color_and_quality

Copyright © 1995/1999 Heidelberger Druckmaschinen AG Kurfürsten-Anlage 52 – 60 D-69115 Heidelberg Telefon + 49-62 21-92-00 Fax + 49-62 21-92-69 99

Colour & Quality

2nd Edition

00.992.6402a E (Colour & Quality)

Colour & Quality

Colour & Quality

Printed in the Federal Republic of Germany.

All rights reserved. No part of this publication may be translated, reproduced, or transmitted, in any form or by any means, without the written permission of the copyright holder. Any offense will be prosecuted.

Photography Heidelberger Druckmaschinen AG Tony Stone, Page 8 Bavaria, Page 31 2nd revised edition Copyright © 1995/1999 Heidelberger Druckmaschinen AG Kurfürsten-Anlage 52 – 60 D-69115 Heidelberg Telefon + 49-62 21-92-00 Fax + 49-62 21-92-69 99 Fee: DM 15.00

TABLE OF CONTENTS Light and colour 1.1 1.2 1.3 1.4

Light is colour Visual perception of colour Colour mixture Systems of colour classification

Colour reproduction in printing 2.1 2.2 2.3 2.4

Ink film thickness The significance of the halftone value in printing Contrast Colour balance/image build-up

2.5 2.6

Ink trapping and colour sequence Print control strips

Densitometry 3.1 3.2 3.3 3.4 3.5 3.6 3.7

Measuring principle of a reflection densitometer The use of filters in densitometry Measuring values in densitometry Measurement Evaluation Standardisation in printing Limits of densitometry

Colorimetrics 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10

Measuring colour Tristimulus values/white reference Standard illuminants Standard observer/colour matching functions Evaluation with a spectrophotometer Colour difference E Munsell Tristimulus method Spectral colour measurement The measuring principle of the Heidelberg CPC 21 spectral quality control 4.11 Proof and colour control strips 4.12 Ink control with Heidelberg CPC 21 4.13 Advantages of colorimetrics for offset printing

1 8 10 12 16

2 22 24 34 35 40 43

3 48 50 53 56 58 60 62

4 68 70 70 72 74 76 86 87 88 90 91 93 98

Light and colour

1

Light and colour

1.1 1.2 1.3 1.3.1 1.3.2 1.3.3 1.4

Light is colour Visual perception of colour Colour mixture Additive colour mixture Subtractive colour mixture Autotypical colour mixture Systems of colour classification

8 10 12 12 13 15 16

7

1.1 Light is colour We live in a colourful world. With the help of colours we brighten up our surroundings to make us feel good.

Interior design and colour schemes directly influence our impressions and feelings. Colours that go well together create a harmonious balance, putting us in a good mood.

The printing industry also uses colours to make presentations more effective. The quality demands on printed materials supplied to customers are increasing all the time.

To meet these new demands, new standards of quality have to be introduced.

In order to judge colours we have to “see” them.

For this purpose we need light.

The sun emits light – it is a primary light source.

Most objects in our environment, however, do not emit light themselves. They are so-called secondary light sources.

We can perceive them and their colours only if they are illuminated by light.

8

Light is radiation which propagates very quickly – at a speed of 300,000 kilometers per second. Strictly speaking, light consists of electromagnetic oscillations spreading out from their source like waves. Like a water wave, each light wave consists of a crest and a trough.

wave crest

wave trough Waves are classified on the basis of either their length or the number of oscillations they perform per second.

Wavelengths are given in units such as kilometers, meters, centimeters, millimeters, nanometers or picometers.

The number of oscillations per second – the frequency – is measured in Hertz.

Waves of different lengths possess different properties.

X-rays, for example, are used in medicine for diagnostic purposes, while many households are already equipped with microwave ovens. Other kinds of waves serve to transmit

red (approx. 700 nm)

telephone calls as well as radio and television programs.

Only a very small range of electromagnetic waves is seen by us as coloured light. The visible portion of the wave spectrum lies between 380 nm (ultraviolet light) and 780 nm

green (approx. 550 nm)

(infrared light). By means of a prism, light can be broken up into its colour components. White light, being composed of all the colours of the spectrum, is broken up into all the colours of the rainbow.

blue (approx. 400 nm)

The following illustration shows how the wavelengths from red to green to blue become shorter and shorter.

9

X-rays

UV

IR Microwaves

Gamma rays

Radiofrequency Radar TV

FM Radio UHF KB MB LB Wavelenght

Visible range

1.2 Visual perception of colour

It is only in conjunction with light that colours become “visible” – but why?

Colour can not be regarded as a characteristic feature of an object, as can its shape. Yet it is a property of objects to either absorb or reflect light of certain wavelengths.

We can only perceive the colours that correspond to the reflected wavelengths.

10

If white light reaches an object, one of the following may occur: ■

All the light is absorbed. In this case we perceive the object as black.

■

All the light is reflected. In this case the object appears as white.

■

All the light is let through the object. In this case the colour of the light does not change.

■

Part of the light is absorbed, the rest is reflected.

We perceive a colour whose hue depends on which wavelengths are reflected and which are absorbed.

■

Part of the light is absorbed, the rest is transmitted.

We see a colour whose hue depends on which wavelengths are absorbed and which are transmitted.

■

Part of the light is reflected, the rest is transmitted.

Under these circumstances the colour of the reflected and that of the transmitted light changes.

The properties of the illuminated object determine which of the above-mentioned effects is likely to occur.

Light reflected or transmitted by an object is received by our eyes and transformed into nervous impulses, which trigger the colour sensation in our brain.

11

The retina of the human eye contains light-sensitive cells.

There are two kinds of cells: rods and cones. The rods distinguish between bright and dark, whereas the cones react to colours. There are three kinds of cones, each of which is sensitive to certain wavelengths. Part of

Paper

them reacts to light within a range of 400 to 500 nm and is therefore sensitive to blue light. Other cones can “see� only within a range of 500 to 600 nm, i.e. only green light. The third kind of cone is receptive to red light, which lies within a range of 600 to 700 nm.

This composition of rods and cones renders the human eye so sensitive that it is capable of perceiving and distinguishing millions of colours.

1.3 Colour mixture 1.3.1 Additive colour mixture An additive mixture of colours is a superimposition of light composed of different colours. If all the colours of the spectrum are added together, the colour white results.

Red, green and blue are the additive primary colours.

They are so-called one-third colours because each represents one third of the visible spectrum.

The principle of the additive colour mixture can be illustrated very well with three diascopes, each of which produces a light spot on a screen in one of the three additive primary colours.

12

green

+

red

=

yellow

green

+

blue

=

cyan

blue

+

red

=

magenta

blue

+

red

=

white

=

black

+

no light

green

Within the overlapping areas of the three light spots the following secondary colours emerge:

The principle of additive colour mixture is used in colour TV and in the theater to produce all the colours of the visible spectrum.

1.3.2 Subtractive colour mixture For subtractive colour mixture individual colour components are taken from white light. If all the colour components are removed, the colour black results.

Cyan, magenta and yellow are the subtractive primary colours. They are two-third colours because each represents two thirds of the visible spectrum.

They can be produced by subtracting an additive primary colour from white light (for example by means of a filter), or by superimposing the light of two additive primary colours.

Printing inks are transparent substances that function like colour filters. Which colour results if a blue-absorbing substance is printed on white paper?

Blue is removed from white light; the other components (green and red) are reflected. The additive superimposition of these two colours produces yellow.

This is the colour we perceive.

The printing ink has thus subtracted one third (i.e. blue) from the white light (consisting of red,

Paper

green and blue).

13

Let us assume that two transparent substances are printed one upon another, for example, the printing inks “yellow” and “cyan”. The substances successively filter the blue and red portion from the white light. As a result, we perceive green light. Together, the printing inks have subtracted two thirds of the colour components.

Paper Paper

When cyan, magenta and yellow are printed in layers one upon another the incident light is totally absorbed, (i.e. there is no reflection); we perceive the colour black.

Paper Paper

14

cyan

+

yellow

=

green

yellow

+

magenta

=

red

magenta

+

cyan

=

blue

cyan

+

magenta

=

black

=

white

+

yellow

no colour

1.3.3 Autotypical colour mixture Colour images are printed using the four printing inks cyan, magenta, yellow and black. The black printing ink improves the sharpness and depth of pictures. This is because, due to the properties of the pigments of chromatic colours, the black colour subtractively mixed from cyan, magenta and yellow, is never really dark black as such.

In offset printing the size of the dots depends on the desired hue. When printed, the dots of the individual colours are partly juxtaposed or partly or totally printed one upon another. If we look at the dots through a magnifying glass (see illustration), we perceive colours which – except for the white of the paper – are the result of subtractive colour mixture. However, without a magnifying glass and from a normal distance, the human eye cannot discern the individual dots. In this case the printed colours are mixed additively.

The composition of additive and subtractive colour mixture is called autotypical colour mixture.

[Illustration: diagram of additive and subtractive colour mixtures.]

15

In subtractive colour mixture the following secondary colours will result when cyan, magenta and yellow are printed one upon another.

1.4 Systems of colour classification Each individual perceives colours in a different way.

A description of hues by several persons will therefore lead to very different results.

Printers, however, need standardised criteria for colour identification. For this purpose different systems of colour classification have been established.

Some printing ink manufacturers produce sample books and give the colours names such as Novavit 4F 434. Others use colour fans like HKS and Pantone. The colour circle is another aid. It may consist of 6, 12, 24 or more parts.

All these systems set out examples of the individual colour hues and give them names. Yet they are never complete and mostly unsuitable for calculations.

As we have seen, our chromatic sensation depends on the stimulation of the receptors in our eye, which are sensitive to red, green and blue. Thus, for an unambiguous classification of various colours three values are needed.

With the help of such a system green, for example, could be described as: green = 0  red + 1  green + 0  blue or, even shorter, G = 0  R + 1  G + 0  B. If one draws the primary colours as the axes of a coordinate system a so-called colour space is obtained.

