Digital Proofing Basics - ICC

1, ICC

ICC (International Color Consortiun) International Color Consortium was established in 1993 and launched eight companies. They are Adobe, Apple, Microsoft, Silicon Graphics, Sun, and Taligent. Six manufacturers provide color management platform. Agfa and Eastman Kodak Home care about open standards for practical color management manufacturers. ICC now has 61 member companies and 3 honorary members in addition to the originating company. These companies are all companies that play important roles in the related color fields. The original standards were all implemented, so most of the systems on the market could not be compatible, limiting the promotion of the company's products and the development of related technologies. In view of this, these companies organized the ICC and decided to create an open, universal, cross-platform, and standard color management system. Applying this standard color management system can enable various products to coordinate with each other when applied in various fields. . This standard color management system has been certified by ISO as the international standard for color management.

The core of this standard color management system is the ICC Profile, which we call the color profile. The ICC Profile is simply a file describing the color characteristics of a particular color device. He describes the correspondence between the color description of this particular device and the standard color space. There are seven kinds of ICC profiles, namely input, output, display, drive connection, color space conversion, spot color and absolute profile. Generally, we can only encounter the first three. The input ICC Profile is for scanners and digital cameras. The output ICC Profile is for printers and film recorders. The ICC Profile is for various displays including LCD and CRT.

2, ICC Profile and color reproduction

Simply put, the ICC profile of a device is the color profile of the device. It is a correspondence between the device's color description and the standard color space color description method. Different devices describe the color through the standard color space and The ICC Profile of these devices is linked.

But just this way, we can not completely match the color of one device with the color of another device, because the projection range of all the colors of the two devices in the standard color space is different. This projection range is called the color gamut. In order to ensure the quality of the original, we must use the color gamut of the replica to completely cover the gamut of the copied product. The gamut of the copy is more manageable than the gamut of the original. Cut it off. However, the color gamut of the copy is less than the gamut of the original. It is usually practiced in two ways. First, proportional compression compresses all the colors of the manuscript to the gamut of the copy. This method ensures the contrast and gradation of the original color, but does not guarantee the accuracy of the color; the second method is boundary compression, compressing the color of the manuscript outside the gamut of the copy to the boundary of the gamut of the copy, and the color in the gamut. Keeping the same, this method guarantees the accurate reproduction of common neutral colors, but the extreme color has a merging phenomenon.

So how did the various ICC Profiles actually come from? As mentioned earlier, we can encounter three types of ICC profiles in practical applications. There are two kinds of display devices, CRT (commonly referred to as a display) and LCD (usually referred to as a liquid crystal display). The display color is generated by the superposition of three kinds of RGB light; the input device is mainly a variety of scanners and digital cameras. Whether picking colors using CCD, CMOS, or photomultiplier tubes, these color picking components are RGB. In software, source image standards are RGB files, which usually correspond to RGB source ICC profiles. Generally, the RGB source ICC profile selected in the digital proofing software is the ICC Profile corresponding to the international standard RBG space, but the ICC profile of the customer-specific equipment is not excluded in special cases. We recommend that customers only use the standard RGB source ICC Profile and use PhotoShop's ColorMatch RGB color space when working with images in PhotoShop because in most cases customers cannot create accurate RGB source ICC Profiles to process images even if the customer Bought a good scanner.

Setting up the ICC Profile of the output device is a bit more troublesome. For the general printing, the CMCK ICC Profile must be established. For a large number of inkjet printers, the ink used by them is very different, unlike the monitor phosphor or the scanner CCD. . An important part of digital proofing is the establishment of a printed source ICC Profile. Because the number of color copying steps in the printing process is not easy to control, coupled with the high speed of the printing press and the high starting cost, sampling is very difficult. To monitor the working status of the printing press for a period of time, analyze the printed matter printed at different printing times at different times, and obtain the dynamic range of the printing press, and then print it out in different environments under strict technical conditions. Standard proofs are analyzed to obtain a standard printed CMYK source ICC Profile. This standard CMYK source ICC Profile has vivid color reproduction, structured, suitable for a variety of printed matter, to meet a variety of printing conditions and environmental characteristics. There is a misunderstanding of color management. We want to establish the color profile of each printing machine in each printing factory. We believe that this cannot be done under realistic conditions. The first is the establishment of a standard. Based on which outcome is the standard, no consensus can be reached in this case. Even with the standard, digital proofing must first be followed by standards, and the process of printing and proofing should be close to the standard. Because the dynamic range of the printing itself has long exceeded the range allowed by the standard, and the standard cannot cover all the originals, digital proofing is used as an intermediary.

At present, there is a trend in the market. In order to save time, the printing factory only fixes its own process technology to a certain extent, does not consider whether the process is reasonable, and then requires digital proofing to reflect the work characteristics of the printing factory at this time. From a color matching point of view, this is feasible, except that the accuracy of the proofing results will change dynamically. Therefore, comparison tests are often performed to ensure the consistency of proofing and the printing environment.

How to establish the ICC Profile of digital proofing equipment? Generally follow the following principles, the first linearization of the device to ensure that the device output maximum density, neutral gray, tone close to the printing, while ensuring that the device's ink is not to pile up ink. Then print out the standard color block for the colorimetric device to read, and then use the ICC Profile Generator to generate the ICC Profile, and finally fine-tune the results to obtain the final output device ICC Profile. The ICC profile generator that we use in Founder's photo is EASTMAN KODAK's COLORFLOW software.

In particular, the error of the entire system to establish the ICC profile should be mentioned here. It mainly refers to the error problem of the colorimetric equipment. According to the color theory described above, we know the color-taking environment of the colorimetric equipment and the environment in which we observe the sample. There is a difference, and based on the physical accuracy of the colorimetric device, we can probably estimate that the difference in color reproduction using only the device color measurement is about 4% (where the spectrophotometer itself has an error of 1%-2%). The human eye can accurately compare color differences of less than 1%, so blind worship of spectrophotometers is not desirable, which also explains why customers are not advised to do their own color profile of printing equipment. So what's the use of color measurement equipment? Admonish everyone that there is no color measurement equipment. The difficulty of color correction is 20 times that of color measurement equipment.



Source: PACK.CN