 Expert Guide on Color & Quality Color in Printing
2 Color in Printing The goal of quality control in printing is to correctly and consistently reproduce colors through the pressrun. Various factors affect this; besides the inks and the color shade of the substrate, the most important parameters are the thickness of the ink films, tonal values, color balance, ink trapping and color sequence.
Less-than-optimum saturation also restricts the range of reproducible colors. In the figure, white is used to show how insufficient saturation of all three chromatic colors reduces the range. In terms of physics, the ink film thickness inf luences appearance as follows:
0.8
0.7 0.6
0.5
2.1 Ink Film Thickness In offset printing, for process-related technical reasons the maximum ink film thickness that can be laid down is about 3.5 micrometers.
Printing inks are translucent, not opaque. This means that light penetrates them. While doing so, it strikes particles of pigment that absorb a fairly large share of certain light wavelengths.
When printing Euroscale colors (as defined in DIN 16539) on art paper, it is advisable to achieve the correct color locations with film thicknesses between 0.7 and 1.1 micrometers. The use of unsuitable plates, substrates or inks can prevent the standardized corner points of the CIE chromaticity diagram from being reached.
Depending on the pigment concentration and the ink film thickness, the light can strike more or less pigment, resulting in different amounts of light being absorbed. The light rays ultimately reach the (white) surface of the substrate and are ref lected by it back through the ink to the observer’s eyes.
0.4 0.3
0.2 0.1 0.0 0.0
0.1
0.2
0.3
0.4
0.5
2.2 Tonal Value After the ink film thickness, halftone (or tonal) value is the most important factor affecting the visual appearance of a color nuance. In reference to a film or digital image file, the tonal value is the share of an area covered by halftone dots. Brighter colors have smaller tonal values. To reproduce different color nuances, the conventional approach is to keep the screen ruling (also known as screen fre-
0.6
0.7
quency) constant while varying the size of the halftone dots as required to obtain the desired tone. In frequency-modulated screening, by contrast, the dots stay the same size while the screen ruling changes. Tonal values are normally given as percentages.
Color in Printing Expert Guide on Color & Quality 11
2.2.1 Changes in Tonal Values When halftone dots are transferred from film via a plate and blanket to paper, various factors can affect their size and shape, which has repercussions on the tonal value. Process-related changes to tonal values (see section 2.2.3) can be compensated for in prepress. Print samples are measured and compared with the originals, which lets transfer curves be plotted. Provided that the same standards are consistently applied throughout the process chain from the scanner to the finished print product, true-to-original results can be expected. Changes to tonal values caused by printing problems are unpredictable. Special attention therefore has to be paid to them. Here are the most important ones:
The path of a halftone dot
Factors influencing halftone dots
Film Assembly Camerawork
Film edges, adhesives
Development
Chemicals, development times
Appearance of halftone dots
Two halftone dots on film (magnified approx. 150x)
Plate
Materials, wear during printing
Platemaking
Exposure time, vacuum, undercutting
Dampening
Amount of dampening solution, pH, surface tension, water hardness, temperature Ink film thickness, consistency, temperature
Inking
Printing
Cylinder rolling
Halftone dots on the plate after inking
12 Expert Guide on Color & Quality Color in Printing
Dot Gain and Sharpening The path of a halftone dot
Factors influencing halftone dots
Blanket
Material, condition, surface
Printing Blanket/paper
Cylinder rolling
Appearance of halftone dots
The dots on the blanket.
Paper Sheet transport Delivery
Surface, paper grade Transfer register Smearing
High magnification clearly shows the first-class results on paper.
• Dot gain When halftone dots grow in size relative to the film or digital image, it is called “dot gain” or occasionally also “dot spread”. This can be caused in part by the printing process, materials or equipment, factors that are relatively difficult for the operator to inf luence, and in part by the inking, which the operator can manipulate. • Fill-in Fill-in is a problem similar to dot gain that is caused by printing ink in the non-image areas between the dots, narrowing the spaces until they disappear entirely. Slurring and ghosting can sometimes be responsible for fill-in. • Sharpening Sharpening refers to a decrease in the tonal value as compared to the film or digital image. In practice, the term is always used to describe a reduction in dot gain, even when the dots are still fuller than on the film or in the digital image.
Color in Printing Expert Guide on Color & Quality 13
Halftone Dot Deformation • Slurring Slurring is when the shape of a halftone dot is distorted during printing by relative motion between the plate and blanket and/or blanket and sheet. For example, a round dot can be stretched to an oval shape. Slurring in the direction of printing is called circumferential slurring, and perpendicular to that it is known as lateral slurring. If both types occur together, the direction of slurring is diagonal. • Ghosting In the context of offset printing, ghosting is when a second, typically smaller, shadow-like ink dot is unintentionally printed next to the intended dot. Ghosting is caused by ink being transferred back to the blanket out of register.
• Smearing When mechanical factors in the press cause the deformation of halftone dots, it is known as smearing. The term is also used as a synonym for offsetting. What the operator has to pay attention to Dot gain and its extent can be monitored visually and with the aid of instruments. Control strips include special patches that are excellently suited for visually detecting dot gain. Sharpening can be easily monitored using measurement targets with a high tonal value.
Right
Wrong
Both dot gain and fill-in are usually caused by excessively heavy inking, insufficient dampening solution feed, too much pressure between the plate and blanket cylinders, or inadequate blanket tension. Sometimes it can also be due to incorrect setting of the inking and dampening form rollers.
Right
Wrong
Even under normal conditions with correctly made plates, a certain amount of dot gain occurs. Sharpening can occur under abnormal conditions such as plate blinding or ink accumulation on the blanket. This can be prevented by washing the blankets and inking units more frequently, possibly changing the inks and sequence of colors, and checking the form rollers and cylinder pressure settings.
Slurring is most conspicuous in patterns with parallel lines. In many cases, this also reveals the direction of slurring. Circumferential slurring usually indicates that the plate and blanket are slipping slightly relative to one another as they turn, or that the cylinders are pressing too hard against one another. So it’s very important to check the printing pressure and cylinder rolling. Frequently, the culprit can also be a blanket that isn’t clamped tightly enough, or excessively heavy inking. Lateral slurring rarely occurs by itself. If it does, pay special attention to the substrate and the blanket.
Right
Right
Dot gain
Sharpening
Slurring
Ghosting
Wrong
Smearing
Wrong
14 Expert Guide on Color & Quality Color in Printing
The same methods are used to check for ghosting and slurring. A magnifying glass should also be used to inspect the halftone dots, because line patterns cannot reveal whether ghosting or slurring has occurred. Ghosting can have many possible causes, but it is usually due to problems with the substrate or something directly related to it.
Right
Wrong
Smearing is extremely rare in today’s modern printing presses. If it occurs, the parts of a sheetfed press in which sheets are mechanically supported on the freshly printed side should be checked first. Stiff substrates increase the risk of smearing. Smearing can also occur in the delivery pile and in perfector presses.
Right
Wrong
Printed control elements like the SLUR strip let you quickly identify the type of dot distortion involved. These elements visually amplify the printing problem so it can be easily seen. Problems like dot gain and sharpening, slurring and ghosting are worse with fine screens than with coarse screen rulings. The reason is that fine halftone dots increase or decrease in size by the same amount — i.e. in absolute, not relative terms — as larger ones. However, many small dots together have a total length several times that of large dots with the same tonal value. Consequently, more ink is used to print fine dots than large ones. Areas with fine screen rulings therefore appear to be darker. Control and measurement targets take advantage of this fact. To illustrate this, let’s look at the structure and functions of the SLUR strip (see figure below). This strip contains both coarse-screen and fine-screen patches.
While the coarse-screened background has a uniform tonal value, the numerals 0 to 9 have a fine screen ruling and an increasing tonal value. On a well-printed sheet, the number 3 and the coarse screened patch have the same tonal value and the number is invisible. With increasing dot gain, the next-higher number disappears instead. The fuller the printed dots get, the higher the value of the invisible number. This works in reverse when sharpening occurs. Then the number 2, 1 or even 0 becomes illegible. However, the numerals only indicate that printing is getting fuller or leaner. The causes must be ascertained by examining the plate with a magnifying glass or checking the press.
Color in Printing Expert Guide on Color & Quality 15
Good
Fuller
Leaner
Lateral slurring Circumferential slurring
The part of the SLUR strip to the right of the numerals mainly shows whether slurring or ghosting has occurred. The word SLUR is equally legible with lean, normal and full printing; the whole patch merely appears somewhat lighter or darker. It is easy to detect the directional spread typical of slurring and ghosting in the word SLUR, however. In the case of circumferential slurring, for example, the horizontal lines forming the word SLUR, which run parallel to the sheet’s leading edge, become thicker. If lateral slurring has occurred, then the vertical
lines forming the background of the word SLUR appear darker. The figure to the right illustrates how changes in the halftone dots affect printing, specifically when there is dot gain. If the dots for just one color are larger than they should be, this results in a new shade — which naturally also inf luences the overall appearance of the printed image. In offset printing, the need to transfer images from the plate to the blanket and from there to the paper usually results in a certain amount of dot gain.