Many experts have discussed systems of colour classification and established different notions of how a colour space should be designed. All of these colour spaces have advantages and disadvantages.

16

Y The most important colour spaces have been standard-

100

ised internationally. They are used in many

80

branches of industry, for example in the dyeing and lacquer industry, in the textile industry, in food production

60

y

and in medicine. The CIE standard

0.8

colour chart has gained worldwide acceptance. (The abbreviation CIE stands for

40 0.6 20

0.4

“Commission Internationale de l’Eclairage”.)

0.8 0.2

This system uses the variables X, Y and Z for the “colour content values” instead of R, G and B. For practical reasons the chromaticity coordinates x and y and the luminance factor Y are determined from these coordinates.

(The luminance factor Y is used as a lightness measure of object colours.) The location of each colour can be precisely defined using these three coordinates.

Colours of the same lightness can be drawn in two dimensions, i.e. in a single plane. A cross section of the CIE colour space in a lightness plane is a CIE chromaticity diagram.

Spectral colours are the most saturated colours that can be produced for a given hue (wavelength). They are located on the border of the CIE chromaticity diagram. The illustration shows the spectrum loci together with the corresponding wavelengths in nanometers. The straight line connecting the wavelengths 380 nm and 780 nm is called the purple line.

All the trichromatic units of colours made through additive mixture of spectrum loci lie within the area surrounded by the spectrum locus and the purple line.

The central trichromatic unit has the coordinates x = 0.333 and y = 0.333. It is abbreviated as E (for “equi-energy spectrum”) for primary light sources and sometimes also A (for “achromatic”) in the case of object colours.

17

E 0.6

0 0.0 0.0

0.4 0.2

x

Visually perceptible colours within a lightness plane of the CIE colour space (standard colour chart).

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

The saturation of all colours increases from the central trichromatic unit towards the spectrum locus.

The Euroscale DIN 16 539 describes the position of the colour locations for cyan, magenta and yellow for threecolour and four-colour offset printing. It also defines the colour locations for the subtractive secondary colours red, green and blue.

The following chromaticity diagram shows the colour locations as laid down in DIN 16 539 as well as the range of colours that can be produced in printing. This distribution is very similar for all lightness values.

Colour hues located within the hexagon can be reproduced in four-colour offset printing using the colours of the Euroscale. Colours outside this area can only be produced with the aid of additional special colours.

18

Range of reproducible colours of Euroscale DIN 16 539.

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

In Euroscale DIN 16 539 the following values for coated paper have been defined for specified printing and measuring conditions: Primary and secondary colours Yellow Magenta Cyan Yellow-magenta Yellow-cyan Magenta-cyan

Colour coordinates 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

Luminance factor Y 77.8 17.1 21.9 16.3 16.5 2.8

The values for x, y and Y are measured using spectrophotometers. They exist as handsets or as computing centers with on-line machine control (as in the Heidelberg CPC 21).

19

Colour reproduction in printing

2

Colour reproduction in printing

2.1 2.2 2.2.1 2.2.2 2.2.3 2.3 2.4 2.4.1 2.4.2 2.4.3

Ink film thickness The significance of the halftone value in printing Halftone value shifts Dot gain Print characteristic Contrast Colour balance/image built-up Chromatic composition Achromatic composition Chromatic composition with under colour addition (UCA) Chromatic composition with under colour chromatic removal (UCR) Chromatic composition with gray stabilization Chromatic composition with gray component replacement (GCR) Five-, six- and seven-colour printing Ink trapping and colour sequence Ink trapping Colour sequence Print control strips Solid patches (fields) Overprint patches (fields) Colour balance patches (fields) Halftone patches (fields) Slur/doubling patches (fields) Plate exposure control patches (fields)

2.4.4 2.4.5 2.4.6 2.4.7 2.5 2.5.1 2.5.2 2.6 2.6.1 2.6.2 2.6.3 2.6.4 2.6.5 2.6.6

22 24 24 31 32 34 35 35 36 37 37 38 38 39 40 40 42 43 44 44 44 45 45 45

21

Quality assurance in printing aims at a correct and constant colour reproduction throughout the whole print-run.

In addition to the printing ink and the colour of the printing stock the most important parameters are ink film thickness, halftone value, colour balance, ink trapping and the sequence of colours.

2.1 Ink film thickness For technical reasons the maximum ink film thickness in offset printing is about 3.5 Âľm.

For coated paper and process colours according to DIN 16 539 the correct colour locations should be achieved with ink film thicknesses between 0.7 and 1.1 Âľm.

If unsuitable lithographies, inappropriate printing stock or unsuitable printing ink are used, however, it may happen that the standardised points at the corners of the CIE chromaticity diagram are not reached.

The range of reproduceable colours also decreases if the saturation is insufficient. In the illustration the white area shows how the range of colours narrows with the underinking of each of the three process colours.

In terms of physics the influence of the ink film thickness on the optical appearance can be explained as follows.

Printing inks do not cover the paper; they are, rather, transparent. The light penetrates the ink. In passing through the ink it encounters pigments which absorb to a greater or lesser extent certain wavelengths.

22

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

The higher the pigment concentration and the thickness of the ink film, the more pigments are hit by the incident light and, consequently, the more of it is absorbed.

Finally, the light rays reach the (white) surface of the printing stock and are reflected. On its way back the light has to pass through the ink film again and only then can it reach the observer’s eye.

A thick printing ink film absorbs more light and reflects less than a thin one; the observer therefore perceives a darker, more saturised, colour hue. The portion of light reaching the eye thus serves as a basis for the assessment of each colour.

23

2.2 The significance of the halftone value in printing Next to the printing ink the halftone value is the most important parameter for the optical appearance of a colour hue.

The halftone value indicates how much of the printing stock is covered by ink. The brighter the colour hue to be reproduced the smaller the covered area will be.

For the reproduction of different colour hues in classical scanning with a constant scanning frequency halftone-dots are used, whose size will depend on the desired hue.

In contrast to this, in frequency-modulated scanning different dot spacings are used to produce different hues (all the dots having the same size). Generally, halftone values are expressed in percent.

2.2.1 Halftone value shifts When a halftone dot is transferred from the film via the plate and the blanket to the printing stock, several factors may change its geometric size, and thus also its halftone value.

The alterations in halftone value caused by processing can be compensated in the pre-press stage. A curve describing the transfer characteristics is obtained by measuring printing patterns and comparing them to the originals. If during the entire printing process (from scanning to the finished print product) the same (standardised) parameters are always used one can expect the print product to be true to the original.

Halftone value shifts caused by difficulties in printing, however, cannot be foreseen. Special attention has to be paid to them during the printing process. The most important ones are:

24

Transfer of the

Features of the halftone dots

Influencing factors

halftone dot

Film

Film tape edges, adhesives

Mounting Exposure Chemicals, Development

duration of development Two halftone dots on the film (appr. 150fold magnification).

Printing plate

Material, abrasion due to printing

Plate exposure

exposure time, vacuum, side lighting

Dampening

Amount of dampening solution, Halftone dots on the plate.

pH value, surface tension, water hardness, temperature Inking

Thickness of ink coat, consistency, temperature

Printing plate/blanket

Rolling/packing Halftone dots on the plate after inking.

Blanket

Material, condition, surface

Printing blanket/ printing stock

Rolling/packing

Halftone dots on the blanket.

25

Transfer of the

Influencing factors

Features of the halftone dots

halftone dots

Printing Stock

Surface, paper quality

Sheet transport

Transfer register

Delivery

Offset

The large magnification clearly shows the first-rate result

Increase/decrease of the halftone value Dot gain means that there is an increase in halftone value Dot gain

during the printing process as compared to the dot on the film. The increase is in part due to processing, material, and machines, and thus cannot be influenced by the pressman (this aspect is also called enlargement of halftone dots).

To a certain degree, the pressman can counteract dot gain, especially by manipulating the inking.

Filling-in

Filling-in means the reduction of non-printing areas up to their complete disappearance. Filling-in may also be caused by slur or doubling.

Sharpening means a decrease in halftone value as compared

Sharpening

to the dot on the film. In practice, the term sharpening is often used to describe the reduced increase in halftone value, though the print may still be fuller as compared to the film.

26

Deformation of halftone dots Slur

Slur means that the form of the halftone dot is changed during the printing process due to relative motions between plate and blanket and/or blanket and printing sheet, that is, a circular point may become oval. Slur in the printing direction is called circumferential slur, and slur at right angles to the printing direction is called lateral slur. Diagonal slur results if both forms of slur occur at the same time.

Doubling

In offset printing, doubling means that a shadow-like and unintended, in general smaller, ink dot appears besides the intended halftone dot. Doubling is caused by an incongruent retransfer of ink by the subsequent blanket.

The term offsetting refers to those deformations of halftone

Offsetting

dots that are caused by mechanical factors after the printing process. The term offset is also used to describe the transfer of ink from freshly printed matter on to another surface.

Points for the pressman to observe With the help of control strips, dot gain can be monitored visually and measured in size. For purely visual checks, signal strips are particularly well suited. Filling-in can be best monitored with the help of screen measuring elements with high halftone values.

Dot gain and clogging are mostly caused by excess ink feeding not enough water feeding, too much pressure between plate and blanket, or by a poorly clamped blanket.

Furthermore, the inking and damping form rollers may not be well adjusted.

27

correct

wrong

correct

wrong

Under normal printing conditions and precise plate exposure the print is generally fuller than the film. Defects such as blind plates and build-up of ink on the blanket may cause sharpening. Remedies might be: more frequent washing of the blankets and inking units; changing the inks and the colour sequence; checking plate rollers, printing pressure, and printing process.

Slur is most obviously signalised by line screens. The parallel lines often indicate the direction of slur. Circumferential slur normally indicates printing differences that have emerged between plate cylinder and blanket cylinder, or excessive printing pressure. This is why each step of the printing process and the printing pressure should be monitored very carefully. Slur may also be caused by a poorly clamped blanket or by excessive inking. Lateral slur is mostly associated with other problems. In this case, the printing stock and the blanket should be carefully checked.

