The principle of color matching and color perception

Color synthesis system

Figure 2: Color perception and reality

The figure above illustrates the strong influence of contrast between adjacent colors on hue. The chromaticity values ​​on each color bar are exactly the same, but from a visual point of view, there is a difference in color from left to right due to the gradient of the background color.

Pantone's color synthesis system designed for the screen printing industry is a big project, mainly because the types of substrates and inks involved in screen printing are too many. Moreover, there are also many process variables involved in the production process, such as the number of meshes and the tension of the stretching net, which change the hue of the printed colors.

All color matching ink systems are based on translucent white substrates that mix the various colors to form the main color. The number of main colors has a lot to do with the type and opacity of the ink. The higher the opacity of the ink, the more main colors that can be obtained. Pantone's proposed color synthesis system is as few as 9 colors, up to 21 colors. Therefore, each Pantone-matched recipe varies from one ink brand to another and from different substrates.

When an ink company identified an ink matching system, they had to submit a certain amount of color samples to Pantone for evaluation. When these colors are rated as being very close to Pantone colors, Pantone will grant the ink company a license for a complete color matching ink system, even though many of the colors in the chromatogram are not directly verified by Pantone. At the time of applying for such franchise, Pantone Company couldn't verify every color of every ink company's printed on coated and uncoated paper, but they developed a reasonable sampling system. Through this system, you can make a more comprehensive judgment on the overall color of the company.

For printers, the safest way to achieve Pantone color mix and match is to perform actual tests based on the recipes on customer-supplied color samples. Although there will be some errors, such an operation is very meaningful.

Pigments and Printing Technology

The screen printing function is very powerful in dealing with a variety of substrates and printing applications, which is unmatched by other various printing processes. In the screen printing process, water based inks, solvent based inks, UV inks and hot melt inks can be used, in addition to porcelain decals printing inks and the like. In terms of ink drying, it is also diverse. It can be air-dried, UV-cured, hot-melted or baked in a kiln. The temperature ranges from room temperature to several thousand degrees in the kiln (making ceramics). The demand for pigments on screen printing is very unique.

Pigments that work well at room temperature can have very bad effects at high temperatures. Solvents, resins, and additives used in pigments to satisfy certain appropriate printing properties can also cause large variations in color. The temperature will change yellow and red. In addition, process principles such as dye sublimation process also affect hue. In the same way, some kind of red and purple pigments that can show excellent results will become very pale under the high-intensity ultraviolet light that polyester UV inks require.

Ink manufacturers have been doing their utmost to find acceptable non-toxic, non-heavy metals, economical, and readily available inks that deliver the ideal color. However, in general, the nature of a print job or substrate makes it difficult to find an ideal ink solution. The result is a far cry from the desired color effect.

As mentioned earlier, the surface properties of the substrate affect our perception of color. In addition, when considering the color change, the thickness of the printing ink should also be noted. The thickness of the ink layer deposited on the substrate will significantly change the hue of the ink. Here, we advocate the aggregation of variables that affect the thickness of the ink layer, such as the number of meshes, the diameter of the network cable, the tension of the network cable at rest, the tension of the network cable during printing (relations with the off-board gap), and the angle of the squeegee. And pressure, the hardness of the squeegee, the sharpness of the edge of the squeegee, and the thickness of the stencil. The number of permutations and combinations of these variables is very alarming, so the changes they bring to color are also very large.

In order to maintain the reproducibility of screen printing, a screen-printable chromatogram should be created to confirm the printability of the color and establish a color matching standard within the company and submit it to the customer along with the chromatogram. Since screen printing has a lot of variables, this may be the only way to accurately reproduce colors in the future.

Measurement and observation

More and more printers have been happy to buy densitometers and spectrophotometers. In recent years, the prices of these devices have dropped drastically. Even some small printing companies can afford it. They designed their own companies. Some very inexpensive measurement solutions, the lowest price of 1,000 US dollars, including hardware and software. With the continuous advancement of digital color management technology, a number of new types of devices with better performance and lower prices have appeared accordingly.

The ability to represent colors with numerical values ​​is very significant, and it provides producers with a target value for tracking and copying. The CIE L*a*b* value is usually used to evaluate the difference between the print and the reference proof, and the Δe value can be further calculated to show how large the difference is from the target value.

Although the Δe value represents the color difference in the entire file, it is more sensitive to changes in color such as pastel, neutral gray, and light brown. Since 1 is the smallest difference that the human eye can discriminate, the difference between some soft colors is usually less than 0.5 delta. On the other hand, it is also very common to see that the bright yellow region is greater than 10Δe, but it is not noticeable that the phase change is outstanding.

For a given Pantone reference manual, the color change from a perfect match of 0Δe to a maximum of 15Δe depends on the manual version and the color sampling conditions. Even a less obvious fingerprint can affect the appearance of many colors. Therefore, once again, the emphasis is on the customer-supplied spectrum, which is the most important part of the program.

When observing printed patches, the observation conditions are varied and vary greatly. The U.S. image industry has developed corresponding ANSI standards for reflected colors and transmitted colors. The standard requires that the color temperature of the observation light source is 5000°K, and the angle of illumination, the color and brightness level of the surrounding environment at the time of observation must be specified.

Contradictory with ANSI standards, Pantone's matching manual was observed under cool-white fluorescent lighting and did not comply with ANSI standards. Although we currently have little access to Pantone's brightness and environmental colors when observing color, Pantone is making chromatograms in a standard viewing box. However, we should determine whether the customer is consistent with our observational conditions. The difference between the two will lead to the failure of matching. (to be continued)