Color control strips can tell you whether the results of printing are good or bad, but they cannot provide any absolute figures or indicate the exact nature of the problem. An objective method is therefore needed for assessing quality by measuring tonal values.
16 Expert Guide on Color & Quality Color in Printing
Note: The measured dot gain Z shows the difference between the tonal value in print (FD) and the tonal value in the film (FF) or the data as an absolute value. In other words, it is independent of the film or data value. 2.2.3 Characteristic Curves The deviation of the tonal value in print (FD) from the tonal value in the film (FF) or data can be clearly represented in a “print characteristic curve”, which can then be directly used to optimize reproduction quality.
Right
Wrong
2.2.2 Dot Gain Dot gain is the difference between the tonal values of a screened film or digital image on the one hand and the print on the other. Differences can result from (1) changes in the halftone dots or (2) the phenomenon known as the “light trap effect” or light gathering (see section 3.4.4).
Like the tonal value (F), the dot gain (Z) is normally given as a percentage (see section 3.5 for the formulas used to calculate it). It is a function of the difference between the measured tonal value in print (FD) and the tonal value in the film (FF) or the data. Because the dot gain can vary depending on the tonal value, when making statements on dot gain it is important to also provide the tonal value in the film. For example: 15% dot gain with FF = 40%, or abbreviated as Z40 = 15%. Modern measuring instruments directly indicate the dot gain.
To determine a characteristic curve, print a step wedge with at least three but preferably five or more tonal levels and one full-tone (solid) patch. Use a densitometer or spectrophotometer to measure all of the levels, and calculate their tonal values. Plot the obtained values on a chart against the corresponding film values; the result is a “transfer characteristic curve”. With standardized platemaking, it is identical to the print characteristic curve. This curve only applies to the same combination of ink, paper, cylinder pressure, blanket and plate for which it has been determined. If the same job is printed on another press with different ink or on different paper, the print characteristic curve may differ somewhat.
In Figure 17, characteristic curve 1 is a straight line running at an angle of 45 degrees. This line is not normally attainable; it represents the ideal state in which the print and the film are visually indistinguishable. Characteristic curve 2 represents the tonal values actually measured in the print. The area between the two curves is the dot gain. The midtones are most useful for determining dot gain in print. In curve 2, it is plain that the tonal value deviations are greatest in that range. This characteristic curve can be used to adjust the screened film while achieving the desired tonal values in print (with the usual dot gain). In practice, however, process-related f luctuations will inevitably result in minor deviations. Because of this, tolerances are always given for the dot gain. To keep the print quality as constant as possible, it is indispensable to continually check the tonal values in a color control strip and with the aid of Mini Spots® from Heidelberg.
Color in Printing Expert Guide on Color & Quality 17
2.3 Relative Print Contrast As an alternative to dot gain, sometimes the relative print contrast Krel (%) is determined, mainly for monitoring the three-quarter tones.
Film
Print characteristic curve
Characteristic curve 2 Characteristic curve 1
DV = 1.50
Film or data
A print should be as contrast-rich as possible. To achieve this, the full tones should have a high color density but the screen should be printed as open as possible (with an optimum tonal value difference). Increasing the ink feed, resulting in a greater color density of the halftone dots, enhances the contrast. But there is a limit to how far this can be taken — too much, and the halftone dots will grow too full and start filling in, especially in the shadows. This reduces the share of paper white and the contrast declines again. If no measuring instrument is available that gives a direct reading of the contrast, an alternative is to calculate the relative contrast (the formulas are given in section 3.5.3) or determine it with the aid of the corresponding FOGRA chart.
If the contrast gets worse during the course of a production run even though the solid density remains constant, this can mean that it is time to wash the blankets. If the solid density is correct, then the contrast value can be used to assess various other factors that affect the results of printing, for example: • Cylinder pressure and rolling • Blankets and packing • Dampening • Inks and additives Because the relative print contrast, unlike dot gain, greatly depends on the momentary solid density, it is unsuitable for use as a standardization parameter. In recent years its importance has greatly diminished.
18 Expert Guide on Color & Quality Color in Printing
2.4 Color Balance/Color Composition As already explained, in the four-color process different color shades are reproduced by mixing varying amounts of cyan, magenta, yellow and black. As soon as their relative proportions change, so does the color. To prevent this from happening, they must somehow be kept in the right balance. If it is only the proportion of black that changes, the color gets lighter or darker, which does not irritate the observer much. The same thing happens if all three chromatic colors change by the same amount in the same direction. The situation is much more critical when the color tone itself changes. This happens when the color components
The extent to which the inevitable f luctuations in each color printed affect the results mainly depends on the color composition approach defined in prepress. The relevant questions are: • Which process colors do the gray areas consist of ? • What approach is used to darken chromatic areas? • How are the shadows created and enhanced?
change by different amounts, and especially if the individual chromatic colors change in opposite directions. These kinds of changes in the color balance are easiest to detect in gray patches. Reference is therefore often made to gray balance in this connection.
2.4.1 Chromatic Composition In this approach, all achromatic shades are created by mixing the chromatic colors, i.e., cyan (C), magenta (M) and yellow (Y). In other words, all gray areas in the image, all tertiary tones, and the shadows contain all three chromatic process colors. Black (K) is only used to enhance the shadows and improve image definition there (skeleton black).
100 %
50 %
0%
70 % C
80 % M
90 % Y
0%K
240 %
The brown shown in the figure consists of 70% cyan, 80% magenta, 90% yellow and 0% black. The total area coverage is therefore 240%. In short: how are the gray, i.e. achromatic, parts generated, and what is the maximum total area coverage that results? Remember, gray (achromatic) shades can be produced either by combining cyan, magenta and yellow, or by using process black. It is also possible to combine both approaches.
How the color components work in combination is illustrated on the right. The brown consists of an achromatic (gray) portion and a chromatic portion. 70% cyan and about 58% magenta and 59% yellow (in the Euroscale) offset one another to yield gray (an achromatic color). Only the other 22% magenta and 31% yellow combine to produce a light-brown chromatic color. Together with the gray portion, this yields dark brown.
C
M
Y
K
Chromatic composition results in high total area coverage, which can theoretically reach 400%, but in practice does not exceed 375%. These high total area coverage levels adversely affect ink trapping, drying and powder consumption, and it is difficult to maintain the color balance during the pressrun.
100 %
50 %
0% C
M
Y
K
Color in Printing Expert Guide on Color & Quality 19
2.4.2 Achromatic Composition In contrast to chromatic composition, with achromatic composition all of the achromatic colors in a multicolor image are produced with process black. In other words, all neutral colors consist only of black, and black is also used to darken chromatic colors and achieve greater saturation. Any given color consists of a maximum of two chromatic process colors plus black. This stabilizes the color balance. With achromatic composition, in theory the brown discussed in section 2.4.1 can be produced by overprinting 0% C + 22% M + 31% Y + 70% K. 100 %
50 %
0%
0%C
C
M
22 % M
Y
However, as the figure shows, merely replacing an achromatic shade produced with CMY by black does not yield an identical color. This is primarily due to the shortcomings of actual printing inks. To obtain truly similar results, it is necessary to modify the proportions, e.g. to 62% M, 80% Y and 67% K. Achromatic composition is equivalent to 100% gray component replacement (GCR; see section 2.4.6 below). 2.4.3 Achromatic Composition with Under Color Addition (UCA) Process black by itself does not always provide sufficient definition in the darker portion of the gray axis. When this is the case, this range and, to a lesser extent, the neighboring chromatic tones can be enhanced by adding CMY. Use of this approach, called “under color addition” (UCA) or “chromatic color addition”, depends mainly on the substrate-ink combination. The illustration on the right illustrates UCA to neutrally enhance the image shadows.
K
31 % Y
70 % K
123 %
2.4.4 Chromatic Composition with Under Color Removal (UCR) The highest area coverages result from using chromatic composition for the neutral three-quarter tones all the way to black. This drawback is offset by “under color removal” (UCR). The proportion of CMY is reduced in the neutral shadows and, to a lesser extent, in the neighboring chromatic tones, while the amount of process black is increased. The example below, the initial area
coverage of 98% cyan + 96% magenta + 87% yellow + 84% black = 355% is reduced by 78% with UCR. This favorably affects ink trapping, drying and balance in the shadows.
