ANALYTICAL TOOLS
540 • inform September 2015, Vol. 26 (8)
Color measurement made easy Björn Hotting In the detergent industry, the color changes standardized stains undergo when they are washed are a strong parameter that can be used to improve products and create new formulas. Traditionally, most labs use spectrophotometers to measure color. Although these instruments are accurate, they are not easy to use and must be operated by skilled technicians. In the detergent industry, a small wash test typically involves 20 different stains with two repetitions and two wash
• color is a strong parameter that can be used to improve products and create new formulas. • Although the spectrophotometers currently used to measure color in the detergent industry are accurate, they are also slow, require a lot of repetition, and can only be operated by skilled technicians. • this article describes a new camerabased color measurement technology that not only measures color accurately, but also quickly, with less repetition, and with easy-to-use software that can be operated by unskilled workers. Inform_September_2015.indd 540
loads. Consequently, a small wash test of four different detergent formulas requires 320 color measurements—possibly even 640, if color is measured both before and after washing. This is a simple but timeconsuming job which, due to the accuracy required and the complexity of the equipment, cannot be done by unskilled workers. The Mach5 is a camera-based instrument that can measure the color of several areas simultaneously (Fig. 1). These measurement areas can be rectangular, round, or any other shape. The user sees a live image of the object and can use the mouse to position the measurement areas. The instrument measures absolute color and gives output in L*,a*,b* and X,Y,Z values (see sidebar on page 542). In the wash test mentioned above, the Mach5 can measure 20 samples in one pass. This means that only 16 cycles of measurement (instead of 320) are needed. Fewer measurements speed up the overall process and make it more user-friendly. Data can easily be exported, and users can even automate their (statistical) calculations (by adding their own Python script, for instance) to combine, move, or prepare data for custom reporting. (Python is an easy to learn programming language that allows users to write complete software or use inside existing software, as one would use a macro within Microsoft Excel. Information is available at https://www.python.org/.) Measuring 320 color samples one after another with a traditional spectrophotometer is physically and mentally tedious, and it can be difficult to find highly skilled technicians who want to repeat the same small task eight hours a day. In contrast, the user-friendly interface of the Mach5 software does not require a high level of skill. Automatic warnings prevent errors, and the software helps users position the measurement locations, which can be stored for future use.
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Dishwasher testing and sub areas The new camera-based system addresses another challenge in color measurement: The stains in dishwashers are not removed evenly during the washing process. The typical dishwasher test material is a small 120mm by 100mm plate. A spectrophotometer with a 9mm or even a 20mm opening will measure only a small area of the plate and average the results. Consequently, the results will vary largely depending on where the measurement is made, and any information about how and where the stain is removed will be lost. While it is possible to measure different locations, adding multiple spectrophotometer measurements to one dishwasher tile only increases the workload. Instead of providing an average measurement for the entire test material based on one location, the Mach5 automatically splits the measurement area into smaller sub-areas and calculates the standard deviation for the various sub-areas, providing critical information about the homogeneity of the total area.
Accuracy Most cameras only divide the spectrum of visible light into 3 parts (red, green, and blue) and are therefore not very good at measuring color in absolute terms. The technology in the Mach5 is upgraded to measure color better by splitting the visible light spectrum into 25 parts instead of only three (Fig. 2, page 542). CONTINUED ON NEXT PAGE
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FIG. 1. Mach5 with open drawer. The object to measure can either be inserted via the drawer or via the two doors. In the image, the standard color checker card is measured. This also gives an idea about the dimensions of the drawer size and measurement area.
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A quick introduction to color measurement There are various ways to measure color. A tri-stimulus instrument divides the fully visible spectrum of light into three parts: usually red, green, and blue. Having only three values allows such instruments to measure quickly. Their chief limitation is that they do not detect subtle color changes within colors, such as different shades of red, blue, or green; only the total sum is given. The human eye can detect visible light ranging in wavelength from blue (380nm) to red (680nm). A spectrophotometer that divides this range into 10nm intervals will have 30 data points over the full range, which is 10 times the data points the tri-stimulus sensor has. These additional data points allow the spectrophotometer to “see� the more subtle changes within red, green or blue. The best instrument to use depends on the application. For example, the granulate used to make polyethylene terephthalate (PET) bottles is checked by the suppliers, so it should be spectrally correct. Because PET bottle manufacturers only add more or less of a color additive to the plastic, it is very unlikely that color changes within red, green, or blue will occur. In most cases, only an intensity change will occur as a result of adding too much or too little of the color additive. On the other hand, in the detergent industry, where multiple stains are washed together, real color changes (not just in intensity) are observed. Also, the stains are not standard dyes but real live food ingredients, so a tri- stimulus approach will not yield the best results. There are various ways to describe color. Most systems give a color value with three numbers. A well-known system is the simple red, green, blue system (RGB) used in PCs and televisions. These are non- absolute systems; for instance, if the backlight of the screen is changed the same RGB value will result in a very different color. In addition to non-absolute systems there are absolute color terms such as L*,a*, b* and X,Y,Z. These allow a designer in the United States to communicate with a manufacturer in China about color in a common language they both understand. Absolute color is defined by three coordinates. In the L*,a*b* color space, L* is the lightness (from black to white), a* is the red-green axis, and b* is the blue-yellow axis. The L*,a*b* system is designed in such a way that a 1-point change is perceived equally by humans. So, a 1-point change in a* (red-green shift) will be seen equally as a 1-point change on either the L* or b* axis. For more info on color spaces like XYZ and L*,a* and b* see: http://en.wikipedia.org/wiki/Lab_ color_space.
The Mach5 is slightly less accurate than a spectro (dE < 2), but the major benefit of the Mach5 over conservative spectrophotometers is its flexibility with respect to the shape and size of the areas that can be measured, as well as the extra information it gives. It can also be operated by an unskilled worker, and is quicker than a spectrophotometer, as it can measure multiple
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FIG. 2. The spectral data of a sample as measured by the Mach5, which show the visible light spectrum from blue to red. The black dots represent measurement points in the spectrum. Normal camera-based systems only have three data points (red, green and blue). The Mach5 has multiple data points per spectrum, which makes it considerably more accurate. areas in one cycle. Full-size images of measured objects are saved for later assessment. The software and some of the hardware can be adapted to provide additional functionalities for specific industries. Although the current version of the Mach5 can only measure the color of non- gloss materials, a new version that handles gloss materials will be released by end of 2015.
Reading barcodes on coded test material Codes are traditionally hand written on test materials so that samples before and after washing can be uniquely identified and their results compared. Since the Mach5 uses a camera, it can also read barcodes. Some test material suppliers have already begun to offer a service in which they add a barcode to the test material. The Mach5 can read that barcode at the same time it measures the color, uniquely identifying the product and saving the data by the code that appears on the test material.
Customers and markets The first Mach5 was sold five years ago. That first customer asked for a market-specific instrument. Colour Consult designed the instrument, wrote the software, and helped the customer sell it. Because of that, most customers are in the detergent industry, but the company is considering expanding into other markets. The instrument is sold worldwide as a turnkey system with the PC included, so the instrument can easily be serviced remotely. A short video is available on request; please contact Colour Consult for more information. Bjorn Hotting is the owner of Colour Consult. He can be contacted at BHotting@ColourConsult.nl.
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