Doubling is monitored on the basis of the same elements as slur. In addition, halftone dots have to be examined with a magnifying glass, since the line screens alone do not allow to distinguish between doubling and slur. Doubling may be caused by various factors, most of which are related more or less closely with the printing stock.

Offsetting problems hardly ever occur with modern sheet-fed presses. Those areas of a sheet-fed press that support the freshly printed side of the sheet mechanically are most likely to cause offset. Stiff printing stock makes offset problems more likely. Offset may also emerge in the pile or in perfector machines.

28

Printed signal elements such as the SLUR strip are a valuable tool for the quick optical evaluation of halftone value alterations. Signal elements such as the SLUR strip optically amplify faults in the printing process.

Faults such as dot gain, sharpening, slur and doubling affect fine screen elements more than coarse ones. The reason is that small dots are reduced or enlarged by the same values as large ones. A large number of small dots, however, has many times the total circumference length of coarse dots of the same tone value. This is why, during printing, more ink is deposited around the fine dots than around the coarse dots. As a consequence, the fine screen image areas will appear darker. This phenomenon is the basis on which signal and measuring elements function.

To give an example, the structure and the function of the SLUR strip will be briefly explained. In the SLUR strip, coarse screen elements (surroundings) and fine screen elements (numbers) are combined.

Compared with the uniform halftone value of the coarse screen, the fine-screen numbers from 0 to 9 have increasingly sharp halftone values. When during the print run of a properly printed sheet the figure 3 and the coarse-screen field display the same halftone value, then the figure 3 can no longer be recognized. If dot gain occurs during printing, however, then the next-higher figure with a sharper screen approaches the halftone value of the surroundings.

The more dot gain takes place, the more the halftone value equality shifts towards higher numbers.

With sharpening, this process is reversed. Here the figures 2, 1 or even 0 may become illegible.

29

O.K

Gain

Sharpening

Lateral slur

Circumferential slur

Yet the figures merely indicate whether dot gain or sharpening occurs. The causes have to be looked for with a magnifying glass on the plate or on the printed sheet itself.

The SLUR section to the right of the numbers shows whether there is dot gain, slurring, or doubling. With dot gain in printing, the word SLUR is not more legible than with good printing, although the entire field appears somewhat darker.

Halftone dots, however, are less suited for detecting slurring and doubling. The typical, direction-related, widening in cases of slur is easier to detect in the SLUR field. In the case of circumferential slur, for example, the horizontal lines forming the word SLUR (parallel to the image start) will be widened. With lateral slur, on the other hand, the area surrounding the word SLUR, which consists of vertical lines, will be darker.

The illustration shows how dot variations affect the print result, using dot gain as an example. Even if the dots of only one colour are larger than desired, a different hue will result.

30

correct

This, of course, is also important for superimposition.

The transfer process used in offset printing usually causes the dots to become larger. This effect is called dot gain.

Signal strips help to assess the quality of the print result, but they do not provide information on absolute values and errors. To assess the quality of halftone values with objectively verifiable numbers an objective measurement method is therefore needed.

2.2.2 Dot gain Dot gain is the difference between the halftone values in the screen film and in print. Deviations result both from geometrical dot variations and from the effect of light entrapment (see Chapter 3.4.4).

Similar to the halftone value F, the dot gain Z is generally expressed in percent. (The formulae for the dot gain Z are given in Chapter 3.5.1).

The dot gain is the difference between the halftone value in print FD and the measured halftone value in film FF.

31

wrong

Since the dot gain is different in the various halftone value ranges, the figures on dot gain should also include the halftone value in film. Example: “15 % dot gain with FF = 40 %” or, shorter, “Z40 = 15 %”.

Advanced measuring instruments display the dot gain directly.

N.B.:

The dot gain Z(%) indicates the difference between the halftone value on film FF and the halftone value in print FD in absolute figures.

It therefore does not refer to the film value!

2.2.3 Print characteristic The deviation of the halftone value in print FD as against the halftone value FF in film can be clearly represented for direct use in repro work in the form of what is called a print characteristic.

To determine the print characteristic, screen step scales with a minimum of three, or even better five or more screen steps and a solid patch element are printed. A densitometer is used to measure the ink densities in the solid patch and in the screen steps, and subsequently the halftone values are calculated. When the values thus obtained are drawn in a diagram versus the corresponding film values the transfer characteristic is obtained. Having used plate making this is a print characteristic.

It is valid only for the particular combination of ink, paper, printing pressure, blanket and plate for which it was determined. If the same job is printed on another press, with different ink or on different paper, then the print characteristic will be slightly different.

The illustration shows how characteristic 1 runs at an angle of 45°. It represents the ideal case in which print and film are optically identical, but which is unattainable under nor-

32

mal conditions. Characteristic 2 reproduces the halftone values actually measured in print. The marked area between the two lines represents the dot gain.

For determining the dot gain in print, the middle-tone range is the most significant. The print characteristic shows that here the halftone value shifts reach a maximum.

By means of characteristic 2 the screen film can be adjusted in such a way that in print (with a normal dot gain) the desired tone values are attained.

In practice, however, this can only partly be achieved.

Film

Print

Characteristic

Print

Characteristic 2 Characteristic 1

DV = 1.50

Film

33

2.3 Relative print contrast As an alternative to dot gain the relative print contrast Krel.(%) is often determined, particularly to check the screen in the three-quarter tone.

A print should have a contrast as high as possible.

This means that the solids should have a high ink density, but the screen should still print open (optimum halftone value difference). When the inking is increased and the ink density of the dots rises, the contrast is increased. However, the increase in ink feed is only practicable up to a certain limit. Above that limit the dots tend to exhibit gain and, especially in three-quarter tone, to fill in. This reduces the portion of paper white, and the contrast decreases again.

If there is no measuring device available with a direct contrast display, the relative print contrast can be calculated or determined on the basis of the FOGRA PMS. (The formulae are given in Chapter 3.4.3).

If the contrast value deteriorates during a production run in spite of constant ink value in solid DV, this may be a sign that the blankets need washing.

If the solid density is correct, the contrast value can be used to assess various factors which influence the print result such as ■

rolling and printing pressure,

■

blankets and underlays,

■

dampening,

■

printing inks and additives.

Since the contrast value, unlike the dot gain, depends to a large extent on the solid density it is not suitable as a variable for standardisation. This is why in the recent past its importance has decreased significantly.

34

2.4 Colour balance / image build-up As has been explained, colour hues are reproduced in fourcolour printing by different portions of cyan, magenta, yellow and black. If their portions change, the resulting colour changes. To avoid this, the colour portions for the desired colour hue must be balanced correctly and reliably.

If only the black colour portion changes, the hue becomes brighter or darker, a phenomenon we do not regard as disturbing. The same is true when the chromatic colours all change relative to their portions and in the same direction.

However, we react critically to shifts in the colour hue. Such shifts occur if the individual colour components do not change together, or, at worst, if they change in opposite directions.

Such impairments of the colour balance can be recognised most clearly on gray balance fields; the colour balance therefore is often termed gray balance.

The extent of the inevitable variations in each of the printing inks during the printing process largely depends on the principle of image build-up chosen in pre-press.

In this connection, the print-relevant questions are: - Which inks form the gray areas?

- Which technique is used to darken coloured image areas?

- How are shadows and image depth produced?

In short: What are the gray and achromatic portions composed of, and which maximum percentage dot areas are the result?

Remember: Gray and achromatic values can either be produced from cyan, magenta and yellow or with black printing ink.

A combination is also possible.

2.4.1

Chromatic composition

In chromatic composition, the achromatic values always consist of portions of the chromatic printing inks cyan (C), magenta (M) and yellow (Y), i.e. all gray image areas, all tertiary hues and the shadow details contain the three chromatic printing inks.

35

Black (K) is only used to intensify image shadows and to 100%

improve shadow details (skeleton black).

50%

80 % M

70 % C

90% Y

0%K

240 %

The brown colour shown in the illustration was built up i chromatic colour structuring from 70 % cyan, 80 % magenta, 90 % yellow and 0 % black. All in all, surface covering amounts 0%

to 240 %.

C

M

Y

K

The effect of the colour portions is shown in the margin. The chromatic colour composition of the brown consists of an achromatic (gray) and a chromatic portion. With inks of the European Colour Scale, 70 % cyan, approx. 58 % magenta, and

100%

59 % yellow will neutralise and result in gray. Only the remaining 22 % magenta and 31 % yellow form the light brown chromatic portion. With the gray portion added, the result will be dark brown.

50%

Chromatic composition results in high dot percentages which theoretically may be 400 %, yet in practice usually do not exceed 375 %. These high dot percentages have a negative effect on the ink trapping behaviour, on drying and on powder

0% C

M

Y

K

consumption. Also, colour balance is difficult to maintain during the print process.

2.4.2

Achromatic composition

In contrast to chromatic composition, in achromatic composition all achromatic contents of multi-colour printed pictures are produced, in principle, with black printing ink. That is why neutral hues only consist of black printing ink, and chromatic hues as well as shadow details are also darkened with black. All chromatic hues are the result of not more than two chromatic printing inks plus black. This is why colour balance is easier to maintain with achromatic composition.

With achromatic composition, the brown as shown in Chapter 2.4.1 theoretically consists of: 0 % C + 22 % M + 31 % Y + 70 % K.

36

100%

22 % M

0%C

31 % Y

70 % K

123 %

The figure shows that it is not possible to produce matching colours by simply replacing the CMY achromatic colour by

50%

black. This is mainly due to inadequacies in the real printing inks. Colour similarity can only be achieved with a changed portion of the chromatic colours and a modification of the black portion, e.g. to 62 % M, 80 % Y, and 67 % K. The achromatic composition corresponds to 100 % GCR (Chapter 2.4.6).

