Correction of Pre-Quality Images in Prepress Production

In the field of books and packaging and printing, color printing has become the mainstream of printing, and the quality of printed matter is also increasing. However, due to subjective and objective factors, there are not many fine-quality products that can still be compared with foreign high-quality products. We know that the quality of color prints is affected by many factors in the process of copying, such as the tone levels of color original images, dot gain values, and color balance, and the color quality of the prepress images is directly determined. The quality, although it can be partially corrected in the process of plate-making, but to print a fine is basically powerless. Therefore, when making images before printing, if you find that the quality of the original is insufficient, you should try to correct it. Below, we briefly introduce some processing techniques and methods.

One. Requirements for manuscripts

(1) Original density

The difference between the maximum and minimum density in the original (ie, the contrast), the maximum print density that can be achieved in today's print is 1.8, the print image can be up to 1.7, and the black ink density of the original is 1.8. In general, all the density of the original can only be reproduced in the range of 0 to 1.8 on white paper. Therefore, for the original, there is a range of density suitable for printing. When the density range of the original document is too large, the sensitivity of the scanner and the extension unit to the out of density range decreases, so the color separation layer is flat. According to practice, the manuscript is 0.3 to 2.1, which is the most suitable when the contrast is 1.8. Color negative original density difference is controlled within 2.4; if the original contrast is less than 2.5, a reasonable compression effect when copying, the effect is also ideal, if the original contrast is greater than 2.5, even if the copy compression time, it will make too much level loss, the effect Not good.

(2) The color cast of the original

Generally divided into the overall color cast, low-key color cast, high-profile color cast and high and low tone each biased different colors (that is, cross-color cast). Therefore, to take a holistic view when correcting, you can't see a color cast and immediately process a color, otherwise it will cause other color casts.

(3) Original Level

To measure the quality of copies, there are mainly three indicators: level, color, and sharpness, and the most important is the level. The level of normal manuscripts should be characterized by the fact that the picture is not bright and dark, high, medium, and low-key, there are many density variations, and the tone is rich.

II. Original color problems and processing methods

The true condition of the original hue can be observed directly from the measurement of the three-color density contrast of the original and the depiction of the three-color density curve. The color print copy divides the density range of the original and the replica into four segments, ie, the density range between the highlight point and the dark tone point is divided into a bright tone, a middle tone, and a dark tone, and the area less than the high light spot density is called an extremely high light.

In the analysis of the original color quality, if the three-color density and contrast are basically the same, indicating that it is not color cast, the tone is basically normal; if the tone is complete, the level is rich, the distribution is reasonable, even if the total contrast is slightly different, can be considered normal manuscript, The color reproduction can be corrected with the normal color error data, and the emphasis is on the reproduction of the tone level.

If the density of the three colors is different, it means that it is biased toward a certain color, but the contrast of the three color densities is basically the same, which is an equilibrium color cast. This partial color draft is best based on the three-color density curve, designed to adjust the level of three-color gray balance of the replication curve, so that the tone of the full tone is unified and corrected.

If the original bright three-color density of the original document is larger, the dark color density is nearly the same, and the color cast is mainly manifested in the high-profile region, which is unfavorable for high-level image duplication. It is better to design a three-color version of the level reproduction curve according to the three-color density curve, change the recording settings of the high-light three-color dots of the color separation image, and correct the high-profile color cast accurately.

If the original highlight three-color density is basically the same, only the dark tone of the three-color density is different, the density contrast is not equal, in the dark tones show a significant color cast, dark tone levels slightly loss, normal level in the high profile. Can change the color picture dark tone three-color dot record settings, or add a full-color full tone background removal (only for mixed double color role). However, when dealing with color casts, it is important to note that some of them are artistic effects that the author intended to create. Backlit photos are examples of this.

In printing, there are generally three different concepts for the reproduction of colors. One is the physical color reproduction, which requires that the spectral distribution of the original color is exactly the same on each color point. The second is the reproduction in the sense of colorimetry, so that the printed reproduction image is consistent with or close to the color point of the original, that is, the different-same-color effect. This is a measure of the objective evaluation of color reproduction. The third kind is the psychological sense of color reproduction, that is, the color of the print reproduction, and the color may be somewhat different from the original color, but in terms of color effect, it may reach the satisfaction of visual psychology.

The fundamental principle of color reproduction lies in restoring the gray in the original. To measure the gray balance printing reproduction, the color density meter can be used to measure the three-color neutral gray ladder attached to the print, or the three-color optical density that should be the neutral gray level in the measurement screen. If observation is used instead of judging, it is necessary to compare the gray level in the print screen with a standard neutral grey block under the condition that the light source and ambient color temperature are very standard.

III. Manuscript level density problems and processing methods

The manuscripts with different levels of density and quality are handled differently depending on the manuscript.

(1) Normal exposure, density contrast standard

The main parts are in bright tone, middle tone, and in direct proportion to the brightness. They are in the 4th to 6th levels of the brightness new standard system.

(2) Ordinary contrast originals

The low density is 0.3 to 0.5, and the high density is 2.4 to 2.8. The manuscript is close to the standard. When the contrast of the original gradation is compressed, the gradation adjustment can be performed according to the reproduction curve of the tone level close to the standard to achieve a good copying effect.

(3) High contrast originals

The high optical density of not more than 0.4, and the highest density of more than 3.0 manuscripts, mostly the contrast of the original scene contrast is too large, the general intermediate tone level is rich and complete, the ratio of the brightest and darkest areas is not large. This type of manuscript, when selected for highlights and shadows, can generally be rounded off at both ends of the secondary level, in order to reduce the scope of its tone reproduction.

(4) Low contrast original

The tone contrast is less than 1.7, and the high light density is less than 0.3. The thin tone of the original is thin, the main part of which is brighter, thinner, and brighter. It is at the 7th and 8th grades of the Shinshin new standard system. The brightness adjustment level is rich, but the density difference is very high. Small, and occupy a large proportion of the area. Make a deeper copy when making, to reduce the brightness, and approach the standard brightness. Such manuscripts need to be enhanced with the reproduction and redistribution of levels in the light and middle notes. The black and white field calibration density values ​​should be smaller, and the light curves of the middle and middle notes should be slightly deepened to achieve better levels of contrast and visual brightness. The shape of the curve should be distributed according to its level, and its main body should be adjusted to be in bright or medium density range. At the same time, it also needs to be supplemented with deeper and longer blacktone tones.

Source: Graphic Arts

Author: Lin water