Analysis of Gray Balance in Four-color Printing (II)

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 in order 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% , Grayscale 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 gray scale, it is recommended that the color density be in the following range: 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.

Sixth, the conclusion

The separation is the key to correctly mastering the gray balance color reproduction. Each set of three primary color inks and color separations corresponds to a gray balance state. Indiscriminately changing the ink brand and color will destroy the entire color balance and gray balance of the image. With the separation of color should always be linked to ensure the ink density of the relationship between several color ink, which is the key to achieve the gray balance, if the gray balance in the separation is not well grasped, then all the work such as silk screen plate, moire control The expansion of outlets is in vain. Each color label to be printed with light, shades, and gray step changes is only necessary for printing high quality images.

Source: <Screen Printing>