2.4.3

0%

C

M

Y

K

C

M

Y

K

C

M

Y

K

Chromatic composition with under colour addition (UCA) 100%

In some cases, black printing ink will yield only insufficient image depth in the darker areas of the gray axis. As a countermeasure, this area and, to a lesser extent, the adjacent chromatic areas as well are intensified by adding an achromatic portion of C+M+Y. UCA („Under Colour Addition“) particularly

50%

depends on the combination of substrate and printing ink. The figure in the margin demonstrates under colour addition in the neutral image depth.

0%

2.4.4

Chromatic composition with under colour removal (UCR)

With chromatic composition, the highest dot percentages will be found in the areas of neutral three-quarter tones to black. This disadvantage is countered by under colour removal (UCR). The

100%

portion of the achromatic composition made up of C+M+Y is reduced in the neutral shadow areas and, to a lesser extent, in the adjacent chromatic areas; the portion of black printing ink is increased. In the example shown in the margin, the original dot percentage of 98 % cyan + 86 % magenta + 87 % yellow +

50%

84 % black = 355 % is reduced to 68 % cyan + 56 % magenta + 57 % yellow + 96 % black = 277 %. This has a positive effect on the ink trapping behaviour, on drying and on the shadow balance.

0%

37

2.4.5

Chromatic composition with gray stabilization

It is difficult to maintain a balance in the printing process with a chromatic composition of gray tones. Colour casts easily occur. They are eliminated by means of gray tone stabilization.

Achromatic portions of C+M+Y are partly or completely replaced by corresponding portions of black along the total gray axis and, to a lesser extent, in the adjacent chromatic areas as well, i.e. not only at the darker end of the gray axis as in UCR. In practice this is also called „long black“.

2.4.6 Chromatic composition with gray component replacement (GCR) For gray component replacement (GCR), the portions of C+M+Y neutralising into gray are replaced by the achromatic printing

100%

ink black both in neutral and chromatic image areas. GCR thus makes it possible to create all intermediate steps between images formed with chromatic and achromatic composition in all shadow areas, and is, therefore, not limited to the gray areas as UCR, UCA or gray stabilization are. Gray component repla-

50%

cement is sometimes also called complementary colour replacement.

With GCR, the brown as shown in Chapters 2.4.1 and 2.4.2 0%

C

M

Y

K

could theoretically also be composed, e.g., of

50 % C

60 % M

70 % Y

20 % K

200 %

As with achromatic composition (Chapter 2.4.2), the same colour can not, in practice, be obtained if only part of the CMY achromatic colour is replaced by black and the portion of chroma remains unchanged. For example, a similar colour is achieved with 49 % C + 70 % M + 80 % Y + 30 % K.

38

2.4.7 Five-, six- and seven-colour printing Advanced four-colour printing meets even high quality standards. Nevertheless, special sets of colour plates may be needed with some originals and for highest quality requirements.

The reproducible range of colours can be expanded by using special colours (besides the four primary colours). The following illustration shows where the colour values measured for a seven-colour print are located in the CIE chromaticity diagram.

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

The hexagon located within the diagram’s borders shows the range of the process colours cyan, magenta and yellow (measured values). The dodecagon surrounding it indicates how the range of colours can be enlarged by the additional colours green (G), red (R) and blue (B).

39

2.5 Ink trapping and colour sequence 2.5.1 Ink trapping Another variable influencing the reproduction of the colour hue is the ink trapping characteristic. It indicates how well an ink is accepted when printed onto another ink as compared to when it is printed onto the printing stock.

A distinction has to be made between wet-on-dry and weton-wet printing.

The term “wet-on-dry” printing is used when an ink is printed directly onto the printing stock or another, dry ink. If, on the other hand, an ink is superimposed on a wet colour, one uses the term “wet-on-wet”. For multicolour presses, the term “wet-on-wet” printing is generally used.

If the coverage is uniform and if the hue is located at the correct coordinates, then one speaks of good ink trapping.

If, on the other hand, the desired hue can not be attained, the ink trapping is faulty. This may be the case with all mixed colours. As a consequence, the range of colours is reduced and certain colour shadings can no longer be reproduced.

40

If the ink film thickness is correct and if the colour locations of the primary colours cyan, magenta and yellow are situated at the correct reference locations, it may nevertheless be the case that the reference locations of the mixed colours red, green and blue cannot be attained due to faults in superimposition during printing.

The following CIE chromaticity diagram demonstrates the effects of a faulty ink trapping or an unfavorable colour sequence on the print result. The white area illustrates the extent of halftone value reduction caused by faulty ink trapping.

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

41

2.5.2 Colour sequence The schematic illustration shows the results of three different superimpositions of the colours cyan and magenta.

C

M

C

M

+

C

=

M

C

M

+

C

=

C M

C

M

C

+

M

=

M C

M

In the first example a layer of magenta was printed on a single-colour press as the first colour. Then a layer of cyan was superimposed after drying (“wet-on-dry�). The ink film thickness of both colours is identical. The ink film trapping is good and the desired colour location has been reached.

The second example was produced on a multi-colour press.

First a film of magenta was printed on the dry paper (weton-dry). Then a layer of cyan was printed onto the still wet magenta ink (wet-on-wet). Whereas the magenta ink film was accepted well by the paper, the ink trapping for cyan was worse (due to the colour separation during superimposition in printing). The result is a red-cast blue.

In the third example the wet-on-wet printing method was also used but with reversed colour sequence (cyan onto magenta). The result is a blue-cast red.

42

In four-colour-print the colour sequence black-cyanmagenta-yellow has been generally accepted as standard.

This colour sequence is also the basis for the adjustment of colour consistency in printing ink manufacturing.

To reduce the effects of faulty ink trapping that might occur in special cases, typon and printing plates should be carefully checked before mounting. For solid fields, for example, it might be advantageous to print the lighter form before the more solid one.

In particular, this applies to the superimposition of screens and solid ink films. The screen should first be printed on the white paper and then the ink film onto it.

2.6 Print control strips For controlling print quality on the basis of measured data, print control strips are printed with the image. They are available from various research institutes and suppliers.

However, only originals may be used, since deviations might occur during copying onto a duplicating film that impair the measurement results.

Print control strips are available for four- to eight-colour presses. With print control strips for more than four colours the number of halftone and slur patches is reduced in favor of the solid and colour balance fields necessary for the control of the ink fountain zones.

All print control strips consist of several elements. In the following, the most important patches of the Heidelberg CPC colour measurement strip, the FOGRA and Brunner print control strips, will be described.

43

2.6.1 Solid patches (fields) B

C

M

Y

Solid patches enable the uniformity of the inking to be checked. It is advisable to use one solid field per printing ink spaced at the distance of the ink fountain zone width (32.5 mm for Heidelberg). This makes it possible to use solid fields for the automatic colorimetric control of solids.

2.6.2 Overprint patches (fields)

MY

CY

CM

These elements are designed for the visual and densitometric assessment of the ink trapping performance.

2.6.3 Colour balance patches (fields) CMY

B

One has to distinguish between solid and halftone colour balance fields. In solid patches, the superimposition of cyan, magenta and yellow must result in an approximately neutral black. For purposes of comparison, a black solid field is printed next to the overprint field.

B

CMY

Given correct ink film thickness, standardised colour sequence and normal dot gain, the superimposition of cyan, magenta and yellow produces an approximately neutral gray. Different halftone values are used by manufacturers for the typons of the various colours.

Colour balance patches are also used for the automatic gray balance control of cyan, magenta and yellow.

Heidelberg:

70 % cyan

60 % magenta

60 % yellow

FOGRA:

28 % cyan

21 % magenta

19 % yellow

Brunner:

50 % cyan

41 % magenta

41 % yellow

44

2.6.4 Halftone patches (fields) Depending on the manufacturer, halftone fields may contain different typon halftone values.

From the measured data of the halftone and solid patches the dot gain and print contrast are calculated.

Heidelberg

Fogra

Brunner

70 %

40 % and 80 %

50 % and 75 %

70 %

40 %

80 %

50 %

75 %

2.6.5 Slur/doubling patches (fields) Line gratings of different screen angling allow the pressman to visually and densitometrically check for slur or doubling faults (see Chapter 2.2.1).

2.6.6 Plate exposure control patches (fields) Plate exposure control fields are designed for visual monitoring of the plate exposure. The control elements shown contain microlines and micro reverse lines as well as fields with dots.

0,5%

0,5%

99.5

1%

99%

2%

98%

3%

97%

1%

6ď‚ľ 8ď‚ľ

2%

3%

11 13

11

4%

5%

16

45

Densitometry

3

Densitometry

3.1 3.2 3.2.1 3.2.2 3.3 3.4 3.4.1 3.4.2 3.4.3 3.4.4

Measuring principle of a reflection densitometer The use of filters in densitometry Colour filters and brightness filters Polarisation filters Measuring values in densitometry Measurement Zeroing on paper white Solid density Halftone density Optically effective area coverage (halftone value in print) Evaluation Halftone value in print Dot gain Contrast Ink trapping Standardisation in printing Limits of densitometry

3.5 3.5.1 3.5.2 3.5.3 3.5.4 3.6 3.7

48 50 50 52 53 56 56 56 56 57 58 58 58 58 59 60 62

47

Transmission densitometer

Densitometry is the cheapest and most widespread measuring method in the field of repro work and printing.

Densitometers are used as hand-held instruments or in the form of automatic measurement devices (scanning densitometers).

There are two kinds of densitometers, used for different purposes: â–

The transmission densitometer is used in repro work to measure the blackening of the film (transparent

Reflection densitometer

substrata). â–

Reflection densitometers are used to measure printed images (opaque substrata).

In the following, the working principles of reflection densitometry will be described in more detail.

3.1 Measuring principle of a reflection densitometer In reflection densitometry the ink to be measured is illuminated by a light source. The light ray passes through the transparent (glazing) ink layer and is partly absorbed.

The non-absorbed content of the light is largely scattered by the printing stock. Part of this reflected light again passes through the ink and is absorbed again. The remaining nonabsorbed light reaches a detector, which converts the light into electricity. The result of the measurement with a reflection densitometer is given in density units.

