Pre-press gray balance knowledge review--application in four-color printing

4, gray balance in four-color printing

The basic relationship between each color plate in the green, yellow, and black color separations is called gray balance, and many color printers do not understand the gray balance very well. The gray balance refers to the generation of neutral grays. The required density values ​​of cyan, magenta, and yellow, these values ​​being high or low can cause color shifts in the entire image.

The color separation workers use the relationship between the yellow, green, and blue color separation sheets that can produce neutral grays as a color separation basis, and the color balance is limited by this color balance when the color separation sheets reproduce various colors. The more information the color separation worker has on the job to be copied, such as ink, printing material, and print test, the smoother the color separation will be. However, if the color balance of the color separation sheet is not well understood, the image will look It will not be good.

The screen printing trade union spent a long time to modify the screen and change the ink in order to achieve accurate reproduction of the four-color image. The printing operation union measured the registration of the printing image to make the image color reproduction realistic. When all of these jobs do not improve the quality of the print job, the responsibility falls on the color separation worker. The problem may also be caused by the poor quality of the film, but most of the time it is due to the separation of the gray balance and the guarantee of the print quality. Color separation films, screens and other variables are not understood.

This article will discuss color theory in gray balance and the control of gray balance in screen printing. Let us first review the chromaticity maps and their role in image reproduction.

First, the basic theory of color

All colors can be represented in a three-dimensional space, such as the Munsell color map, Oswald color map, etc., which are all based on the CIE standards. There are several color coordinate systems such as L”a”b”, XYZ, RGB, or HSB in the color map. Of course, we use the CMYK color system when printing in four colors.

The shape of any three-color product map is determined by its corresponding mathematical relationship. The use of mathematical methods to calculate color values ​​is the basis for color separation. We do not discuss mathematical calculations of color here. We only need to understand how colors are measured and described so that color separation is more accurate and color reproduction is achieved on each tear-printing material.

The RGB and XYZ color spaces represent the colors in the additive method, based on the color perceived by the human eye. As early as 100 years ago, people analyzed and studied the colors perceived by human eyes, and they have profound research theories about the sensitivity of human eyes to light. The RGB primary colors can be reproduced by exposure and development on the photosensitive paper and film, but in most cases, the RGB three primary color light signals are converted into a limited CMYK signal by computer processing.

In Photoshop color image processing software, the color representation system uses the simplest method of hue, saturation, and brightness (HSB). In the HSB system, the color is expressed on a circle with an angle change of 0 degrees and 360 degrees. The specific allocation is red (0), yellow (60), green (120), cyan (180), blue (240), and the product. (300) Finally 360 returns to red. Color saturation refers to the purity of the color, regardless of hue and saturation. The HSB system was originally used for color correction. Technically speaking, unlike the RGB and CMYK color system, it is not a color reference standard provided by the taste printer.

In the color image processing software in the Mac, the HSL color system is used, L represents lightness, H represents hue, and S represents saturation. In the HSL system, the lip with the highest saturation color is placed around the circumference of the circle, and the color saturation decreases toward the center of the circle until reaching the gray component area. In this area, the brightness is expressed, and all colors can be decolorized to black, white or Certain gray, pure white brightness is 100%, black brightness is 0%.

Second, color and non-color gray balance

There are two ways to reduce the color saturation: add a color complement or add black. The most basic theory of achromatism is called color gray balance, because it uses one color to decolorize another color, and the "decolorizer" is the complementary color or gray component of the color.

The complementary color of any color can be obtained by adding or subtracting 180 degrees from the angle value of the color on the hue, such as red (0), and the complementary color is cyan (180). In the gray balance, the neutral gray is passed. Mix any color with its complementary color.

Another method to obtain gray is called achromatic gray balance, which is achieved by adding a percentage of black to achieve the desired gray. You can add black color with the Photoshop color scale. In this color scale, the saturated solid color is located in the center. The more you can see the color changes with the amount of black added, some colors such as blue, green and purple are added in black. The impact is small, while yellow and orange are affected.

Because the color ink superimposed image is always defective, people have come up with many improvement methods, such as ground color removal (UCR), which can reduce the amount of green, yellow, and yellow in the center gray or black, and reduce the amount of each color. According to the amount of ink printed on the exact calculation. The smaller the amount of blue, yellow and yellow in the black component, the more vivid the color. If superimposed printing of inks in printing, the amount of ink on the printing material will be very large, which will produce serious moiré patterns for UV ink printing, and will tend to be dark and unclear for textile printing. It should be noted that the amount of ink overprinted on the surface of printing materials is limited. We call it the total ink limit. For textile printing, the total limit of the total ink is 240% - 260%, and the general limit of total ink is 260%. --280%. In order to keep the total ink volume strictly within the limit, it is necessary to remove the background color.

Gray Component Substitution (GCR), also known as achromatic gray balance, uses a certain amount of black ink instead of the gray component that is superimposed on the image. GCR achieves color printing by reducing the total amount of ink and adding a part of black ink. When and where gray components need to be used for substitution require very rich practical experience.

Third, the realization of the gray balance in the separation film

After the establishment of the three primary color theory 150 years ago, there were a lot of achromatic and gray methods, initially obtained by mixing equal amounts of cyan, magenta, and yellow colorants, but it was found that the result of their superposition was blurry brown. Not gray, why is this? Theoretically, an equal amount of three primary colors is superimposed on neutral gray, but actually the ink raw material is impure. No class of pigment can absorb the red, green and blue primary colors of the additive process as theoretically. The appearance of color is the result of absorbing its complementary color. Cyan pigments absorb red light and reflect the complementary colors of red light to the human eye. The problem is that the most accurate cyan, magenta, and yellow cyan materials absorb different amounts of light. Because the actual tricolor is not as perfect as theoretically read. Therefore, the amount of each colorant must be accurately calculated to achieve an ideal neutral gray balance.