100 %
100 %
50 %
50 %
0%
C
M
Y
K
0%
C
M
Y
K
20 Expert Guide on Color & Quality Color in Printing
2.4.5 Chromatic Composition with Gray Stabilization Gray shades created with chromatic composition are hard to keep balanced in the print process. Color casts readily occur. This can be prevented by gray stabilization. Achromatic components generated with C + M + Y are partially or entirely replaced along the entire gray axis and to a lesser extent in the neighboring color ranges — i.e., not just at the darker end of the gray axis like with UCR —by an equivalent amount of black. This is often referred to as “long black”. 2.4.6 Chromatic Composition with Gray Component Replacement (GCR) “Gray component replacement” (GCR) involves using process black to replace CMY components in both chromatic and neutral image areas. GCR can be used for all intermediate stages between chromatic and achromatic composition in all image areas — and is not, like UCR, UCA and gray stabilization, limited to the gray areas. Gray component replacement is sometimes also referred to as complementary color reduction.
2.4.7 Five-, Six- and Seven-Color Printing The modern four-color process ensures high-quality image reproduction. However, with some originals and when extremely high quality is needed, it can be necessary to use additional special colors. The use of additional colors (alongside the four process colors) or special process colors can extend the range of
The brown in sections 2.4.1 and 2.4.2, for example, could theoretically be produced as follows with GCR:
60 % M
50 % C
70 % Y
Like with achromatic composition (section 2.4.2), the colors obtained with the two methods are not identical if black is merely substituted for part of the achromatic CMY without adjusting the chromatic components as well. Similar colors are achieved with, for example, 49% C + 70% M + 80% Y + 30% K.
20 % K
200 %
0.8
0.7
0.6
0.5 100 %
0.4
0.3 50 %
0.2
0.1 0%
C
M
Y
K
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Color in Printing Expert Guide on Color & Quality 21
reproducible colors. The previous figure shows the measured values for a seven-color print entered in the CIE chromaticity diagram.
ink that is still wet, then one speaks of “wet on wet� printing. With multicolor presses, it has become standard to talk about wet on wet printing.
The hexagon on the inside shows the color gamut reproducible with the process colors cyan, magenta and yellow (as measured). The surrounding dodecagon shows the extended color space that can be printed with the additional colors green (G), red (R) and blue (B).
When inking is uniform and the colors are accurate, this indicates that there is good ink trapping.
2.5 Ink Trapping and Color Sequence 2.5.1 Ink Trapping Another parameter that inf luences color reproduction is ink trapping. This is a measure of an ink’s ability to transfer equally well to unprinted substrate and a previously printed ink film. Two different cases occur: wet on dry, and wet on wet. Wet on dry printing is when an ink is laid down directly on the substrate or onto a previously printed and dried ink film. If the second color is printed on
In contrast, if the target color cannot be achieved, then the ink trapping is inadequate. This can be the case with every tone involving overprinting of two or more process colors. This restricts the printable range of colors, and certain color nuances cannot be reproduced. Even if the right ink film thicknesses are printed with a given set of colors and the primary colors cyan, magenta and yellow are accurate, it can still happen that the secondary colors red, green and blue can still be poor, due to overprinting problems. The CIE chromaticity diagram above shows the effects of disturbed ink trapping or an unfavorable color sequence on the printed result. The white area corresponds to the extent of the tonal reduction caused by the ink trapping problems.
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
22 Expert Guide on Color & Quality Color in Printing
2.5.2 Color Sequence The schematic diagram shows three different overprints of the colors cyan and magenta. The second example was printed on a multicolor press. First magenta was printed on the dry paper (wet on dry), then cyan on top of the still-moist magenta (wet on wet). Whereas the magenta was accepted well by the paper, the cyan was accepted less well (due to the ink splitting that occurred during overprinting). This caused the resulting blue to have a red cast. The third example was also printed wet on wet, but in the reverse order (magenta on cyan). The result was red with a blue cast. In the four-color process, the color sequence black – cyan – magenta – yellow has prevailed as the standard. In order to reduce the effects of ink trapping problems in special cases, the original and the plates should be carefully inspected before mounting the latter on the press. When there are solid areas, it can be advantageous to print the lighter form before the heavier one.
This applies especially when overprinting screened areas and solids. The screened areas should be printed first on the white paper, then the solids.
2.6 Color Control Strips So that the print quality can be assessed by performing measurements, color control strips are included in the printed sheets. These are available from various research institutions and suppliers. It is important to always use the original strips, because copying them onto duplicate films results in deviations that can falsify the results of measurements. Color control strips are available for
four- to eight-color presses. Strips for more than four colors have fewer tint patches and patches for detecting slur, and more of the elements that are needed to adjust the solids and color balance.
Color in Printing Expert Guide on Color & Quality 23
All color control strips have multiple elements. In the following, the most important elements of the Heidelberg CPC color strip and those from FOGRA and Brunner are illustrated. 2.6.1 Solid Patches Solid patches are used to check the uniformity of inking. It is expedient to use one solid patch for each ink printed, spaced to correspond to the width of the ink zones (in the case of Heidelberg, 32.5 millimeters). The solid patches can then be used for automatic regulation of the solids. K
C
M
CY
When the colors cyan, magenta and yellow are overprinted in a solid patch, the result should be a fairly neutral black. For purposes of comparison, a solid black patch is printed alongside the overprint patch. CMY
CM
In the standardized process as described by ISO 12647-2 (identical with the standard offset process), the proper gray balance must be mainly achieved by applying an ICC color profile to generate the separations. 2.6.4 Tint Patches The tonal values of the tint patches on film vary depending on the manufacturer.
K
The values measured in the tint and solid patches are used to calculate the dot gain and relative print contrast. With the correct ink film thicknesses, the standard color sequence, and normal dot gain, the tint patches for cyan, magenta and yellow should yield a fairly neutral gray when overprinted.
Y
2.6.2 Solid Overprint Patches These elements are used to assess ink trapping by means of visual inspection and measurements.
MY
2.6.3 Color Balance Patches There are solid and tint color balance patches.
Color balance patches are intended to be visually checked; they are also used for automatic gray balance control for the colors cyan, magenta and yellow.
K
CMY
Today the FOGRA color control strips with 40% and 80% patches are most widely used.
2.6.5 Slur and Ghosting Patches Line screens with different angles are used to check for slurring and ghosting by visual inspection and measurement (see section 2.2.1).
2.6.6 Platemaking Elements Platemaking elements are used to visually check the results of platemaking. The elements shown have microlines and microcolumns as well as fine dots.
24 Expert Guide on Color & Quality Densitometry
3 Densitometry Densitometry is an effective method for monitoring solid density and tonal values in the print process. It works reliably with black-and-white reproductions and with the process colors cyan, magenta, yellow and black. There are two types of densitometers: • Transmission densitometers are used to measure film blackening (i.e., with transparent materials). • Ref lection densitometers are used to measure light ref lected from the surface of a print (i.e., with ref lective originals). The technology of ref lection densitometry is described in detail below.
Transmission densitometer
Reflection densitometer
3.1 Reflection Densitometry In ref lection densitometry, the color to be measured is illuminated by a light source. The light beam penetrates the translucent ink film, which attenuates it. The remaining portion of the light is greatly scattered by the paper underneath. Part of this scattered light is ref lected back through the ink film, being further attenuated in the process. What is left then finally reaches a sensor, which converts the light into an electrical signal. The result is indicated in density units. Lens systems are used to focus the light to facilitate measurement. Polarizing filters suppress the wet gloss (see section 3.2.2). When measuring chromatic colors, color filters are placed in front of the sensor (see section 3.2.1).
Densitometry Expert Guide on Color & Quality 25
The figure shows how ref lection densitometry works, taking the example of a printed chromatic color. White light — ideally consisting of equal parts of red, green and blue — shines onto the press sheet. The printed ink contains pigments that absorb red and ref lect green and blue, which is why we call it cyan. The densitometer measures the absorbed light of a certain color, because density and ink film thickness correlate well. In this example, a red filter is therefore used; it filters out blue and green and only allows red to pass. The density of a printed ink primarily depends on the type of pigment it contains, its concentration, and the ink film thickness. The density of a printed ink reveals the film thickness but tells us nothing about the color itself.
Color filter Polarizing filter Color filter Polarizing filter Polarizing filter
Paper
Lens system
26 Expert Guide on Color & Quality Densitometry
1.0
3.2 Densitometer Filters 3.2.1 Color and Brightness Filters The color filters in a densitometer are optimized for the absorbed wavelengths corresponding to cyan, magenta and yellow. The relevant standards, such as DIN 16536 and ISO/ANSI 5/3, stipulate the spectral passbands and the wavelengths of the pass maxima. They define narrow- and wideband color filters (in the case of ANSI, designated A and T, respectively); narrowband filters (DIN NB) are preferable because different spectrometer makes deliver more consistent measurement values with them. Always choose a color filter that is the polar opposite of the colors being mea-
sured. Black is evaluated with a visual filter that is adjusted to the spectral brightness sensitivity of the human eye. Special colors are measured with the filter that yields the highest measurement value. The three figures on the right show the ref lection curves for cyan, magenta and yellow using the corresponding color filters as defined by DIN 16536.
