The necessity of color management in the printing process

Abstract: The ultimate goal of print copying is to obtain batch copies of originals that are as consistent as possible with the original image. With the development of electronic science and technology, printing is increasingly dependent on electronic products used in a series of printing processes such as scanners, digital cameras, monitors, and printers. Therefore, these devices undoubtedly all contribute to the final display of prints. Very important role. The reality is that many printing companies, especially small and medium-sized enterprises (accounting for more than 90% of the printing team), lack understanding of the necessity of color management. The author hopes to let the owners of printing companies re-understand the necessity of color management through this article.

Keywords: color space color management

Color management is a hot word in the printing industry today. For a printer to get the ideal print copy, one cannot ignore such a concept and act in the actual printing operation. Indeed, with the rapid development of optoelectronic technology, printing technology has also shown a "new day" situation, with prepress technology being the most obvious. More and more technology products are involved in the prepress process and play an important role in the printing field. It is precisely because of the involvement of these products that the complexity of the printing process and the possibility of color transfer between different devices are increased, and naturally the color fidelity problems caused by the transmission of colors between different devices are naturally caused. What to say about color management.

First, the printing process

Original - Imposition - Proofing - Output - Printing This is a simple print job flow diagram. First, the printing company obtains the printed manuscript from the customer, and then the printer digitizes the manuscript with the corresponding scanner according to the type of the manuscript and the printing requirement and some other factors, and stores it on the computer for use; the operator then uses desktop publishing software to put various layouts. The elements are organized together and previewed on a computer monitor. After the grouping is completed, the worker then proofs through a color printer, selects the appropriate output device after proofreading (including color), and outputs it in various forms, and then prints. Finally get the printed product.

From this process, we can clearly see that the color-related devices mainly involved in the printing operation system are scanners, digital cameras, monitors, color printers, digital proofers, digital printers, and traditional printers. While customers and staff must and can only rely on these devices to check the color of images, the accuracy of the color rendering of each device is crucial. To realize the accurate transfer of image colors between these devices, it is necessary to understand the color space and color rendering principle of these devices.

Second, the scanner. Digital Camera Color Space and Color Rendering Principle

The scanners currently used in DTP systems are mostly flatbed scanners and film (transmission) scanners, and the color space of the scanner is the RGB additive space. The photosensors of this type of scanner are CCD photocouplers. There are three kinds of RGB color material coating on the CCD photoelectric coupling device. Through the decomposition of the color during scanning, the corresponding color data values ​​are obtained after the conversion of the photoelectric signal, and then the color characteristics are recorded in the form of data to reproduce the color.

Due to hardware factors, the operator is prone to the following problems in digitizing the manuscript:

1. Different brands of scanners scan the same original and will get different image data;

2. Different models of the same brand scanner scan the same original, will get different image data;

3. The same scanner scans the same original at different times and will get different image data;

In recent years, digital cameras have become the main source of manuscripts after scanners. Digital cameras also use RGB color space, use CCD as a light-sensitive device, and record image data after photoelectric conversion. Therefore, the problem of color reproduction that easily occurs is comparable to the scanner. At the same time, digital cameras are different from scanners in that they rely on external light sources to sense image information. Therefore, the external light conditions have a decisive effect on the appearance of image data.

Third, the monitor's color space and color theory

Current displays used for prepress are mainly cathode ray (CRT) displays and liquid crystal (LCD) displays. Their color space is RGB color space.

The main working principle of cathode ray displays is:

1. The three cathodes can emit three electron beams and the emission is controlled by three primary color signals. The phosphor must be R, G, B and have very close distribution (the distance must be within one pixel).

2. The electron beams controlled by the three color signals of R, G, B can correctly reach the corresponding R, G, B phosphors after passing through the color selection plate.

3. Phosphor is a kind of high luminous efficiency, high color purity, high saturation, can produce standard white and neutral gray.

CRT monitors display colors based on the RGB color space. Due to the differences among various manufacturers, the phosphors used are different, the defined deflection center is different from the center of the exposure plane, and the phenomenon of non-coincidence is likely to occur. Therefore, the display will produce errors when the colors are reproduced. The difference between the display's color space and the color space used by the printing equipment can also directly lead to color deviations, which are seen as non-obtainable.

The display principle of the liquid crystal display is that, under the action of the external electric field, the alignment direction of the liquid crystal molecules changes, R, G, B three-color filter film and the liquid crystal molecules together make the display color.

Therefore, regardless of whether it is a CRT display or a liquid crystal display, since they all use an RGB color space, the CMYK color reduction space used in the final printing is different, and the similarly existing hardware characteristics are problem, the staff performs image processing, making a layout, etc. on the display. During the operation, the following problems will inevitably occur:

1. Different brands of monitors show that the same image will have different performance;

2. Different models of the same brand display show the same image will have different performance;

3. The color expression capabilities of displays and printing inks are different, and thus WYSIWYG cannot be fully realized.
4. Different image processing software lacks consistency when converting R, G, B data into C, M, Y, K data.

Fourth, the printer's color space and color reproduction principle

In the DTP and CTP systems as soft proofing tools, the printer output device has the same color space and the same color rendering principle as the printing press. To be exact, color inkjet printers and color laser printers use FM. Printing method. Each ink dot is of the same size, and the tone level of the color is represented by the irregular distribution of ink dots. Although the working principle of the thermal wax printer and the sublimation printer is different from that of the former two, the color space is the same and both are CMY color reduction spaces.

Although various printers use the same CMY color reduction space as the printing press, the coloring materials they use are not exactly the same as the printing inks, so there are still different degrees of inconsistency with the printed matter. The performance is:

1. Different types (color inkjet, color laser, thermal sublimation, hot wax) printers print the same image for different color effects.

2. Different types of printers of the same type print the same image for different color effects.
3. The same printer uses different inks to print the same image to get different color effects.

Fifth, the color space and color rendering principle of the printing machine

The printed color space belongs to the CMY subtractive space. In printing, dots are the most basic element of color expression, and different colors are formed by the overlapping of dots in different proportions.

Plane printing is based on the subtractive method and is usually printed on the substrate (eg paper). In fact, after the surface of the paper is printed with clear ink, the colors of the red, green, and blue light waves are filtered by multiple bonds. When the cyan, magenta, and yellow inks are combined, the wavelengths absorbed by the combined three-color ink are not uniform. The red light partially reflects more and results in a brownish tone instead of black. In order to compensate for this defect in printing, black ink is used in the color printing process.

At the same time, paper also has a significant effect on color reproductions.

Therefore, the printing process itself also has many factors such as ink, paper, pressure, etc. that affect the final print's color presentation, and it is easy to have the following problems:

1. Printing using different brands of ink will produce different printing effects;

2. Printing on different papers will produce different effects;

3. Various settings of the printing process (pressure, ink balance, etc.) will produce different printing effects.

Sixth, the conclusion

Printing is a complex process that requires scanners, digital cameras, monitors, printers, proofers, printers and many other devices to work together. These devices all display the same image data in their own links. However, due to various reasons such as hardware, they do not use a uniform way to represent the color of the image. Therefore, although the color parameters of the image itself are in every aspect, It is the same, but it conveys a different appearance to the reader through the presentation of each device, and ultimately leads to the fact that the colors displayed by various devices are not such a condition of “what you see is what you get”. Therefore, in the image system, in order to obtain excellent copy prints in the printing process, it is necessary to pay full attention to the problems of color transfer between devices during the entire printing process, and adopt a practical and practical color management system to implement effective colors. Management to make the image color appear as consistent as possible between devices. Otherwise, the printing color control will step into the position of Xinmayouyu.