Offset principle is not enough to improve color reproduction quality

The development of traditional lithographic offset printing technology has become a mature technology, but it still has some limitations. As an applicator, we should try our best to avoid the weaknesses of technology and use its strengths to serve our work to achieve satisfactory results.

1 Compression of color gamut - RGB color gamut is larger than CMYK color gamut

As we all know, the scope of CMYK rendering color is much smaller than that of RGB color space. Why is this? This is mainly caused by the printing of raw materials, the first is the ink, we compared the yellow, the product, and the blue three-color ink ideal absorption light map and the actual absorption of light, you can find the difference, the ink absorption of light in the absorption of the spectrum is not complete Absorbed, the reflected spectral segments are not completely reflected. In this way, the actual ink produces “grayscale”, and this “grayscale” greatly compresses the range of the print's expression color; followed by paper, paper is the basis for color rendering, and the whiteness of the paper determines the color brightness of the picture at high light. And saturation, which affects the contrast of the color; as well as the absorptivity and smoothness of the paper, play a crucial role in color rendering. Therefore, print cannot truly reflect the rich colors in the natural world. What we are doing is just simulation. Here, on the one hand, we hope that ink factories and paper mills can produce better inks and papers to better represent colors; on the other hand, as print designers, in order to allow prints to reflect bright colors, the cost allows Use inks and papers with good printability as much as possible.

2 Tone compression and adjustment - the original contrast is much larger than the print contrast

The ideal reproduction condition of the tune is a linear copy, but the reproduction density that the print can achieve is only 1.8 to 2.0 at the highest, and the normal density of the natural color original has a maximum density of about 3.0, and most of the contrast is above 2.2, so the original must be In order to compress the gradations, it is necessary to lose some of the subtleties. In practical operation, a reasonable level curve should be adjusted according to the contrast of the manuscript and the specific content of the tone, that is, the level of the minor part of the manuscript should be compressed. Make the main site level emphasized. Now we can use PHOTOSHOP's curve palette to adjust RGB channels for different themes of the image. As shown in Figure 1, the vertical axis represents the output density, and the horizontal axis represents the input density. (A) The concave curve represents the highlight of the emphasized image, and the (B) concave curve represents the shadow of the emphasized image. (C) Positive sigmoidal curves represent the midtones of the emphasized image, and (D) the inverse sigmoidal curves represent the highlight and shadow levels of the image at the same time.

From the (A) diagram, we can see that if the adjustment point is shifted upwards to become the (E) diagram, the level of the dark tone can be compressed, and the adjustment point can be shifted down (as shown in Figure F). Gradually, some layers are opened; according to (B), shifting the adjustment point upwards (as shown in Figure G) can open up some of the darker levels, and moving down (as in Figure H) can pull some of the mid-level highlights off.

From the above it can be seen that the level adjustment in PHOTOSHOP is very convenient, but there is one point to note that adjusting a part of the hierarchy will affect the other part of the hierarchy. We should adjust a reasonable level curve according to the characteristics of different manuscripts and correct it. The insufficiency of the manuscript maintains the main part, giving up the minor part and making the picture better.

3 Gray balance--CMY three-color dot ratio curve does not coincide

The same amount of CMY blending results in a gray color, while the actual ink yellow, product, and cyan blending results in not a gray color but a partial brown color. To make the composite colors of the three primary colors at all levels be neutral gray, the gray balance data of the three color versions of yellow, magenta, and cyan can not be controlled on a single curve. Through practice, it is proved that three kinds of printing primary ink dot sizes of gray printing Usually different, cyan dots are usually larger than yellow and magenta dots. As shown in Table 1, a set of neutral gray reference data.

The gray balance in color reproduction has become the scientific basis for our prepress production. As long as the proportions of the three color version dots on the gray part of the print are correct, the three color dot ratios of other colors can be surely correct. Based on the gray balance data, we can use the dropper tool on the PHOTOSHOP's curve palette and refer to the values ​​displayed on the information palette to find the point where the gray balance appears in the image, determine whether the image is color cast or neutralize the pole. .