Printed color
Filter color
Cyan
Red
Magenta
Green
Yellow
Blue
Cyan
0.5
0.0
1.0
Magenta
0.5
0.0
1.0
0.5
0.0
Yellow
Densitometry Expert Guide on Color & Quality 27
3.2.2 Polarizing Filters When press sheets are pulled freshly printed from the delivery and measured, the ink is still wet and has a shiny surface. While drying, the ink penetrates into the paper (absorption) and loses its gloss. This changes not only the color’s tone but also its density. If the press operator wishes to use densitometry to compare the wet sheets with the reference values, which also refer to dry ink, the results will inevitably be wrong.
Filtering out the light ref lected by the ink’s glossy surface has the effect of making the densitometric measurement values for wet and dry ink roughly equivalent. However, due to absorption by the polarizing filter less ref lected light reaches the sensor. Consequently, the values measured with instruments of this kind are usually higher than with other apparatus, depending on the gloss.
3.3 Densitometric Values The result of measurement with a densitometer is a logarithmic number: density (D). This is expressed as the logarithm (base 10) of the opacity, which is the reciprocal of the transmission or ref lection of a tone. Density is calculated by applying the following formula:
D = lg To make this possible, two linear polarizing filters at right angles to one another are placed in the path of the densitometer. Polarizing filters only permit light waves oscillating in a certain direction to pass. Part of the resulting aligned beam of light is ref lected by the surface of the ink but its direction of oscillation remains unchanged. The second polarizing filter is rotated perpendicular to the first, which blocks out these ref lected light waves. However, if the light isn’t ref lected until after it penetrates into the ink film, either by the ink or the paper, it loses its uniformly aligned direction of oscillation (polarization). Consequently, the second polarizing filter allows part of it to pass and strike the sensor.
1
LeW
Paper Direction of scattering Direction of oscillation
β=
The ref lectance (β) indicates the ratio between the light ref lected by a sample (the printed ink) and a standard white (reference value). The value β yields the following density:
D = lg
β
The ref lectance (also called the beta value) is calculated as follows:
β=
LeP is the light ref lected by the printed ink, and LeW is the light ref lected by the reference white.
LeP LeW LeP
LeP 50 % = = 0.5 LeW 100 %
1 β
= lg
1 = lg 2 = 0.30 0.5
28 Expert Guide on Color & Quality Densitometry
There is a close correlation between the ink film thickness and the ink density. The diagram shows that ref lection diminishes and density increases with thicker ink films.
Density
Please see page 27 for the calculation formulae.
Black 2.5
Cyan Magenta Yellow
2.0
1.5
1.0
0.5
Ink film thickness 0.0 0
0.5
1.0
1.5
2.0
2.5
The diagram shows how the ink film thickness and density correlate for the four process colors used in offset printing. The dotted vertical line indicates the usual ink film thickness in offset printing, or about one micrometer. The diagram shows that the density curves do not f latten until considerably higher values are reached. Then, at even higher thicknesses the density barely increases any further. Even if you were to measure a full can of ink, the value obtained would be only negligibly higher. Of course, ink films that thick have no relevance to the standard four-color process.
3.4 Measurement 3.4.1 Calibration to Paper White Before any measurements are made, the densitometer is calibrated to the applicable paper white (reference white) in order to eliminate the inf luence of paper coloring and the paper’s surface when assessing the printed ink film thickness. To accomplish this, the density of the paper white is measured in reference to “absolute white� and this value then set to zero (the reading is D = 0.00).
Densitometry Expert Guide on Color & Quality 29
3.4.2 Solid Density The values measured in a solid area indicate the solid density. It is measured in a color control strip that is printed on the sheet perpendicular to the direction of travel and has a number of patches, including solid patches for all four process colors (and special colors if required).
3.4.4 Optically Effective Area Coverage (Tonal Value) When using densitometry to measure halftone images, it is not the geometrical area coverage (the percentage of the patch’s surface covered by halftone dots) that is measured but rather the “optically effective area coverage”.
The solid density can be used to monitor and ensure a uniform ink film thickness across the entire width of the sheet and throughout the pressrun (within certain tolerances).
The difference between the geometrical and optically effective area coverage is that, regardless of whether they are assessed by a visual check or measurement with a densitometer, part of the light shining onto the sheet penetrates into the paper in the blank areas between the halftone dots, and part of what is ref lected strikes the rear of the dots and is absorbed by them.
3.4.3 Halftone Density Halftone density is measured in the tint patches of the color control strip. These round patches, which are typically three to four millimeters wide, contain a mix of halftone dots and paper white, corresponding to the inner structure of the human eye. The measured value is the halftone density. This value increases with the tonal value of the halftone dots and the ink film thickness.
This effect is known as “light trapping”. It makes the halftone dots appear larger than they actually are. The optically effective area coverage thus consists of the geometric area coverage plus the optical magnification effect.
Paper
30 Expert Guide on Color & Quality Densitometry
3.5.4 Ink Trapping Ink trapping is calculated from the densities measured in single-color solid and two- and three-color overprint patches, while taking the color sequence into account.
3.5 Evaluation The values measured for the solids and halftones can then be used to calculate the tonal values, dot gain and contrast. A prerequisite for doing this is that the densitometry must be calibrated to the paper white beforehand. 3.5.1 Tonal Values The measured solid and halftone densities (DV and DR) can be used as follows with the Murray-Davies equation to determine the printed tonal value (FD): 3.5.2 Dot Gain The dot gain (Z) is the difference between the measured printed tonal value (FD) and the known tonal value in the film (FF) or data. 3.5.3 Relative Print Contrast The relative print contrast is also calculated from the measured solid density (DV) and the halftone density (DR). The DR value is best measured in the threequarter tones.
–DR FD (%) = 1 –10 · 100 –DV 1–10
Z (%) = FD–FF
Krel. (%) = DV – DR · 100 DV
The ink trapping calculated with the following formulae tells us what percentage of a color is overprinted on another. It is compared with the firstdown color, the trapping of which is assumed to be 100%. 3.5.4.1 Overprinting Two Colors Where D1+2 is the density of the over printed colors, D1 is the density of the first-down color, and D2 is the density of the seconddown color.
Note: All density values must be measured using the color filter that is diametrically opposite the last-down color. The formulas given here are also used by the quality control systems Prinect Axis Control and Prinect Image Control from Heidelberg. Other methods also exist for determining ink trapping. All of the methods are controversial, so the results they give should not be taken too seriously. However, they are useful for comparing pressruns with one another (and especially for comparing sheets pulled from the same run).
FA 2 (%) = 1
D1+2 – D1 D2
· 10
Note: All density values must be measured using the color filter that is diametrically opposite the second color. 3.5.4.2 Overprinting Three Colors Where D1+2+3 is the density of all three over printed colors and D3 is the density of the last-down color.
FA 3 (%) = 2 1
D1+ 2 + 3 – D1 + 2 D3
· 100
Densitometry Expert Guide on Color & Quality 31
x = suitable for process colors • = suitable for process colors ( ) = limited suitability
Densitometer
Colorimeter Tristimulus colorimeter
Spectrophotometer •
Mixing of special colors
Inking setup • By standards
x
(•)
x
•
x
•
• Using color control strips in test prints
x
(•)
x
•
x
•
x
•
• Based on specified values
(x) (•)
• Using proofs
x
•
x
•
• Based on a specimen
x
•
x
•
• Based on image data (repro)
(x) (•)
x
•
• Assessment of color suitability
(x) (•)
x
•
Inking adjustment (by comparing)
x
•
x
•
Pressrun control • Based on solid patches
x
(•)
x
•
x
•
• Based on single-color tint patches
x
(•)
x
•
x
•
• Based on multicolor tint patches
x
•
x
•
• Based on in-image measurements
x
•
x
•
• Detection of ink soiling
x
•
x
•
• Detection of changes in substrate
x
•
x
•
Measurement values • Solid density
x
(•)
(x) (•)
x
•
• Tonal values and increases
x
(•)
(x) (•)
x
•
• Relative ink trapping
x
(•)
(x) (•)
x
•
x
•
x
•
x
•
• Absolute ink trapping • Metamerism • Subjective impressions
x
•
(x) (•) x
•
32 Expert Guide on Color & Quality Densitometry
1.0
3.6 The Limits of Densitometry Similar to the method used to create color separations, densitometers work with filters geared to the four process colors. They provide a relative measure of the ink film thickness, but do not reveal anything that directly correlates with human color perception. This fact limits their usefulness. The table on page 31 shows their typical applications as compared to tristimulus colorimeters and spectrophotometers. One major constraint on densitometry is that the same ink densities do not necessarily create the same visual impression. This is always the case when the colorants being compared differ, which is why proofs, test prints on different paper and/or with different ink than will be used in the production run, or other samples cannot serve as reliable references for setting the inking. The restriction to red, green and blue color filters is also significant. As soon as color sets comprising more than the four process colors come into play, problems arise for measuring the additional colors. Usually no suitable filters are available for them, which leads to excessively low ink density and incorrect dot gain values.