In the measurement, lens systems are used to focus the light. Polarisation filters serve to prevent differences in the measured values obtained from a shining wet surface and from the surface of a dry ink. Colour filters are inserted for measurements of colours (see Chapter 3.2.1).

48

Colour filter Colour filter Colour filter Polarisation filter Polarisation filter Paper

Lens system

The illustration explains this principle, taking a coloured ink as an example. Ideally, the incident white light consists of equal portions of red, green and blue. The printed colour contains pigments which absorb the red portion and reflect the green and blue portions, which is why we call it “cyan�.

Densitometers are intended for measurement within the absorption range of each colour, where density and ink film thickness closely correlate. In our example a red filter is used which allows only red light to pass, whereas blue and green are blocked.

The density of a given ink mainly depends on the pigmentation, its concentration and its ink film thickness. For a given ink the density is a measure of the ink film thickness, yet it does not tell us anything about the hue.

49

3.2 The use of filters in densitometry 3.2.1 Colour filters and brightness filters The colour filters in a densitometer are tuned to the absorption performance of cyan, magenta and yellow.

Common standards such as DIN 16 536 and ISO/ANSI 5/3 define the spectral transmission bands and the positions of the transmission maxima accordingly. Of the narrow and broadband colour filters listed there, referred to as status A and T in ISO respectively, narrow band filters should be used since the difference in measurement resulting from the use of different brands of filters is smaller than in broad band filters.

Colour filters must always be chosen complementary in colour to the printing inks which are to be measured.

The colour black is measured with a visual filter tuned to the spectral of brightness sensivity of the human eye.

Special colours are measured with the filter which returns the highest measurement value.

Printing inks

Filter colour

cyan

red

magenta

green

yellow

blue

The following three illustrations show the reflexion curves for cyan, magenta and yellow, together with the respective colour filters according to DIN 16 536.

50

Cyan

1.0

0.5

0.0

Magenta

1.0

0.5

0.0

1.0

Yellow

0.5

0.0

51

3.2.2 Polarisation filters Densitometers can be used to measure both dry and wet printing inks. Wet colours have a smooth, shining surface.

During the drying process, the ink adapts to the irregular structure of the paper surface, and the reflection effect decreases. If a given ink is measured first in wet and then in dry condition, different readings will result.

In order to eliminate this effect, two crossed linear polarisation filters are inserted in the path of the rays. Polarisation filters allow the light of only one particular vibration direction to pass, while blocking all light waves which are vibrating in other directions. Part of the light rays polarised by the first polarisation filter are reflected by the ink surface specularly, i.e. without altering their direction of vibration.

The second polarisation filter is aligned at an angle of 90째 to the first so that the reflected light waves are inhibited from passing.

Light rays, however, which penetrate into the ink film and are reflected either by the ink or by the printing stock, lose their original polarisation.

They are therefore able to pass through the second polarisation filter and reach the detector.

By thus blocking the portions of light reflected by the wet colour surface, approximately equal readings for wet and dry inks are obtained.

Paper

Due to the absorption by the polarisation filters less light reaches the detector; the readings obtained with such

Direction of scattering

devices are therefore generally lower than measurements

Direction of vibration

made with other instruments.

52

3.3 Measuring values in densitometry Densitometers display their readings for the ink density D as a logarithmic number. It is the logarithmic ratio of the absorbed light for a “reference white” to that obtained from the measured ink film. In practice, the ink density reading is mostly referred to as the “density”.

The ink density value is calculated using the following formula: 1 

D = lg

The reflectance factor  is calculated as follows:



LeP LeW

LeW

LeP

100 % 50 %



LeP LeW



50 % 100 %



where LeP is the light reflectance of the printing ink and LeW the light reflectance of the reference white.

The reflectance factor  gives the ratio between the light reflectance from a measuring sample (printing ink) and from a “white” (reference value).

With the – value calculated above the ink density is: D = lg

1 

= lg

1 0.5

= lg 2 = 0.30

53

There is a close correlation between ink film thickness and ink density. The illustration shows that with the ink film thickness increasing, light reflectance decreases and the ink density value increases.

90 %

  = 0.9

D = 0.05

 = 0.5

D = 0.30

 = 0.1

D = 1.00

 = 0.01

D = 2.00

 = 0.001

D = 3.00

 = 0.001

D = 3.00

50 %

10 %

1%

0.1 %

0.1 %

Formulas for calculating the ink density values are given on page 53.

54

The diagram illustrates the correlation between ink film thickness and ink density for the four process colours in

Density

offset printing.

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

!m

The vertical line marks the ink film thickness range of about 1 Âľm customarily used in offset printing. The diagram also shows that the density curves do not start to flatten off until significantly higher ink film thicknesses are reached.

From these ink film thicknesses upwards there is hardly any further increase in the ink density; even if the measurement were performed in a full ink container, the density value would not be higher. However, these ink film thicknesses are no longer relevant for offset printing.

55

3.4 Measurement 3.4.1 Zeroing on paper white Before measurement is started, densitometers must be calibrated to zero against the paper white (reference white) of the printing stock in order to eliminate the influences of paper colouring and surface characteristics on the evaluation of the printed ink film thickness.

For this purpose, the density of the paper white in relation to “absolute white� is measured, and this figure is set to zero (reading D = 0.00).

3.4.2 Solid density The readings of a solid area, are referred to as solid density (DV). It is measured on a print control strip, which is printed on the sheet at right angles to the print direction.

Besides other control elements the print control strip contains solid fields for all four process colours and, if necessary, for additional colours.

The solid density value allows a regular ink film thickness to be checked and maintained (within a certain tolerance) throughout the whole sheet width and print-run.

3.4.3 Halftone density The halftone density is measured in the halftone patches of the print control strip. In the measurement spot of three to four millimeters, a combination of dots and paper white is included, similar to that seen by the human eye.

The measurement value is the ink density in halftone (DR). The larger the ratio of the area of dots to the total area on a measured surface and the higher the ink film thickness of a given printing ink, the higher is the measured halftone density value.

56

3.4.4 Optically effective area coverage (halftone value in print) When screens are measured with a densitometer, it is not the geometrical area coverage, i.e. the area ratio between dots and paper white on the measuring spot, but the “optically effective area coverage” which is measured.

The difference between geometric and optically effective area coverage is due to the fact that both in visual observation and in densitometric measurements, part of the arriving light penetrates into the paper between the dots at the unprinted points, but is trapped under the dots during reflection and thus absorbed.

Paper

This effect is called “light gathering”. It causes the dots to appear optically larger than they are in reality. The optically effective area coverage is composed of the geometric area coverage plus the optical gain of the area.

57

3.5 Evaluation From the measurement values of solid and halftone densities halftone value, dot gain and contrast can be calculated.

First, however, all measuring devices must be calibrated to zero against the paper white.

3.5.1 Halftone value in print Given the DV and DR readings, the halftone value in print FD can be calculated using the Murray-Davies formula.

FD (%) =

1–10–DR 1–10–DV

· 100

3.5.2 Dot gain The dot gain Z(%) is obtained from the difference between the measured halftone value in print FD and the known halftone value in film FF. Z (%) = FD–FF

3.5.3 Relative print contrast The relative print contrast is also calculated from the readings of the solid ink density DV and the screen ink density DR. The DR value here is best measured in the three-quarter tone.

Krel.(%) =

58

DV – DR DV

· 100

3.5.4 Ink trapping The ink trapping is calculated from the solid density values for every individual colour in the solid fields, for all two-colour superimpositions and for the three-colour superimposition in the solid superimposition fields of the print control strip in accordance with the colour sequence involved.

The ink trapping calculated with the following formulae indicates which percentage of an ink is superimposed onto another. The value is given relative to that of an isolated ink printed on the paper whose trapping is set to 100 %.

3.5.4.1 Two-colour superimposition FA2 % = 1

D1+2 – D1 D2

· 100

where D1+2

is the ink density for the superimposition of both colours,

D1

is the ink density of the colour printed first and

D2

is the ink density of the colour printed last.

N.B.:

All ink densities must be measured with a filter which is complementary to the second colour.

3.5.4.2 Three-colour superimposition FA3 (%) = 2 1

D1+ 2 + 3 – D1 + 2 D3

· 100

where D1+2+3

is the ink density for the superimposition of all the three colours and

D3

is the ink density of the colour printed last.

N.B.:

All ink densities have to be measured with a filter which is complementary to the third colour.

59

The given formulae are also used in the Heidelberg CPC Quality Control. In addition, there are other methods of calculating the ink trapping. All these methods are controversial and, for this reason the values obtained should not be interpreted too stringently. However, for a comparison from run to run, and especially within the same run, they are in fact meaningful. The higher the FA value, the better is the ink trapping performance.

3.6 Standardisation in printing In offset printing there are many steps between the typon and the final print product e.g. reproduction (the making of progressives), proof, plate exposure and print runs.

In each of these processing steps the size of the elements of the image changes: halftone dots become larger or smaller, lines become thinner or thicker.

The typical performance for each of these process steps can be described by transfer characteristics, the most common of which are the plate exposure characteristic and the print characteristic.

The whole reproduction process aims at making the print look like the typon. At the pre-print phase all the transfer characteristics must be known. It is only then that variations of the elements of the printed image that result from the characteristics of the process can be compensated.

For reasons of economic efficiency, however, this is only possible if the number of transfer characteristics is low.

60

Standardisation in printing therefore aims at defining only a small number of transfer characteristics along with their tolerances in order to obtain low-cost and high-quality repros without having to take into account the properties of individual plate-exposure devices or printing presses.

All process steps must aim at this goal, and their constancy must be continuously monitored. Print control strips, plate exposure control fields and, in particular, colorimeters at the printing press are valuable tools in achieving this goal.

3.6.1 Standardisation systems There are various standardisation systems. Yet all of them aim at the same goal: producing cost-effective prints of constant high quality.

