Color Matching Authenticity - Color Matching and Perceptual Dynamics

As a printing practitioner, we all know that color is the most sensitive and the most difficult to control. Accurately mixing and matching colors is the most daunting challenge before us. For this reason, you may have been rejected by customers more than once. Product experience. When matching colors, related technical issues to be considered include color description, digital display, color synthesis system, toner (pigment), printing technology, measurement, observation conditions, and the surface of the substrate, etc., in which each factor changes. Both have a great influence on the ability to match colors. It is almost impossible to fully consider each of the above-mentioned factors, but in the production of production should take care of some major aspects, and their relationship with each other.

1: Using color measurement equipment, color accuracy can be evaluated based on data. Such as a densitometer or spectrophotometer

Color description

The core of color management is the way color is described. The most common and popular method at the moment is Pantone's Pantone Matching System. The system has the ability to provide color descriptions and descriptions to designers and users, and printers use various methods to copy the illustrated colors as accurately as possible. The process of copying is related to whether the selected ink system is in compliance with Pantone's standards. On the surface, this process is simple and perfect, but there are actually many potential problems with screen printing.

Many printers are familiar with 1012 Pantone color guides: Pantone Color Selection Manual. It includes all colors that can be printed on coated paper or uncoated paper, as well as optional Pantone blending formulas.

For screen printers, the problem of color matching was faced from the very beginning: Almost all chromatograms were produced using offset printing, and their ink formulations were designed for offset printing inks. In fact, the Pantone colors matched by the screen printing process can only be called "Pantone analog color".

When trying to use the Pantone Color Reference Manual, screen printers must keep in mind a few limitations. First of all, we must look at the time of publication of the reference manual. For manuals that have been published for a long time, oxidation may occur on the surface. It is recommended not to use manuals that were published a year ago. In recent years, Pantone began to use the date on the manual as a reference. Due to the relatively high prices of these manuals, giving up them is always repulsive, but their color is very different from the new version. Continued use will cause a serious decline in product quality. Therefore, when making a decision, it must be Weighing again and again. This applies to both the designer and the production department, and the version used by the printer should be the same as the version suggested by the customer. This is the only way to ensure an exact match of color.

Another major challenge facing screen printers is the fact that the range of printing materials is too wide, and these materials have a great influence on the appearance of printed colors. For example, if printing on textiles, use coated Pantone chromatography as a reference? Still using the uncoated Pantone chromatography as a reference, it is difficult to make a choice, requiring the printer to have considerable experience. If printing with UV inks, glossiness issues need to be considered because the gloss of UV inks is much higher than that of coated materials. In either case, the customer is the key figure in the final judgment of whether the color is accurately matched. If you use the ink mixing values ​​indicated in the Pantone chromatograph for production, you should also be aware that many of the factors for color reproduction go beyond what Pantone chromatography can control.

In addition, when the operator controls the color, it is largely related to other factors that will be described below.

Digital Display

Since we have now entered the world of digital printing, all image manuscripts are stored as electronic files in vector format or raster format. On the monitor we can see the color of the image. The appearance of the color depends on what the human eye sees. But no two monitors or observation conditions are exactly the same, so when looking at the color, it needs to be done under the control of a color management system.

Digital color management achieves exactly the same color by characterizing and correcting each display. As a printer, our job is to look at the documents and make sure that the observation conditions are as close as possible to these parameter descriptions, and to confirm the colors specified by the customer.

Although we will not discuss the ICC color management profile here, the various components that make up the ICC monitor profile have a great influence on the color appearance we observe. The following points need to be considered in monitor observation factors:

· Color temperature when observed (5000° K or 6500° K);

· Work color space (sRGB, Adobe 1998, Apple, Color Match, etc.)

· Display gamma

· White and black spots

· Display contrast

The above factors vary from monitor to monitor, resulting in changes in the RGB values ​​that make the appearance of the same Pantone reference color very different; similarly, the L*a*b* values ​​of the same color will also be different. In addition, there is a big difference between the light emitted by the display and the brightness reflected on the printed surface, which further exacerbates the inability of the colors to achieve a perfect match. In the case of hard color matching in this case, the color code provided by the customer is also the only basis.

With the continuous improvement of the design and color management technology, the display parameters and characteristic description files have become correspondingly more complete. Most color management systems allow feature descriptor files to be embedded in graphic image files. Of course, in order to ensure that the characteristics of the document can have a certain degree of accuracy, require each person operating the computer should have knowledge of digital color management.

Color synthesis system

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.

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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 distinguish