0.5
0.0 Color specimen: Pantone Warm Gray 1
The use of densitometers is also difficult for regulating inking based on multicolor tint overprint patches (e.g., gray patches). Measuring a gray patch with all three color filters yields different ink densities than if each color were measured by itself. Each of the three colors makes a more or less substantial contribution to all ink densities. This is because the process colors are not genuinely pure primary colors with each representing two-thirds of the spectrum; they also absorb light from other wavelengths. Densitometers are useful for monitoring quality in pressruns using the four-color process. In all other cases, their suitability is limited.
The color tone shown here (Pantone Warm Gray 1) has — as can be seen in the adjacent diagram — relatively high remission, which drops off slightly in the blue spectrum (380 to 500 nanometers). So the highest density value (0.27) is measured with a blue filter.
Densitometry Expert Guide on Color & Quality 33
1.0
The special colors HKS 8 and HKS 65, shown in the second and third examples, have radically different tones. This is also evident in their remission curves. However, both colors have the greatest absorption in the blue spectrum (380 to 500 nanometers), for which reason the highest density value (1.6) is measured with a blue filter in both cases. This illustrates the fact that there is no correlation at all between density values and color tones.
0.5
0.0 Color specimen: HKS 8
Only colorimetric measurements can tell us something about a color’s appearance.
1.0
0.5
0.0 Color specimen: HKS 65
34 Expert Guide on Color & Quality Colorimetry
Light source
4 Colorimetry
Measurement instrument an ce ct tra lr ef le ec
ce
an
Sp
ct
To perform a measurement, a printed sample is illuminated. The ref lected light passes through one or more lenses and strikes a sensor. The sensor measures the received light for each color and relays the results to a computer. There the data is weighted using algorithms that simulate the action of the three types of cones in the human eye. These algorithms have been defined by the CIE for a standard observer. They yield three standardized color values: X, Y and Z. These are then converted into coordinates for the CIE chromaticity diagram or some other color space (e.g., CIELAB or CIELUV).
le ef lr
A color (specimen) is illuminated by a light source. Part of the light is absorbed by the specimen while the rest is ref lected. The ref lected light is what our eyes register, because it stimulates the red-, green- and blue-sensitive cones (color receptors) in the retina.
Person This natural process is emulated in colorimetric instruments. tra
The principle of operation of all colorimetric instruments is based on how human beings see and perceive color (see figure).
Radiation
ec
4.1 Measuring Colors Tristimulus colorimeters and spectrophotometers are used to measure colors. They are described in sections 4.8 and 4.9 below.
This stimulation results in electrical signals being sent via the optical nerve to the brain, which interprets them as colors.
Sp
As explained in the section on color systems, three parameters are needed to unambiguously describe a color. Colorimetry tells us how to obtain these values and how they are interrelated — provided, that is, that colors are measurable. So there is a direct connection between color measurement and colorimetry.
Eye
Lenses with sensor
Cones
Blue
Green
Red
Spectral value algorithm for standard observer
Stimulation
Standard color values
Color perception
Color coordinates
Colorimetry Expert Guide on Color & Quality 35
4.2 Standard Color Values/ Reference White Before colors can be measured, it is necessary to determine standard color values based on measured ref lectance and emissions under standardized conditions. Most instrument manufacturers have fixed or applied these so that the user doesn’t need to worry about them. However, three factors are usually variable when measuring body colors, and therefore have to be set by the user: reference white, the type of light (illuminant), and the observer. Normally, colorimetric values are based on “absolute white”. They are calibrated to the measuring instrument’s white standard, which is in turn calibrated to a (theoretical) absolute white. In contrast to densitometry, measurements are only referenced to the paper in special cases. 4.3 Standard Illuminants Without light there is no color. The type of light also codetermines how we perceive a color. The color of the light itself is defined by its spectral composition. The composition of natural sunlight is inf luenced by the weather, the season and the time of day.
The spectral composition of artificial light also varies. Some lamps emit reddish light, while others give off slightly greenish or bluish light. Lighting conditions affect spectral ref lectance and thus color perception. Standard color values therefore have to be based on standardized light, which is called an illuminant. For standardization purposes, the spectral distribution (intensity) of different illuminants has been defined within the wavelength range from 380 to 780 nanometers. The figure above shows the spectral distributions of the standardized illuminants A, C, D50 and D65. The standard illuminants C, D50 and D65 resemble average daylight, with the greatest radiation intensity in the blue region. The figure below shows the spectral composition of D65. The standard illuminant A is most intense in the red spectrum and therefore appears reddish (like evening light and the light from light bulbs).
36 Expert Guide on Color & Quality Colorimetry
4.4 Standard Observers/ Spectral Value Functions We are all equipped with three spectral value functions for interpreting red, green and blue. In persons with normal color vision, they are approximately the same. Consequently, only borderline colors are perceived differently from person to person. For example, what one individual sees as bluish green may be perceived by another as greenish blue. For colorimetric purposes, it is therefore indispensable to define a theoretical average person, the “standard observer”. In the 1920s a series of experiments was carried out with subjects having normal color vision. The findings were used to derive the standard spectral value functions x, y and z, which the CIE specified in 1931 in a
number of national and international standards including DIN 5033 and ISO/CD 12647. The experiments were conducted using a circular split screen 2 degrees across (see the diagram on the right). This corresponds to a screen 3.5 centimeters in diameter at a distance of one meter. In 1964 the tests were repeated with a screen 10 degrees across and the results were also standardized, giving rise to the “10-degree standard observer”.
1m
^ 3.5 cm 2° = 10° ^ = 17.5 cm
Colorimetry Expert Guide on Color & Quality 37
4.5 Evaluation with a Spectrophotometer The standard color values are calculated based on the spectrum of the S(λ) illuminant, the measured spectral ref lectance of the color b(λ) and the standardized spectral value functions x(λ), y(λ) and z(λ) for the standard observer.
Illuminant
times The lambda in brackets (λ) indicates that the calculation depends on the wavelength l of the light. The first step is to multiply the radiation function of the standard S(λ) illuminant for each of its wavelengths (i.e., for each spectral color contained in it) by the ref lectance values β(λ) measured for the color. This yields a new curve, the color stimulus function ϕ(λ).
Reflectance yields Color stimulus function times
The second step is to multiply the values of the color stimulus function by those of the standard spectral value functions x(λ), y(λ) and z(λ). This yields three new curves.
Standard spectral value function
Finally, integral calculus is applied to determine the areas beneath these curves, which are then multiplied by a standardization factor to obtain the standard color values X, Y and Z, which precisely describe the measured color.
Integral and standardization factor
and
yields Standard color values
38 Expert Guide on Color & Quality Colorimetry
4.6 Color Distance (ΔE) The color distance (ΔE) is a measure of how far apart two colors are within a color space (for example, between an original and a printed reproduction). The CIE color space is explained in section 1.4 on color systems. But this color space has one serious drawback: equal distances in the chromaticity diagram do not correspond to equal perceived visual differences between different color tones. The American D.L. MacAdam studied this in many experiments and developed what are known as MacAdam ellipses that define regions of color in a chromaticity diagram that are indistinguishable to the observer. The figure shows them enlarged by a factor of ten. Because the CIE diagram is actually three-dimensional, in reality they are ellipsoids. It turns out that these regions vary widely in size depending on the color.
As a result, the CIE color space is not suited for evaluating color distances. Using it would mean accepting different tolerances for every color tone. In order to reliably and usefully calculate color distances, a color space is needed in which the distance between two colors actually corresponds to the perceived difference between them. Two such systems are CIELAB and CIELUV, which are mathematically derived from the CIE chromaticity diagram. The transformation applied maps the MacAdam ellipsoids onto spheres of nearly identical size. As a result, the numerical distances between colors matches the perceived distances between them. In 1976 the CIELAB and CIELUV color spaces, which are now the ones most widely used in the printing industry, were internationally standardized.
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Colorimetry Expert Guide on Color & Quality 39
The figure shows the a* and b* axes of the CIELAB color space in the x-y color chart.
0.8 Other color spaces are also used in the United States, such as the CMC system and the Munsell color space.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
40 Expert Guide on Color & Quality Colorimetry
4.6.1 CIELAB The CIELAB color space is the one most often used to measure body colors (for instance, for developing color mixing formulae and measuring printed colors). The hue and saturation are plotted on the a* and b* axes. The a* axis runs between –a* (green) and +a* (magenta), and the b* axis runs from –b* (blue) to +b* (yellow). The lightness axis, L*, runs from 0 (black, below) to 100 (white, above).
The figure on the right shows the CIELAB color space for body colors. Because it was derived by a mathematical transformation, it is shaped differently from the CIE chromaticity diagram. The shape of the individual lightness levels also changes with L*. The figure below shows a cross-section of the CIELAB color space for body colors at a lightness value L* of 50. The smaller green and enlarged blue ranges are clearly visible.
Colorimetry Expert Guide on Color & Quality 41
White
It is very practical to use a schematic diagram for this.