Instructions for standardisation in printing are available from various research institutes and suppliers. As an example, the reader is referred to the standardisation guidelines compiled by the FOGRA, the German Research Society for Printing and Reproduction Technology, on behalf of the Bundesverband Druck BVD (Association of the German Printing Industry).

This concept is described in great detail in the illustrated publication “Manual for Standardisation of the Offset Printing Process – Instructions for “Platemaking and Printing”.

This publication (in an A 4 folder) and a video cassette with the same title are available from the Bundesverband Druck e. V. in Wiesbaden in English and German.

61

3.7 Limits of densitometry Like the colour separation technique, densitometers work with filters tuned to the four process colours. They provide a relative value for the ink film thickness, i.e. they do not measure the optical appearance of the colour. x = suited for process colours • = suited for special colours ( ) = partially suited

Densitometer

Colour Measurement Unit Tristimulus Spectral •

Ink formulating Ink setting (adjusting) • on the basis of standards

x (•)

x •

x •

• on the basis of proof control strips

x (•)

x •

x •

(x) (•)

x •

• on the basis of proof

x •

x •

• on the basis of random pattern

x •

x •

• on the basis of image data (repro)

(x) (•)

x •

• identifying suitable ink

(x) (•)

x •

x •

x •

• on the basis of key numbers

Colour matching (comparing) Controlling production run • on the basis of solid patch

x (•)

x •

x •

• on the basis of monochrome halftone-patch

x (•)

x •

x •

x •

x •

• in the image

x •

x •

• recognizing ink spoilings

x •

x •

• recognizing changes in the printing stock

x •

x •

• on the basis of multicolor halftone-patch

(x)

Measurement • solid density

x (•)

(x) (•)

x •

• dot gain

x (•)

(x) (•)

x •

• ink trapping (relative)

x (•)

(x) (•)

x •

x •

x •

(x) (•)

x •

x •

x •

• ink trapping (absolute) • metameric index • sensational

62

This fact sets certain limits to their application. The table lists the typical fields of application compared to the tristimulus colorimeter and spectrophotometers.

One essential disadvantage of densitometry is that the same colour densities do not necessarily lead to the same optical impression. This is the case when the colour substances to be compared differ from each other. Therefore setpoint values can not be taken from proofs or other samples.

The restriction to the three colour filters for red, green and blue is of similar importance. When colour sets are composed of more than the four process colours, the measuring of the additional colours becomes problematic. In most cases there are no appropriate filters for the additional colours, as a result of which the values measured for ink density are too low and those for dot gain are incorrect.

The use of densitometers is also critical for colour control on the basis of multicolour halftone patches such as gray balance patches. If a gray balance patch is measured with the three-colour filters the resulting ink density values are different than the values that result when each of the colours is measured by itself. This is because each of the three printing inks contributes to all ink densities. The reason for this is that the process colours are not perfect two-third inks and thus also absorb light from other spectral ranges.

Densitometers are useful in monitoring the print-run of a four-colour print. In all other cases densitometers are of limited use.

63

The following two examples illustrate how additional colours are measured with a densitometer.

1.0

0.5

Colour sample Pantone Warm Gray 1

0.0

The hue (light beige) shown here – has a relatively high reflectance, slightly decreasing in the blue range (380 to 500 nm). Accordingly, the highest density value (0.27) is measured with a blue filter. This low value cannot be altered easily since changes in ink film thickness only lead to insignificant shifts in the density. In practice light pastel colours are therefore assessed visually on the basis of an OK-sheet and corrected manually.

64

1.0

0.5

0.0 Colour sample HKS 8

1.0

0.5

0.0 Colour sample HKS 65

The additional colours HKS 8 and HKS 65 shown in the second example have completely different hues as can be seen from their reflectance curves. For both colours the absorption within the blue range (380 to 500 nm) is greatest.

As a consequence, the highest densities (1.60 for either colour) are measured by the blue filter. Equal density values measured by the same filter thus do not necessarily mean that the hues are equal!

The appearance of a colour can thus only be evaluated colorimetrically.

65

Colorimetrics

4

Colorimetrics

4.1 4.2 4.3 4.4 4.5 4.6 4.6.1 4.6.2 4.6.3 4.6.4 4.7 4.8 4.9 4.10

Measuring colour Tristimulus values/white reference Standard illuminants Standard observer/colour matching functions Evaluation with a spectrophotometer Colour difference E CIELAB CIELUV CIELCH CMC Munsell The tristimulus method Spectral colour measurement The measuring principle of the Heidelberg CPC 21 spectral quality control Proof and colour control strips Proof control strips Colour control strips Ink control with Heidelberg Colorimetric control by gray fields Colorimetric control by solid fields Densitometric control by solid fields Advantages of colorimetrics for offset printing

4.11 4.11.1 4.11.2 4.12 4.12.1 4.12.2 4.12.3 4.13

68 70 70 72 74 76 79 82 83 84 86 87 88 90 91 91 92 93 93 95 97 98

67

As described in the chapter “Systems of colour classification�, three numbers are needed to unambiguously define a colour. Colorimetrics describes how these figures are determined and how they relate to each other.

One prerequisite is, however, that colours are measurable.

Thus colour measuring and colorimetrics are directly connected with each other.

4.1 Measuring colour Colours are measured with tristimulus colorimeters or spectrophotometers. Their function is described in Chapters 4.8 and 4.9.

In principle, the construction of colour measuring devices follows the visual and sensoric model of the human eye (see illustration).

The ink (sample) is illuminated by a light source (radiation).

Part of the light is absorbed by the sample, the rest is reflected. The reflected light is captured by the human eye. There the red, green and blue sensitive cones (visual receptors) are stimulated. Via the optic nerve, this stimulation will trigger off the perception of colour in our brain.

This natural process is imitated in the measuring device.

In the measuring process, the light is sent onto the printed sample. The reflected light passes through a lens system and to a sensor, which measures the intensity of the incident light for each colour and transmits the measured readings to a computer. There they are weighted with functions that imitate the weighting functions of the three types of sensitive cones in the human eye, and which have been defined by the CIE for the standard observer. The result is the tristimulus values X, Y and Z. These are finally converted into chromaticity coordinates or coordinates of other colour spaces (such as CIELAB or CIELUV).

68

light source

radiation

measuring device

man

io ct le ef lr

blue

n

sp

io

ec

ct

le

tra

ef

lr

tra

ec

sp

n

sample

eye

optics with receiver

visual receptors

colour matching functions of the standard observer

green

red

stimulation

tristimulus values

colour sensation

colour coordinates

69

4.2 Tristimulus values/white reference In the measuring of colours, the identification of tristimulus values from measured reflections and emissions requires standardised conditions. Most of them have been laid down by the manufacturer of the equipment and have been taken care of in such a way that the user need not pay further attention to them. In the measuring of body colours, however, three factors are usually variable and must be adjusted by the user: the white reference, the type of light and the observer.

Normally, colorimetric values are determined relative to “absolute white�. The calibration is thus set to the calibration standard of the measuring unit, which is in turn calibrated against a theoretical absolute white. In contrast to densitometry, the paper is used as a reference only in exceptional cases.

4.3 Standard illuminants Without light there is no colour. But this also means that the type of light influences our colour perception. The colour of light is determined by its spectral composition.

In natural sunlight, the weather as well as the season and time of day influence the spectral composition. Photographers and film directors often have to wait a long time until the lighting conditions are the way they want them to be.

Likewise, there are differences in the spectral composition of artificial lamp light. Some lamps produce reddish light whereas others emit a greenish or bluish light.

Spectral reflection and consequently colour perception change depending on the light conditions. Tristimulus values must, therefore, be based on standard light.

70

In standardisation, the intensity distribution has been laid down for different types of light in the range between 380 and 780 nm (at intervals of 5 nm). The illustration shows the spectral distributions for the standard illuminants A, C, D50 and D65.

The C, D50 and D65 standard illuminants are similar to average daylight with a peak radiation intensity in the blue area.

The illustration shows the composition of the type D65 illuminant. An A standard illuminant has a peak intensity in the red area; it thus appears reddish (evening light and electric light).

71

4.4 Standard observer/colour matching functions Each individual has three colour matching functions to evaluate red, green and blue. In the case of persons with normal chromatic vision, they will be almost identical.

Thus colours are perceived differently only in boundary areas. For example, what one individual still perceives as a bluish green, another will see as a greenish blue.

That is why it was necessary to define, for colorimetric purposes, an individual with average visual perception, i.e. the “standard observer�. A comprehensive series of tests with a large number of persons with normal chromatic vision was carried out in 1931. On the basis of these tests, the colour matching functions x, y, and z were defined and laid down as binding by the CIE both in national and international standards such as DIN 5033 and ISO/DC 12 647.

72

The study was carried out for an observer angle of 2°.

The observer angle in the sense of the standards of colorimetrics is the visual angle at which a colour area is viewed (see illustration). For example, if an area with a diameter of 3.5 cm is viewed at a distance of 1 m, the visual angle will be exactly 2°.

In 1964, the same test was repeated for an observer angle of 10°, and the results were likewise laid down in a supplementary standard. Hence the “standard observer 1964” came into being.

1m

^ 3.5 cm 2 = 10 ^ = 17.5 cm

73

4.5 Evaluation with a spectrophotometer The standard colour values are calculated from the radiation function of the illuminant S(), the measured degree of spectral reflection of the sample () as well as the colour matching functions x (), y () and z () of the standard observer.

The lambda in brackets () shows that the calculation depends on the wavelength  of the light (e.g. in a wavelength range between 400 and 700 nm at intervals of 5 nm).

In the first step of the calculations, the values of the radiation function of the standard illuminant S() are multiplied by the measured degrees of reflection () of the sample for each wavelength  (i.e. for each spectral colour of a specific type of light). The result is a new curve, the colour stimulus function ().

In a second step the values of the colour stimulus function are multiplied by the colour matching functions x (), y () and z (). This results in three new curves.

