Correction of poor quality originals in prepress production
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In the field of books and packaging and printing, color printing has become the mainstream of printing, and the quality of printed materials is also increasing. However, due to subjective and objective factors, there are not many quality grades, and it is still not comparable to foreign quality products. We know that the quality of color prints is subject to many factors such as the tone level of the color original image, the dot gain value, and the color balance. Among them, the quality of the prepress image is directly determined. The quality, although some corrections can be made in the printing and printing, but there is basically no way to print the boutique. Therefore, when making an image before printing, if the quality of the original is found to be insufficient, you should try to correct it. Below, we briefly introduce some processing techniques and methods.
I. Requirements for originals
(1) Original density
The difference between the maximum and minimum density in the original (ie contrast), the maximum density of prints available today is 1.8, the maximum print image is 1.7, and the black ink density of the finished original is 1.8. In general, all the densities of the original can only be reproduced in the range of 0 to 1.8 on white paper, and therefore, for the original, there is a density range suitable for stencil printing. When the density range of the original is too large, the sensitivity of the scanner and the electric extension to the portion exceeding the density range is lowered, so the color separation plate is flat. According to practice, the original is 0.3~2.1, which is most suitable when the contrast is 1.8. The color negative document density difference is controlled within 2.4; if the original contrast is less than 2.5, reasonable compression is performed when copying, and the effect is also ideal. If the original contrast is greater than 2.5, even if the tone is compressed during copying, the layer will be lost too much. Poor.
(2) Original color cast
Generally, the overall color cast, low-key color cast, high-tone color cast, and high-low key are different colors (ie, cross color cast). Therefore, when reviewing, it is necessary to take a comprehensive look at the overall consideration. If you do not see a color cast, you will immediately process one color, otherwise it will cause other color casts.
(3) Original level
The quality of the replicas is mainly composed of three indicators: level, color and clarity, and the level is the most important. The level of the normal manuscript should be such that the picture is not bright and dark, high, medium and low-key, and the density changes are many, and the tone is rich.
2. Original color problem and processing method
The actual condition of the original color tone can be directly observed from the original three-color density contrast measurement and the three-color density curve. Color printing replication breaks down the original and replica density range into four segments, which divides the density range between highlight and shadow points into bright, midtone, and dark, and the region that is less than the high dot density is called extreme highlight.
In the analysis of the original color quality, if the three color density is basically the same as the contrast, it means that it is not color cast, and the color tone is basically normal; if the tone is complete, the layer is rich, and the distribution is reasonable, even if the total contrast is slightly different, it can be regarded as a normal manuscript. The color copy can be corrected according to the normal color error data, and the focus is on the tone level copying.
If the density of the three colors is different, it means that the color is biased toward a certain color, but the contrast of the three color densities is basically equal, which is a balanced color cast. This kind of color cast is preferably designed according to the three-color density curve to adjust the horizontal reproduction curve of the three-color gray balance, so that the tone of the full-tone tone is uniformly corrected.
If the difference between the three colors of the original light is large, the density of the three colors of the dark end is nearly the same, and the color cast is mainly displayed at the high-level end, which is unfavorable for the high-level reproduction of the image. It is better to design a three-color version of the hierarchical copy curve based on the three-color density curve, and change the record setting of the high-light three-color dot of the color separation picture to accurately correct the high-tone color cast.
If the high-light tri-color density of the original is basically the same, only the three-color density of the dark-adjusted end is different, the density contrast is not equal, the medium-to-dark tone is obviously color cast, the dark tone level is slightly lost, and the middle-high-profile level is normal. You can change the color separation picture tones the three-color dot recording setting, or add a full-tone background color removal (only for mixed color). However, when dealing with the original color cast, it is important to note that some of the artistic effects that the author intentionally created are examples of backlighting.
In printing, there are generally three different concepts for copying and reproducing colors. One is the color reproduction in the physical sense, which requires that the spectral distribution of the original color at each color point is exactly the same. The second is the reproduction in the sense of colorimetry, which makes the printed reproduction image coincide with or close to the chromaticity of the original color point, that is, the heterogeneous color effect, which is a measure standard for realistic objective evaluation of color reproduction. The third type is the color reproduction in the psychological sense, that is, the color of the print reproduction, which may be somewhat different from the original color in the chromaticity, but the color effect may reach the visual psychological satisfaction.
The fundamental principle of color reproduction is to restore the gray in the original. To measure the gray balance print reproduction, the color density meter can be used to measure the three-color neutral gray scale attached to the printed matter, or the three-color optical density in the picture should be neutral gray level. If observation is used instead of judging, it is necessary to compare the gray level in the printed picture with a standard neutral gray block under the condition that the light source and the ambient color temperature are very standard.
3. Original layer density problem and processing method
For originals with different gradation densities, the processing method varies depending on the manuscript.
(1) normal exposure, density contrast standard
The main part is in bright and medium tone, which is directly proportional to the brightness, and is in the 4~6 level of the new brightness system.
(2) Medium contrast original
The low density is 0.3~0.5, and the high density is 2.4~2.8. The standard manuscript is close to the standard. At the same time, the original tone is contrast-compressed, and the curve is adjusted according to the standard gradation reproduction curve, which can achieve good copying effect.
(3) High contrast original
The high optical density is not higher than 0.4, and the highest density is more than 3.0. Most of the originals are too bright and contrast contrast. Generally, the intermediate level is rich and complete, and the ratio of the brightest and darkest areas is small. Such manuscripts, when selected for highlights and shadows, generally have the secondary levels at both ends reduced to reduce the range of their tone reproduction.
(4) Low contrast original
The tone contrast is below 1.7, and the low-density manuscript with a high optical density of 0.3 or less has a bright and thin main body, and has a high brightness. It is in the 7th and 8th grades of the new brightness system, and the brightness level is rich, but the density difference is very good. Small, and a large proportion of the area. Deepen the copying during production to reduce the brightness and approach the standard brightness. For such manuscripts, it is necessary to strengthen the horizontal and medium-level copying and redistribution. The black and white field calibration density value should be smaller, and the bright and medium-level hierarchical curves should be slightly deepened to achieve better level contrast and visual brightness effects. According to the shape of the hierarchical distribution curve, the main body should be adjusted in the range of bright or medium density. At the same time, you need to use a deeper and longer black tone to complement.