Yellow
Green
Example:
Red
Blue
Black
Specified
Actual
target color
measured color
L*
70.0
75.3
a*
55.0
51.2
b*
54.0
48.4
L* = 75.3 means that it is a light color, the position of which is situated between yellow and red with a* = 51.2 and b* = 48.4. So in this example, it is a light yellowish red or orange. Conclusion: The actual measured color deviates from the specified reference color.
The color distances are calculated using the following formulae: ΔL* = L*actual – L*target Δa* = a*actual – a*target Δb* = b*actual – b*target ΔE*ab = √ ΔL*2 + Δa*2 + Δb*2
42 Expert Guide on Color & Quality Colorimetry
White In terms of their visibility, color f luctuations can be classified as follows:
Actual
ΔE between 0 and 1
Deviation that is not normally visible
Target
ΔE between 1 and 2
Very small deviation, only visible to a trained eye
ΔE between 2 and 3.5
Medium deviation, also detectable to an untrained eye
The calculation yields: ΔL* = 75.3 – 70.0 = 5.3 Δa* = 51.2 – 55,0 = – 3.8 Δb* = 48.4 – 54,0 = – 5.6 ΔE*ab= √5.32 +(–3.8)2 +(–5.6)2 = 8.6
ΔE between 3.5 and 5
Obvious deviation
ΔE over 5
Significant deviation
Because the transformation used is not linear, the CIE chromaticity diagram and the CIELAB color space are not interchangeable. The fact that the latter is widely used around the world speaks in its favor, however.
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4.6.2 CIELUV The CIELUV color space is another attempt to linearize the perceptibility of color differences. It is also derived from the standard CIE chromaticity diagram but using other formulae. Its three coordinates are designed L*, u* and v*. Because the CIELUV and CIELAB color spaces result from different transformations, they differ in shape. Both are used for body colors.
The figure shows a cross-section of the CIELUV color space for body colors with a luminance value L* of 50. The green colors are located further inward than in the CIELAB color space, and the blue range is enlarged by comparison (see section 4.6.1).
The CIELUV color space is frequently used to assess the light colors of television screens and computer monitors. Its advantage is that it is derived from a linear transformation, so that color relationships are the same as in the master CIE space (this is not the case with CIELAB).
44 Expert Guide on Color & Quality Colorimetry
4.6.3 CIELCH CIELCH is not a color space in its own right; it merely refers to the use of the cylindrical coordinates C (chroma, as the distance from the center) and h (hue, as an angle) instead of Cartesian coordinates in the CIELAB or CIELUV color space. The calculations involved correspond to those in CIELUV. Here is a schematic diagram with the same measured color locus as in section 4.6.1:
The lightness L* remains unchanged. The chroma C*ab is calculated with C*ab = √a*2 + b*2. The hue angle h*ab is equal to h*ab = arctan ( b* ). a*
4.6.4 CMC CMC, a system based on the CIELAB color space for evaluating color distances, was developed in Britain in 1988 by the “Colour Measurement Committee of the Society of Dyers and Colourists” (CMC). Unlike CIELAB and CIELUV, it describes how well differences in color are accepted by an observer, not how they are perceived. It addresses the fact that, generally speaking, color f luctuations near the lightness axis are perceived as much more irritating than deviations in more saturated colors. It is also true that f luctuations in chroma (saturation) are tolerated much better than in the hue angle.
Measured color: L* = 75.3 C* = 70.5 h* = 43.4°
The figure below illustrates application of the CMC principle to assess color differences in the CIELAB color space. Each ellipse shows colors with acceptable deviations around the target locus based on the CMC formula.
Colorimetry Expert Guide on Color & Quality 45
It can be clearly seen that the ellipses (representing tolerances in the CMC color space) are smaller near the central lightness axis than in regions of greater saturation. They are also shaped to ref lect the fact that the permissible deviations in the hue angle are smaller than in the chroma value. They allow for f lexible adjustments for assessing lightness and color tone deviations; these adjustments are made using two weighting factors, l and c (where l is the weighting factor for lightness; the weighting factor c for the color tone is usually left equal to 1). The textile industry often uses weighting factors with a ratio of l : c = 2 : 1, meaning that lightness deviations are more readily accepted than color tone deviations by a factor of two. Δ Lightness
This relationship can be adjusted to suit each application. This means, however, that color distance values are only informative and comparable in conjunction with the same weighting factors.
Δ Chroma ΔH ue
Δ Lightness Δ Chroma ΔH ue
46 Expert Guide on Color & Quality Colorimetry
4.7 Munsell In 1905, Alfred Munsell developed a system for quantitatively and objectively representing color distances as they are perceived. He used the terms hue, chroma (saturation) and value (lightness) to describe the attributes of color. Five basic hues make up the notation system: red, yellow, green, blue and purple. It was published in 1915 as the “Munsell Book of Color� for 40 color tones with the C illuminant, including both glossy and matt samples. Each of the five basic hues is subdivided into up to 100 even-numbered color tones, each of which has a grid comprising 16 chroma and 10 lightness levels. The figure shows a cross-section of the Munsell color tree with 40 color tones. Because not all of the slots in each grid are occupied, the result is an irregular color space. Munsell coordinates cannot be mathematically converted into CIE coordinates. Other color systems are the DIN color atlas (defined by DIN 6164), the Natural Colour System (NCS), the OSA system (from the Optical Society of America), and the RAL design system (RAL-DS).
4.8 Tristimulus Photometry Tristimulus photometers resemble densitometers. However, instead of three color filters for red, green and blue plus a visual filter, they use filter combinations that reproduce the three standard spectral value functions x, y and z.
for determining color distances, an application in which absolute precision is not critical.
The absolute measurement precision of tristimulus photometers is less than that of spectrophotometers, typically because they fail to accurately model the standard spectral value functions and the required standard light source is unavailable. They are useful, however,
Using a lamp that emits light with a spectral composition approximating that of a standard illuminant, a control patch is illuminated. In the example shown on page 47, the color cyan is being measured.
Tristimulus instruments are also considerably cheaper than spectrophotometers.
The spectral ref lectance is measured using three different filters for x, y and z. Behind filter x (red), the standard color value X is measured, behind filter y (green) the standard color value Y, and behind filter z (blue) the standard color value Z. These standard color values can then be converted to a system that linearizes perceived color distances (CIELAB or CIELUV).
Colorimetry Expert Guide on Color & Quality 47
4.9 Spectrophotometry Spectrophotometry measures the visible spectrum, for example from 380 to 730 nanometers. The light ref lected by a printed color is split into its spectral constituents, for instance using a diffraction grid, and these are captured by a large number of sensors. The measured ref lectance values are used to calculate the standard color values X, Y and Z. This is done on a computer using the standard spectral value functions. Because these functions do not need to be modeled with filters, the absolute precision of spectrophotometers is very high. A major advantage of spectrophotometry — besides its high absolute precision — is the fact that spectrophotometers can output the standard color values for all standardized illuminants and observers, provided that the corresponding values have been stored. They can also calculate densities for any desired filter standards.
Paper The principle of operation of a tristimulus photometer
48 Expert Guide on Color & Quality Colorimetry
Ink manufacturers have to make their products to precisely match specifications. This is very important for standardized colors (as per DIN ISO 2846-1), but also for all HKS and special colors. They achieve this by measuring a specimen with a spectrophotometer and then using an appropriate formulation program to calculate the proportions for mixing the ink. Previously, it was impossible to optimally use spectrophotometers in print shops. They were expensive and awkward to use, and the measurements they provided were not directly applicable to the process colors. They therefore tended to be used only for one-off measurements of special colors and for checking materials (e.g. substrates and inks). They played no role in quality control.
At IPEX 2006, Heidelberg unveiled the world’s first system that measures inline in the press: Prinect® Inpress Control. A measuring head travels over a color control strip and/or the printed image and spectrally measures all of the control elements or image pixels. This can be done as desired with the standard illuminants A, C, D50 or D65 and either the 2° or 10° standard observer. The principle of operation of a spectrophotometer is shown below.
First the light source is directed via an annular ref lector at a 45° angle onto the printed specimen. The light ref lected at an angle of 0°is relayed from the measuring head to the spectrophotometer via a def lecting mirror and a fiber-optic cable. There it is split into its spectral color by a diffraction grid (similarly to a prism).
puter. There the measured values are colorimetrically evaluated and output as the standard color values X, A and Z and also as the standard color value components x, y and Y.
Photodiodes then measure the radiation distribution across the entire visible spectrum (between 380 and 730 nanometers) and pass the results to a com-
Press CP2000 Center Computer
Light source
Spectral remission Diodes
4.10 Spectral Quality Control with Heidelberg At drupa 1990, Heidelberg became the first manufacturer to exhibit a spectral measurement system for offset printing that could be directly interfaced with the press via the CPC®1 automatic remote inking control system: the CPC 21. It was joined at IPEX 98 by the spectral image measurement system Prinect Image Control. The CPC 21 was replaced at IPEX 2002 by Prinect Axis Control.