Finally, by integrating and multiplying with a normalisation factor, the tristimulus values X, Y and Z are calculated from the areas under these curves by integration, which makes it possible to exactly describe the measured colour.

74

Illuminant

times Reflection

is Colour stimulus function

times Colour matching function

times Integration and normalisation

is Tristimulus values

75

4.6 Colour difference E The colour difference is a measure of the distance between two colour locations in the colour space (e.g. between original and printed sheet).

In the chapter “Systems of colour classification”, the CIE colour space was explained. But this colour space has one major disadvantage: Not for all colours does the human eye perceive colour location differences of the same value as being identical.

MacAdam, an American, studied this fact in a long test series, analyzing and illustrating the results. The illustration shows the so-called MacAdam ellipses in tenfold enlargement. Since the CIE colour space is three-dimensional, the ellipses are really ellipsoids, i.e. ellipse-shaped three-dimen-

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0 0.0

76

0.1

0.2

0.3

0.4

0.5

0.6

0.7

sional bodies. The size of the ellipsoids is a measure for the perception threshold of colour deviations (each viewed from the center of the ellipsoid and for the individual hue).

This system is therefore of no practical use in the evaluation of colour differences, since it implies that the acceptable tolerances are different for each hue. For reliable and powerful calculations of colour differences, a colour space is needed in which colour differences that are perceived as being identical have the same numerical values. CIELAB and CIELUV are two such systems; they were developed by mathematical transformation from the CIE colour space.

Through this transformation, the MacAdam ellipsoids of varying size were mapped on spheres of almost identical size. In this way, the human eye perceives identical colour differences for all colours as almost identical.

In 1976 the CIELAB and CIELUV colour spaces, the most commonly used in the printing industry, were standardised on an international basis.

77

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

The illustration shows the location of the a*- and b*-axes of the CIELAB colour space in the x-y colour table.

Other colour spaces such as the CMC system and the Munsell colour space, are also used in the U.S.A.

78

L* 4.6.1 CIELAB 100

The CIELAB colour space is most frequently used for measuring body colours (printing inks), for example, in preparing ink formulae or for quality control in

80

printing. Colour tone and colour saturation are drawn on the a* und b*axes. The a*-axis runs from –a* 60

(green) to +a* (red), the b* axis from –b* (blue) to +b* (yellow). The lightness axis L* runs from 0 (black, at

b*

the bottom) to 100 (white, at the top).

40

The illustration shows the CIELAB colour space for body colours. Since it is the result of a transformation, its shape is different from that of the CIE colour

20

–a*

0

–100 –100

space. Likewise, the shape of the individual

–b*

lightness levels changes with L*.

In the illustration, a cross section through the CIELAB colour space is shown for body colours at a lightness level of L* = 50. The scaled down green area and the enlarged blue area can clearly be seen.

79

100

a*

White

The colour differences are calculated using the following formula: L* = L*act – L*ref

Yellow

a* = a*act – a*ref b* = b*act – b*ref

Green

E*ab =  L*2 + a*2 + b*2

Red

Blue

Black

For the user in practice, a schematic illustration is especially handy. pre-set

measured loca-

reference

tion of colours

L*

75.3

70.0

a*

51.2

55.0

b*

48.4

54.0

Example:

L*= 75.3 means that it is a bright colour the location of which is between yellow and red with a* = 51.2 and b* = 48.4. The present example is therefore a bright yellowred or orange.

Result:

The pre-set reference location of the colour and the measured location of the colour differ.

80

White

Results of the calculation: L* = 75.3 – 70.0 = 5.3 a* = 51.2 – 55.0 = – 3.8 b* = 48.4 – 54.0 = – 5.6

actual value

E*ab =  5.32 +(–3.8)2 +(–5.6)2 = 8.6

set value

In accordance with their perceptibility, the colour location differences can be classified as follows: E between 0 and 1

in general, this deviation cannot be perceived.

E between 1 and 2

very small deviation; only perceivable by an experienced eye.

E between 2 and 3.5

medium deviation; perceivable even by an unexperienced eye.

E between 3.5 and 5

large deviation

E exceeding 5

massive deviation

Since the transformation used is not linear, the regularities of the CIE colour space cannot readily be applied to the CIELAB colour space. One argument in favor of its use is its worldwide application.

81

4.6.2 CIELUV

L *

The CIELUV colour space was also derived through transfor-

100

formulae than for the former. The three coordinate axes

mation from the CIE colour space but by using other are denoted by L*, u* and v*.

Since the CIELUV and CIELAB colour spaces are

80

the result of different transformations, they also differ in shape. Both are used for body colours (see

60

illustration).

v*

40 20

–u*

–100

0

100

u*

–100

–v*

The illustration shows a cross section through the CIELUV colour space for body colours at a lightness of L* = 50 (see also Chapter 4.6.1). The green area in the CIELUV colour space is located closer to the center than in the CIELAB colour space; furthermore, the blue area is larger.

82

The CIELUV colour space is often used for the evaluation of colours on colour monitors (e.g. on scanners or computers).

Its advantage lies in the linearity of transformation so that all the regularities of the CIE colour space remain unchanged.

(This is not the case with the CIELAB colour space.)

4.6.3 CIELCH The term CIELCH is used when the polar coordinates C (distance from the center) and h (angle) are used instead of the Cartesian coordinates a, b or u, v in the CIELAB or CIELUV colour space. CIELCH is thus not an additional colour space.

The lightness L* remains unchanged.

The chroma C*ab is calculated by C*ab = a*2 + b*2.

The hue angle h*ab is calculated from = arctan

( ) b* a*

.

For CIELUV, the calculation is the same.

Here a schematic representation with the same measured colour location as in Chapter 4.6.1.

Actual colour location: L* = 75.3 C* = 70.5 h* = 43.4°

83

4.6.4 CMC CMC, an evaluation of the colour location differences based on the CIELAB colour space, was developed in Great Britain in 1988 by The Colour Measurement Committee of the Society of Dyers and Colourists (CMC). It does not describe the perception of colour deviations (as CIELAB or CIELUV do), but their acceptance by the observer.

In general, colour deviations near the lightness axis are perceived to be much more disturbing than those in saturated colours. Likewise, deviations in chroma (saturation) are more readily tolerated than in the hue angle.

The illustration shows the principle underlying the CMC evaluation of colour location differences in the CIELAB colour space. Each ellipse shows the locations with constant colour location differences in accordance with the CMC formula with respect to the centre of the circle (reference colour location). It can be clearly seen that the ellipses (tolerance ranges in the CMC colour space) are smaller in the achromatic area than in the region of higher saturation.

In addition their shape is such that the admissible deviations in the hue angle are smaller than in the chroma (saturation).

The ellipses also make it possible to individually adjust the evaluation of deviations in lightness and hue. This adjustment is made by means of the two weighting factors l and c. (l is the lightness weighting factor; the weighting factor c for the hue is as a rule equal to 1.) The textile industry often operates with weighting factor ratio of l : c = 2 : 1, which means that deviations in lightness will be twice as acceptable as deviations in colour hue.

84

This ratio can be adapted to the needs of the application in question. As a consequence, the values for colour location differences are significant and comparable only in connection with the weighting factors.

Lightness

Croma H ue

Lightness Croma H ue

85

4.7 Munsell

V

C

Munsell developed a system of colour classification with equidistant location differences as far back as 1905. In this system, the colours are arranged according to hue, brightness and chroma. The basic hues are red, yellow, green, blue and purple. The system was published in 1915 as the “Munsell Book of Colour� for 40 hues, light type C and brilliant and matt specimens.

The five basic colour hues were subdivided into 100 hues of even numbers each having 16 chroma and 10 brightness levels. The illustration shows a cross section of the colour body for 40 hues. The result is an irregular colour body since for some colours and lightness values not all fields are covered.

5G

Y

2,5

5Y 7,5Y

2,5Y 10Y R 16 14

7,5 YR 5Y R2

V ,5

YR

12

R 5R

8

5G 2,5 G

80

7,5

10

90 R

10

2,5R

6 4

10RP 7,5RP 5RP 2 ,5R P

2

10G 7,5G 2,5BG BG 5 BG 7,5 BG

100

10

GY 7, 5G Y

GY

10Y

2 4 6

10 P

8 10

70 60 50 40 30 20

7, 5P

10

12

P

16

2,5

14

5P

2,5PB 10 B 7 PB 5PB ,5B B7,5 10P

5B

2, 5B

H

10 0

The Munsell coordinates cannot be converted into CIE coordinates.

Further systems of colour classification are the DIN colour card (DIN 6164), the Natural Colour System (NCS), the OSA system (of the Optical Society of America) and the RAL Design-System (RAL-DS).

86

4.8 Tristimulus method The construction of tristimulus colorimeters is similar to that of densitometers. Instead of the three colour filters red, green and blue and the visual filter, combinations of filters are used which imitate the three colour matching functions x, y and z.

Tristimulus colorimeters, however, have a lower absolute measuring precision than spectrophotometers since, as a rule, neither can the colour matching functions be imitated exactly nor is the required standard illuminant available.

They are suited, however, for determining colour differences since in this case the absolute values do not necessarily have to be precise.

In addition, tristimulus devices are considerably cheaper than spectrophotometers.

The measuring field is illuminated with a lamp whose spectral composition is close to that of a standard illuminant.

In our example, cyan is to be measured.

The spectral reflection is measured by means of the three different filter combinations, and the tristimulus value X is measured behind filter (red), the tristimulus value Y behind filter (green) and the tristimulus value Z behind filter (blue).

After the measurement the tristimulus values can be converted into a colour space (CIELAB or CIELUV) in which colour differences are perceived to be equidistant.

87

Paper Measurement principle of the three-range photometer

4.9 Spectral colour measurement In the spectral measuring process the total visible spectrum from 380 to 780 nm is measured. The light reflected from a printing ink is separated into its spectral components by means of a diffraction grate and measured by an array of sensors.