We think of the percentage of error and chrominance as the amount of impurities contained in each eye piece. Print samples are measured with a very precise densitometer. The common hue error and gray percentage in silk printing are: the hue error of the yellowish product is 2%-5%, the gray level is 2%--5%; the product yellow wants error 8%--26% The gray scale is 18%--26%.

Very early on the printing, it was found that even if the ink ratio was accurately adjusted each time, neutral ash was not successfully superimposed each time. By simply mixing a certain color with its complementary color, black cannot be obtained. To solve this problem, the printer prints blacks with a tone change using color inks, just like the CMYK color map we use today.

The disadvantage of adding black to the color is that it is impossible to accurately duplicate the tone portion of each color, and the tone portion in the color separation sheet tends to be black without the color component. Most prints do not care about this loss, but artists and designers will argue about the matching of colors.

Fourth, gray ladder foot printing

In order to measure the precision of the color separation film, some scales need to be printed as reference standards. The three most important gauges are the field, the lightest color scale, and the neutral gray scale. If these three kinds of scales are not printed on each print sheet, it is as if they were shot in the dark and they do not know whether the color is deviated or not.

A good printer is often consulted against the printed color scale. If the ink is compatible with the printed sheet and the ink is of the correct color, consistent with the gray scale color scale, the image will be reproduced as well.

The choice of ink and the color sequence of the print will affect neutral gray and color balance. Neutral gray is the skeleton of the color separation film, and all color printing should be based on neutral gray.

To achieve the gray balance, consider the three characteristics of the ink. The first is the color of the ink. Any two sets of three primary colors of ink will not be exactly the same, if the ink is not measured when the color separation, the printing is sure to encounter the problem of color matching. If there is separation and business, take the seal to be printed and let the color separation worker look and measure the color density. After the color separation work is done, do not change the brand of the ink unless the measurement is performed with a colorimeter and a reflection densitometer to ensure that the replacement ink is consistent with the proof. The second is the color density of the ink. The uniformity and blackness of the printed ash scale depends on the color density of the ink. The density of the inks in each color must maintain the correct relationship to obtain a neutral gray balance.

The darkest portion of the color separation sheet has an optical density of 2.0, that is, almost 99% of the light irradiated to this portion is absorbed, and the density of the darkest portion of screen printing is larger than 2.0. When the color is increased, the density is in a logarithmic relationship. That is, after two colors with a density of 1.0 are superimposed, the density is not equal to 2.0. In order to obtain the ideal grey scale, it is recommended to use the following ranges for each color density: Yellow 0.85--1.0 Product 1.15--1.3 Green 1.25--1.4 Black 1.35--1.6.

The density range is selected according to the ink used. If the density is set low or high, the ink density should also be low or high. When each overlay is printed, the final density approaches 2.0. If the density of each color is different, the color of the step will be color cast. The color density of that color is high, and the grey ladder ruler will correct that color. For example, if the cyan density is 1.45, the color of the gray scale will be blue. If the density of a certain color is low, the color of the gray ladder rule is biased to the complementary color of the color. If the density of cyan color is too low, the color of the gray ladder rule is biased to red or orange.

The last influence in the ink characteristics will be the transparency of the ink, if the transparency of the ink is not good, the grey ladder wine will show the color superimposed in sequence. To a good three-primary ink is very good, if you want to save money on the purchase of ink, then gray in the printing adjustment time-consuming and costly.

The non-color gray balance public welfare is to eliminate all the color of the tone. In order to make up for this defect, it is recommended to print the black pattern first, and then print the color ink on the black color. This can have two advantages. The first is to adjust the part. Provides a black background on which color inks look more natural. Second, for UV ink printing, the coverage of black ink is small, and printing black first can reduce the moiré. If black is on the top, the image looks rough and unbalanced compared to other colors.

Five, screen and screen variables

To get a good gray scale, it is important to use a screen version. A good printing screen should be able to print the smallest dot, the amount of ink accumulation is correct, and the registration is accurate.

Print 65 line halftone images, engage the light to adjust the top 5%, dark set 95%, to get the high light part of the gray balance, the ink percentage is Y = 5%, M = 5%, C = 8%, which means that To be able to print 5% of the outlets, otherwise yellow, with the loss of red ash, will balance damage, the color of the ladder will be biased toward the blue.

The choice of screen is important. The more you know about the parameters of the silk grid, the easier it is to select a screen for a certain range of color images. The number of screen meshes, wire diameter, and screen weaving are the physical characteristics of the screens. They determine the thickness of the entire screen and the open rate of the screens. They affect the printing and ink transfer of the smallest dots, and the color of death affects the screen. As for the quality and time of general exposure, the photosensitive film is also very important.

The correct control of the screen and screen variables can be achieved with good color separation. If the surface of the printing material is changed to a light part color mark, it can be checked whether the minimum dot is printed with high light. The 65 line/inch halftone image was printed with 355 lines/inch and a flat weave with a diameter of 34 microns. The finer the screen wire diameter is, the smaller the dot coverage is. Of course, it is still necessary to print a color mark on the printing material. Otherwise, it will not be known whether the inks to be printed in each color are all printed.