Annual mirror Deflecting mirror Fiber-optic cable
Sensor Paper
Diffraction grid
Colorimetry Expert Guide on Color & Quality 49
After comparing the measured values with previously entered target values, the system calculates adjustment recommendations for the various colors involved and relays these to the Prinect® CP2000 Center® press control system. There the data is converted into precise values for controlling the individual ink zone motors and sent to them. 4.11 Color Control Strips Heidelberg also offers a library of digital print control elements (Dipco) for all Prinect products used to monitor and control inking and color. This comprehensive package contains all the digital elements needed to check and control the results obtained at each stage of the print process, from prepress to printing. The Heidelberg color measurement systems Prinect Axis Control and Prinect Image Control measure and evaluate all color control strips included in the Dipco package, provided that they are aligned with the ink zones of Heidelberg presses. The results of measuring every element of a color control strip are compared with stored reference values. Based on this comparison, the Heidelberg color measurement systems calculate adjustment recommendations for the individual ink zones in each of the printing units.
How to mount the Color Control Strips • Do not place diagonally on the sheet;
mount parallel to a sheet edge. • Mount the strip so it is pointing
toward the center of the sheet.
The individual patches in the strips measure 6 mm high by 5 mm wide. The ink zones are 32.5 mm wide in all Speedmaster® presses, so there is room for 13 patches across two ink zones.
• Mount all parts of the strip together
in one row, without separating them. • Select the correct strip for the print job (process colors only, process and special colors, special colors only). • Select the correct strip for subsequent measurement and control with color measurement systems: – Full-tone/gray-patch control: use 4GS, 6GS, 6GS99 or 8 GS – Full-tone control: use 6S or 6S+ • Select the correct strip for the halftone tint patches to be evaluated: – 70%: Prinect strips – 40% and 80%: Prinect/FOGRA strips • Do not cut strips so the patches are smaller than 6 mm high by 5 mm wide. • Position strips so they will not be where the grippers grab the sheet. • Strips can be placed at the leading or trailing edge or in the middle of the sheet (B&W printing). • When working with Prinect Image Control, do not position strips directly adjoining the print image (space about 1 mm away).
4.12 Color Control with Heidelberg 4.12.1 Color Measurement and Control Systems from Heidelberg Prinect Axis Control measures color control strips, doing so along one axis, which explains its name. Measurements are performed on the press control console. Prinect Image Control measures the entire image and can then control inking accordingly, which is also reflected in its name. Measurements are made on a separate console. Prinect Inpress Control measures color control strips in the press and controls inking inline.
50 Expert Guide on Color & Quality Colorimetry
Prinect Axis Control
Prinect Image Control
Prinect Inpress Control
4.12.2 Colorimetric Control Methods Color measurement and control systems from Heidelberg let you select from three different control modes: • Colorimetric based on full-tone (solid) patches • Colorimetric based on gray patches* • Colorimetric based on in-image measurements**
image and control the ink zones on the basis of the data obtained. It is ideal for making sure that sold product measures up.
4.12.3 Prerequisites for Measurement and Control on Printing Presses Before looking at how different measurement systems work, it is important to describe the most important prerequisites that must be met to ensure reliable measurement and control. The focus is on presetting the ink keys and priming the ink train. How the inking is preset depends mainly on the job's area coverage values and the material parameters (these are characteristic curves stored in the press control system). Ideally, the area coverage values are ascertained using CIP4 PPF data from prepress that is transferred to the press either online or with the aid of a memory card. The purpose of presetting the ink zones is to get the colors as close as possible to the target values right from the start. The ink keys and ink stripe widths are appropriately set in every zone of each inking unit. To
Originally only two colorimetric control modes were available: full-tone control using a color control strip (for process and special colors), and gray-patch control using an autotypical gray patch (CMY) and additional solid and tint patches for the chromatic colors CMY. Heidelberg has added a third mode that is based on measurement of the print image itself. Prinect Image Control from Heidelberg is the world’s first system able to measure the entire printed
All three control modes use colorimetric reference values. The ink zones are adjusted to bring the print results into optimum alignment with these reference values. In other words, the goal is a perfect color match between the press and reference sheets. The colorimetric approach underlying color measurement systems from Heidelberg means that a technology is used that emulates the color perceptions of the human eye to minimize the detectible color discrepancies between the OK sheet and the press sheets.
determine this, the characteristic curves are applied to convert the area coverage values into presetting values. This makes sure that the fountain rollers supply exactly the amount of ink that will be accepted by the substrate. A frequently underestimated factor is priming of the ink train. Before the first sheet is printed, the amount of ink is introduced to the ink train that will later result during the production run under stable conditions. It's best if the first sheet pulled is very close to the target colors. Experi-ence has shown that larger deviations from the reference values necessitate a larger number of adjustments. If inking is set well to begin with, there's no need to adjust it. When starting to print, the steps described here determine the point at which color measurement and control can begin.
*Not with Prinect Inpress Control **With Prinect Image Control only
Colorimetry Expert Guide on Color & Quality 51
4.12.5 Determining Target Values: A Practical Example Let’s say that you want to print according to Media Standard Print 2004 (MedienStandard Druck 2004). This standard defines dot gain as well as colorimetric reference values expressed as CIE-L*a*b* coordinates. Due to the inf luence of various factors, the CIE-L*a*b* can never be perfectly matched, so tolerances are also given for the individual process colors under production conditions. It is important for the press operator to know how closely he can approximate the reference values to the inks he is using. The colorimetric control process. Both the relevant values and the adjustment recommendations are directly derived from the spectrum of the measured color.
4.12.4 How Color Measurement and Control Systems from Heidelberg Work Heidelberg uses spectrophotometers for all of its modern color measurement systems, regardless of whether they capture ink density or L*a*b* values. During measurement, the generated spectra are relayed to an integrated computer, where special software uses them to calculate the required values. The spectral color values are the basis for colorimetric control, in other words, the recommendations for adjusting the ink zones are calculated directly without taking an indirect route via the density.
For control purposes, it is vital for the spectral values to be stored in the measuring instrument as reference or target values. In the cases of Pantone and HKS colors, this requirement has been met in all Heidelberg equipment. No spectral values are stored for process colors (4C), highly pigmented and other colors. There are two reasons for this: the large number of ink makes and types used in practice, and the fact that the colors of process inks often vary considerably. This makes it necessary for the press operator to determine the spectral values of these inks by measuring a (full-
tone) print specimen to ascertain a new target color locus. This only takes a few minutes and has the advantage of generating target values that can realistically be achieved with the ink used in the print shop. Quality control by monitoring color deviations is also feasible, for instance between different batches of ink.
There are two practical approaches for determining the achievable target value (= pressrun standard): 1. Make a series of prints ranging from underinked to overinked and measure them. The sheet in which the color is closest to the target value within the permissible tolerances is suitable for use as the standard for the measurement system. 2. Have the ink manufacturer prepare a laboratory test print on the same paper that will be used for the job. Scan it to serve as the standard for the measurement system.
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4.12.6 Inline Measurement and Control After the target values have been determined, measurement of the pressrun can begin. The first sheet pulled provides the first actual measured values, which — as mentioned before — should not be too far from the target values. The goal is now to adjust the ink zones and thus the ink film thicknesses to achieve the target values in a minimum of steps.
Flow chart showing the conversion of spectral values into control signals
This approach may seem simple at first glance, but it is based on a complex color model that describes how changes in the ink film thickness affect the color of the ink used. Colorimetry by itself can only tell us where the color achieved so far is located within the color space (the actual measured value) and where we need to get it to (the target or reference value); what it doesn’t tell us is how to accomplish this. But this is not the job of colorimetry. That is what the color model used is for. It can be used to work out how the color changes if, for example, the ink film thickness is increased by five percent. If the film thickness on the paper is changed by applying more or less ink, its visual appearance also changes by a certain amount, as we know. Imagine a series of prints ranging from very light inking to full saturation within the CIE-L*a*b* color space. They
Colorimetry Expert Guide on Color & Quality 53
or whether it is necessary to take steps such as washing the inking rollers. When using an ink whose target value is not stored, the color measurement system also shows, right from the very first pull, whether the colors can be kept within the tolerances or not. This can be the case, for example, when working with a different make or type of ink and a previously stored reference value. This is a situation in which one of the measurement system’s most important functions comes into play: the ability to determine and display the smallest
achieve color deviation (ΔE0). It can also happen that different batches of the same type of ink let you attain the same CIE-L*a*b* but with different densities. If you only printed based on the reference densities, the visual appearance of the prints can diverge afterward. This is why the ISO standard does not provide any reference densities.
The operator sees at a glance where the inking needs to be corrected. The black line shows the reference colors. The bars show the recommended adjustments, expressed as percentages, for each ink zone.
