Printing image information processing - plate photographic process (2)

Third, monochrome continuous tone image manuscript photography process

　 The process of photographing monochromatic continuous-tone image manuscripts is basically the same as that for lined manuscripts, except that during shooting, the screen is placed in front of the photosensitive plate, as shown in Figure 3-20. Beams of different brightness reflected or transmitted from the document, after passing through the lens, meet the screen before reaching the photosensitive sheet. Since the mesh has the effect of redistributing the light flux, the beam passing through the mesh is divided into light corresponding to the document. The small part of the pass forms a dot on the photosensitive sheet. The bright part of the original forms a large dot on the negative film negative. The dark part of the original forms a small dot on the negative.

Figure 3-20 Screen capture

Monochrome continuous tone originals, generally using photographic film, using contact screen, select the angle of 45 ° screen shot, the process flow is:

Loading documents → loading photosensitive film, screens → screening light → exposure → developing → fixing → washing

During exposure, main exposure and auxiliary exposure are performed according to the characteristics of the original. Auxiliary exposure includes flash exposure and highlight exposure.

In the main exposure, the original is photographed through a screen, and the basic situation of hierarchical reproduction is determined by the main exposure. Flash exposure, the light emitted by the light source, passes through the screen without going through the original document. The purpose is to make the dark portion of the original fill in small dots on the negative film and extend the tone of the middle tone to dark tone. The highlight exposure is performed after the completion of the main exposure and the flash exposure, and the screen is removed during the exposure, so that the level of the highlight portion is opened and the contrast of the highlight portion is improved.

Negative photographs of negatives added to the film shall meet the following quality requirements: the layout shall be clear and not yellow, no dirty spots, no scratches on the membranous film, and the mesh shall be clean and smooth, and the density shall conform to the printing requirements.

Negatives plus negative film, according to the needs of the plate can be remake or copy into Yang Tu plus negative film. Negative or positive negatives with halftone dots are referred to as "screened" negatives when compared to continuous originals or negatives. The screened web can also be filmed with a wet sheet or a gelatin sheet, but due to the complexity of the process, the quality is not easy to control, and it is only applied under special circumstances.

In the process of screening, there are terms to express the reproduction of the tone, such as: deep, flat, light, bright and so on.

Depth: refers to the image area is too large.
Flat: The area of ​​the dot of the bright-toned part of the image is too large, and the area of ​​the dark-tone part of the dot is too small and the level is not obvious.
Light: The image area is too small.
崭: It means that the dot area of ​​the bright part of the image is too small, the dark part of the dot area is too large, and the contrast between light and shade is too large.

Fourth, the photographic process of color image manuscripts

Color image manuscripts can be photographed using the indirect color separation screening method and the direct color separation screening method to obtain color-graded negative or positive negatives.

(I) Indirect color separation screening method

The indirect color screening screening process separates color separation and screening. Therefore, it is also called “two-step process method. The process is shown in Figure 3-21.

Figure 3-21 Indirect color separation screening process

Indirect separation and screening process, although there are many opportunities for manual correction, but the operation is complicated, the consumption of photosensitive materials is more, the production efficiency is low, and gradually eliminated.

(B) Direct color screening method

The direct color separation and screening process is to install the color filter and the contact screen at the same time on the plate making camera. The order grid on the manuscript is directly recorded on the color separation plate, so that the two processes of color separation and screening are completed at one time. Do "hanging." The process is shown in Figure 3-22.

Figure 3-22 Direct Color Separation Screening Process

With the direct color separation and screening process, masks need to be produced to compress the contrast of originals, and to correct chromatic aberrations. It is also necessary to use a strong light source, a touch screen with the correct level, and a full-color film with high sensibility. At present, in China, it is mainly used for color screening of Chinese paintings and manuscripts similar to Chinese painting styles.