4 outlets - there is a phenomenon of level jump

The role of the outlets are as follows: 1 The task of reproducing hue, lightness, and saturation is responsible for printing; 2 is the minimum unit for sensing and repelling water, and is the basic element of image transmission; 3 is the color, level, and color of images in color synthesis. Contour organizer. The most important problem of outlets is expansion. The amount of expansion is not only related to printing pressure, the number of dots and the number of dots, but also related to the dot shape of the dot. Now the dot shapes commonly used are square dots, circular dots, and chain dots. The square dots are at the 40% angle, and under the influence of the printing pressure, the 50% point has the largest amount of expansion. From the perspective of the level transition, a more obvious level jump is formed at the 50% point; round dots have to reach about 70%. When an edge occurs, an imprint jump will also occur. Only at the level, the level jump will be postponed; the chain points will take two angles, 25% and 75%, respectively. A smaller jump. The generation of level jumps will make the transcription of prints not continuous. Therefore, in the prepress production, we should use different shapes of dots according to the features of the original level. For example, you can use circular dots to represent the screen where the dark tone level is not very important. Use the chain points to represent the theme of the midtones. When making figure figures, it is best to use chain-type dots to avoid the enlargement of 50 percent points because most of the faces are about half of the dots. When making scenery maps, you can use square points or chain points.

5 "Water" - A Double-edged Sword

The main function of the fountain solution is to form a water film that repels ink in the blank part of the printing plate, and to keep the blank part of the printing plate from being dirty. It can be said that there is no fountain solution, and conventional offset printing cannot be discussed. However, the use of fountain solution in offset printing is a double-edged sword. Why do we say that here we do not talk about the various printing failures caused by “small water and large water”, that is, under the condition of “ink balance”? When the fountain solution competes with the ink, “water” will scour and corrode on the basis of the ink, so that the fine dot (especially the small dots below 3%) is difficult to ink, causing this part of the dot in the printing process. Missing, resulting in high light leveling. Therefore, as a printing operator, the amount of water should be controlled, and the amount of water in the layout should be reduced as much as possible without the print being dirty. As a prepress creator, you can't just pursue the exquisiteness of the image. You should consider the actual situation and select the appropriate number of screens, because 3% of 120 lines/inch screens are much larger than 3% of 175 lines/inch screens. Large outlets are easier to locate on plates.

6 Total ink limit

The total amount of ink can be set in the color separation option in PHOTOSHOP's CMYK palette. This parameter specifies the total percentage content of cyan, magenta, yellow and black in the black area of ​​the printed image. The theoretically darkest area should be printed with 400% of the maximum total ink. However, in four-color wet-overprinting, this situation can cause the following problems in the actual production operation of the printing press;

1 It is easy to cause dirt on the back of the printed sheet;

2 ink is not easy to dry;

3 Overprinting difficulties, ink overprinting is not good;

4 ink reverse transfer.

In the wet-overprinting, the total ink limit is not more than 340%. It is better to replace the gray component of the image in newspaper printing. The total amount of ink can also be reduced. The actual situation should be based on the actual situation, and the experience should be summarized to find the right one. The color separation parameter setting value? But under the monochromatic overprinting condition, I think that the gray component substitution is only in the image dark tone, the total amount of ink can be set larger, the other color part should be used to express CMY To increase color richness and image glossiness.

7 stamping problems

In CMYK four-color printing, overprinting is the most basic requirement for color printing. The stability of the machine, the suitability of the raw and auxiliary materials, the temperature and humidity of the workshop, etc. may all lead to misprints, even if all aspects are in good condition. There may still be minor misalignment between two color blocks that are trapped in the printing process, resulting in overprint deviations, which are manifested at the junction of different colors. The missing or overlying color of missing or overprinted edges is now compensated for. The defect method is trapping during color separation. The principle of trapping processing is to appropriately expand the color of the light-colored side to the dark side, and the specific expansion can be set according to the actual conditions of the printing (for example, according to the registration accuracy of the printing press), which is now common in many software applications. With trapping settings, some large companies also have professional trapping software that provides better control over trapping. Trapping processing should pay attention to the use of color, and the final trapping value setting cannot destroy the subtle level of the image or blur the boundary of the image. Sometimes trapping may not be done, such as printing in one color only. In addition, when using CMYK to design color blocks, it is necessary to pay attention to minimize the use of color blocks or texts of M+C=B, because M and C are all strong colors, and they are only slightly inaccurate.

I would like to make some suggestions to printing colleagues who produce newspaper advertisements, and try to do as little as possible prints that have thin lines or hollowed out trumpet characters. I often read newspapers. Due to my professional habits, I often need to double-check the coloring papers. Advertisements rarely register to satisfy people. Therefore, prepress designers should not only pursue their own design results, but also pay attention to the actual situation of some printing to make your design work.