CIE-L*a*b* color space lie along a line that varies not only in terms of lightness, but also in its position along the a and b axes. This is called a color line. When using full tones to control inking, the achievable color tones are fixed by the ink’s pigmentation, color intensity and variable film thickness. This color model can be used in this example to calculate which film thickness comes closest to yielding the target value, and where the target value is located within the color space.
4.12.7 How Colorimetry Helps In practice, this means that the operator sees at a glance whether or not he can attain the desired color results. If all parameters of the print process are optimally coordinated, he can expect to achieve them. If the printing conditions change, for instance resulting in blackening of the chromatic colors in the pressrun, the colors can deviate significantly from the targets. Colorimetry can then be a major help by revealing whether the desired color results can continue to be reached within the specified tolerances under these conditions,
Actual
ΔE
a
Reference
Target (Magenta) b
Color line
ΔE0
Colorimetric control always shows two results: the distance (ΔE) to the target color that still needs to be overcome to get as close as possible to the reference value, and the remaining discrepancy (ΔE0) between the actual and reference values that cannot be eliminated. The color line is shown in red.
54 Expert Guide on Color & Quality Colorimetry
4.12.8 Summary The biggest advantage of colorimetric control is that it lets you consistently bring the results of printing as close as possible to the visual appearance of the original, letting you know quickly if the deviation becomes too large. Colorimetric evaluation corresponds to the color perceptions of the human eye, with the advantage of being free of subjective inf luences and variable environment inf luences and therefore able to deliver objective readings. The measurement data can be stored and documented and used for quality certificates. Measurement results can also be automatically evaluated with the Quality Monitor software from Heidelberg, which is part of two Prinect products: the Prinect® Profile Toolbox und Prinect® Calibration Toolbox. 4.13 Standardization of Printing The standards of the graphic arts industry discussed below play key roles.
ISO-Compliant Inks The Euroscale, originally defined by DIN 16539 in 1975, has been improved since then. In 1996, ISO 2846 succeeded in establishing a common process standard that incorporated the ideas of the U.S. SWOP and the Japanese TOYO standards. Part 1 of this standard defines tolerances for colorimetric properties and transparencies for process inks for four-color and web offset printing that may not be exceeded when making test prints on APCO paper with a defined reference ink film thickness. However, the color values given in this standard are binding only for ink manufacturers, not for printers. ISO 12647-2 and ProzessStandard Offsetdruck (German Offset Printing Process Standard) In 1981, the German Printing and Media Industries Federation (bvdm) issued its first publication on standardizing sheetfed offset printing. The practical experience gained and the relevant scientific research findings made subsequently were incorporated into the international standard ISO 12647-2 “Process control for the production of half-tone color separations, proof and production
prints”. A revised edition of ISO 12647 was published in November 2004. ISO 12647-2 provided the basis for the ProzessStandard Offsetdruck (the German Offset Printing Process Standard) that the bvdm published in 2003. It can be ordered from the bvdm as a binder (193 A4 pages with supplementary sheets). Because its scope goes beyond that of ISO 12647-2, many printing companies in Germany and elsewhere are taking advantage of it as the basis for achieving faithful color reproduction.
The ProzessStandard Offsetdruck (German Offset Printing Process Standard) of the German Printing and Media Industries Federation (bvdm) Screen ruling Screen angle
Halftone dot shape Total area coverage Gray balance Quarter tones Midtones Three-quarter tones
60 lpc Nominal angular difference between C, M, K = 60° (chain dots), = 30° (circular or square dots) Y = 15° from another color, dominant color at 45° or 135 Color control strip: circular dots, image: chain dots with 1st dot touch ≥ 40%, 2nd dot touch ≤ 60% ≤ 340 % Cyan Magenta Yellow 25 % 18 % 18 % 50 % 40 % 40 % 75 % 64 % 64 %
Colorimetry Expert Guide on Color & Quality 55
Paper type
1/2 L*/a*/b*
3 L*/a*/b*
4 L*/a*/b*
5 L*/a*/b*
20/0/0 55/–36/–44 46/70/–3 84/–5/88 45/65/46 48/–64/31 21/22/–46
31/1/1 58/–25/–43 54/58/–2 86/–4/75 52/55/30 52/–46/16 36/12/–32
31/1/2 59/–27/–36 52/57/2 86/–3/77 51/55/34 49/–44/16 33/12/–29
Black backing Black Cyan Magenta Yellow Red Green Blue
16/0/0 54/–36/–49 46/72/–5 88/–6/90 47/66/50 49/–66/33 20/25/–48 Substrate backing
Black Cyan Magenta Yellow Red Green Blue
16/0/0 55/–37/–50 48/74/–3 91/–5/93 49/69/52 50/–68/33 20/25/–49
20/0/0 58/–38/–44 49/75/0 89/–4/94 49/70/51 51/–67/33 22/23/–47
31/1/1 60/–26/–44 56/61/–1 89/–4/78 54/58/32 53/–47/17 37/13/–33
31/1/3 60/–28/–36 54/60/4 89/–3/81 53/58/37 50/–46/17 34/12/–29
Paper types
1 115 gsm Glossy Coated Art reproduction
2 115 gsm Matt-coated Art reproduction
3 65 gsm LWC Web offset
4 115 gsm Uncoated White offset
L*a*b* reference values for the five paper types
5 115 gsm Uncoated Yellow offset
56 Expert Guide on Color & Quality Colorimetry
AF (%)
Dot gain ΔA (%) for paper type
40 50 70 75 80
1+2 09 – 13 – 17 10 – 14 – 18 10 – 13 – 16 09 – 12 – 15 08 – 11 – 14
3 12 – 16 – 20 13 – 17 – 21 12 – 15 – 18 10 – 13 – 16 08 – 11 – 14
4+5 15 – 19 – 23 16 – 20 – 24 13 – 16 – 19 11 – 14 – 17 09 – 12 – 15
Reference dot gain values for the five paper types
Media Standard Print Specifications and tolerances for digital proofing
Mean ΔE for all L*a*b* color distances of color patches Maximum ΔE for all L*a*b* color distances of color patches Tolerance for primary colors Maximum deviation of substrate Specifications and tolerances for digital proofing
ΔE 4 10 5 3
The Media Standard Print (MedienStandard Druck) The Media Standard Print (MedienStandard Druck) first appeared in 2004 at the initiative of the German Printing and Media Industries Federation (bvdm). In addition to technical guidelines for digital data for printing, based on ISO 12647, it defined specifications and tolerances for digital contract proofs. This established a set of rules for agencies, prepress studios and printing companies, providing a basis for improving communication and optimizing workf lows. In 2004 the fourth, revised edition of the Media Standard Print was issued. It primarily established the following rules: • A proof must simulate one of the five reference print conditions defined by the ProzessStandard Offsetdruck (German Offset Printing Process Standard). • A proof must include a line of text indicating the file name, the output date, and the color management settings used. • A UGRA/FOGRA media wedge must be included. • The conditions for measurement and evaluation must be defined.
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4.14 Benefits of Colorimetry for Offset Printing To sum up, here is an overview of the principal advantages that colorimetry offers to offset printing: • The measurement values very closely match visual perception of the colors. • Colorimetry is a process-independent color evaluation method that can be used throughout the print process from prepress across all kinds of proofs to final quality control of finished products. • Colorimetric reference values can also be expressed as figures. Linking to prepress is possible. • Colorimetric reference values can be taken from specimens. • Colorimetry is the only way to ensure objective evaluation. • Colorimetry makes image-relevant color control possible (for instance using gray patches) without calibration of individual colors and stored conversion tables. • All colors, including very light special colors, can be correctly and dependably controlled with colorimetry.
• Dot gain is precisely captured by spectral measurement, also with special colors. • Control of the production run is more reliable, because changes in the substrate, ink soiling and metamerism can be captured and taken into account. • Halftone printing with more than four colors can also be correctly controlled. • Print quality can be described and documented more effectively. There is a color-tone-independent measure of color deviations: ΔE. • Spectral measurement enables the development of better color models possible. • Colorimetry lets the printing industry move into line with all other industries in which color plays an important role. • Densitometry is an integral part of spectral color measurement. • Print image fragments can also be compared with originals.
Heidelberger Druckmaschinen AG Kurfürsten-Anlage 52 – 60 69115 Heidelberg Germany Phone +49-62 21-92 00 Fax +49-62 21-92 69 99 www.heidelberg.com Publishing Information Printed: 10/06 Photographs: Heidelberger Druckmaschinen AG Platemaking: Suprasetter Printing: Speedmaster Finishing: Stahlfolder Fonts: Heidelberg Gothic, Heidelberg Antiqua Printing in the Federal Republic of Germany Copyright © 2006 by Heidelberger Druckmaschinen AG
Trademarks Heidelberg, the Heidelberg Logo, Prinect, Axis Control, CP2000 Center, CPC, Image Control, Speedmaster and Mini Spots are registered trademarks of the company of Heidelberger Druckmaschinen AG in Germany and other countries. Other product names used here are trademarks of their respective owners. Subject to technical and other changes.
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