Depending on the required accuracy, the identity of the incoming light is measured in steps of one, five or ten nanometers. The tristimulus values X, Y and Z are calculated from the measured reflections. For this, the colour matching

88

functions, are stored in the computer. Since these functions need not be simulated by filters, the absolute accuracy of spectrophotometers is very high. However, they are more expensive than tristimulus colorimeters.

Apart from the high absolute accuracy, one major advantage of spectral colour measuring is the fact that spectrophotometers can read out the tristimulus values for practically all standardised types of light and observers, if their values are stored in the computer. Furthermore, they can calculate colour densities for all filter standards.

So far spectral measuring has been applied most consistently in the ink industry.

In ink grinding, ink manufacturers have to comply strictly with given targets. This is very important in the case of standardised inks (Euroscale), but also in the case of HKS inks and all special grindings. In these cases, the specimen is measured with a spectrophotometer, and the mixture ratio for the printing ink is calculated on a personal computer with an ink program.

Previously it was not possible to make optimum use of spectrophotometers in printing shops. They were expensive and cumbersome, and it was not possible to use them directly for process colours. They were therefore only used for the individual measuring of special inks and the testing of materials (such as printing stocks and inks). They were unimportant for quality control in printing.

89

4.10

The measuring principle of the Heidelberg CPC spectral quality control

At DRUPA 1990, Heidelberg was the first and only manufacturer to present a spectral measuring unit for offset printing directly linked with the offset printing machines via the CPC 1 automatic remote colour control: the measuring unit CPC 21 and, since IPEX 98, the CPC 24 spectral image measurement unit.

During the measuring process, a measuring head scans the print control strip or the image, making a spectral measurement of all control elements. Alternatively, the standard illuminants A, C, D50 or D65 and the standard observers 2째 and 10째 can be used. The measuring principle of a spectrophotometer is illustrated in the diagram below.

First, the illuminant is directed to the printed probe via ring catoptrics at an incidence angle of 45째. The reflected light at

Printing units CPC Computer

Light source Spectral remission

Diodes

Ring catoptrics

Deflection mirror

Light guide

Printed sample Paper 90

Diffraction grating

an angle of 0° is directed via a deflection mirror and a fiberoptical light guide from the measuring head to the spectrophotometer. There it is split into its spectral colours by means of a diffraction grating which has an effect similar to that of a prism.

Photodiodes measure the radiation distribution in the entire visible spectrum (between 380 and 730 nm) and send the results to a computer.

There the measured colour values are evaluated colorimetrically; the result is given in the tristim-ulus values X, Y and Z and the chromaticity coordinates x, y and Y. After the measured values have been compared to the previously set reference values (allowing for the pre-set tolerance DE), the required modifications are transmitted via CPC 1 to the ink ducts of the printing units, where they are realised immediately.

4.11 Proof and colour control strips 4.11.1 Proof control strips Off-press tonal proofs are increasingly being used instead of press proofs. The reason is that an off-press tonal proof is cheaper and can be produced more rapidly than a press proof.

There are different proofing methods, all of which operate without offset printing ink. Furthermore, the colouring materials of the proofs (i.e. the toners) and offset printing inks differ in their composition.

Heidelberg has developed a special proof control strip (see illustration).

Black CMY CMY

70%

Š 1990

B

Cyan 70%

C

Magenta 70%

M

HEIDELBERG CPC

Yellow 70%

Y

CM

CY

MY

Proof

91

It has solid patches of the colours black, cyan, magenta and yellow, plus one halftone patch with 70% area coverage per colour, ink trapping elements and a gray field consisting of 70% cyan, 60% magenta and 60% yellow. In addition there is an interface element for the automatic reading of reference values into CPC 21.

4.11.2 Colour control strips The colour control strip for spectral measuring has also been developed by Heidelberg and contains (except for the interface element) the same measuring elements as the proof strip. Additionally, exposure measuring elements for standardised plate exposure are available.

Heidelberg offers four different colour control strips: The print control strip type 4 GS (Gray field and Solid control) for four printing inks, print control strip type 6GS and 6 GS for five and six printing inks and the print control strip type 8 GS for seven and eight printing inks.

The data of these colour control strips and the print control strips of the older densitometric measuring unit CPC 2-01 are stored in CPC 21 and CPC 24.

92

4.12 Ink control with Heidelberg Heidelberg offers four types of ink control: ■

colorimetric control on the basis of the gray field,

■

colorimetric control on the basis of solid or halftone fields,

■

densitometric control on the basis of solid or halftone fields (CPC 21 only),

■

colorimetric control in the image (CPC 24 only).

4.12.1 Colorimetric control by gray fields As mentioned in Chapter 2.4, colour balance is a decisive criterion for the optical impression of a printed image.

Faults in the colour balance are especially obvious in gray fields. It therefore seems sensible to use gray fields for measurement-based tuning as well as for monitoring and controlling the print run stability.

Colorimetrics is ideally suited for this. The inks cyan, magenta and yellow should therefore be controlled colorimetrically on the basis of gray fields (if possible with three-quarter tone patches). As reference values, both inhouse standards and values from proof control strips may be used.

93

The illustration shows a monitor display of CPC 21. The location of the reference colour is shown in the a-b plane at the upper left. In our example it is located in the center, i.e. on the gray axis. The center of the illustration shows an enlargement around the reference colour location. The three circles mark the boundaries of the three E-tolerance classes close, medium and wide. The lightness axis is situated near the right border of the screen, also with respect to the reference colour location. Here, too, the three tolerances are marked.

Each cross marks a reading. In the example shown, the measured colour locations of the colour zones deviate towards yellow-green and brighter.

If the deviations are larger than the allowed E-tolerance, the processing unit will automatically compute the necessary corrections for cyan, magenta and yellow. In addition to the spectral readings of the gray field, those of the single colour solid patches and halftone patches of cyan, magenta and yellow as well as the spectral readings of solid superimposition patches are evaluated. In this way, all

94

relevant factors involved are taken into consideration.

Corrections in the press will automatically be made via the CPC 1 press control unit.

4.12.2 Colorimetric control by solid fields Colorimetric solid control is generally preferred for black and for special colours. Black primarily has an effect on brightness. Since the human eye tends to tolerate deviations in brightness more readily than chroma deviations, black can be controlled on the basis of solid fields. Experience has shown that the influence of black on the colour balance is thus sufficiently accounted for.

Additional colours are mostly printed as isolated and solid areas. It is, therefore, sensible and correct to monitor them on the basis of solid fields.

But in solid patch control, too, spectral measuring and colorimetric evaluating have important advantages over density control: it can be ascertained exactly whether a preset hue has been reached.

Furthermore, reference colour locations can be entered either as numerical values or as specimen measurements.

This is not possible in colour density measuring.

The Heidelberg colour measuring units indicate after the first measurement whether or not the reference colour location can be reached with a specific colour. If not, the expected colour location difference Epossible is output.

If there are deviations in excess of the allowed tolerance, the necessary modifications will again be computed. The spectral readings of the single-colour solid and single-colour halftone patches are used in the calculation.

95

4.12.3 Densitometric control by solid field In addition to colorimetric data, spectrophotometers can also establish density values for any colour filter. As an aid to the user, Heidelberg therefore also provides colour density values independent of the type of control.

Especially for repeat orders, for which colour density reference values have already been calculated, solid density control may be an alternative.

4.12.4 Colorimetric image control The CPC 24 logs the entire printed sheet using a spectral photometer and then divides it up into measurement points of 2.0 mm ďƒ— 2.4 mm. If the printed sheet is 1020 mm ďƒ— 720 mm in size, then there are 160 000 measurement points. This means that colorimetric image control on the basis of the printed image is possible for the first time. For four-colour printing, this takes place on the basis of an accurate defined off-press tonal proof or an OK sheet, while

96

a colour pattern or an OK sheet is used for other colour ranges. In addition to the image control, colour control strips can also be used for control. The two control processes can be combined with one another at any stage.

As it is only possible to control the printing machine during image control if the structure of the image is known, the printed image is automatically separated into its component colours. Any other colours are controlled separately and must therefore be defined manually. This can be done either by selecting an area of the image which is printed with the special colour or by selecting the special colour from a database.

Once the special colours used have been entered, the special colour tones used are recognised automatically. The colour settings are calculated such that the discrepancy in colour of the image points from the set image is minimised. Visible deviations of critical colour tones and homogeneous colour areas are taken more into consideration as colour deviations are easier to see and recognise as an error in these cases than they are in saturated colour tones and busy areas of the image.

Limitations of image control: ■

Printing special colours over one another

■

Images with more than 4 colours

■

5th colour in image

■

Hexachrome

■

Hi-fi colour

97

4.13 Advantages of colorimetrics for offset printing In conclusion, a survey of the essential advantages of colorimetrics for offset printing: ■

The measurement readings match as closely as is possible the subjective perception of colour.

■

Colorimetrics is a colour evaluation technique which is independent of the printing process and can be used throughout the printing process from the pre-press stage via all proof stages and, finally, for quality control.

■

Colorimetric reference values can also be given as numerical values. An interface to pre-press units is available.

■

Colorimetric reference values can be taken from specimens.

■

Only with colorimetrics is it possible to objectively tune colours.

■

Colorimetrics makes image-related colour control possible (e.g. by means of gray fields) without colour-specific calibration procedures and without stored values.

■

By means of colorimetrics, all inks, even very light special inks, can be controlled correctly and consistently.

98

■

Dot gains are reliably detected by spectral colour measurement even if special inks are used.

■

The production run control is safer since changes in the printing stock, ink soilings and metamerism can all be detected.

■

Halftone printings with more than four colours can also be controlled reliably.

■

The printing quality can be defined and verified better.

There is a measure for colour deviations independent of the hue.

■

Spectral colour measurement makes the development of better colour control models possible.

■

The printing industry will adapt to the colour measuring principle currently used in all colouring industries.

■

Densitometry is an integral part of spectral colour measurement.

■

The tendency toward the use of more than four inks is accounted for.

■

Colorimetrics also makes it possible to objectively compare parts of printed images with the originals.

99