I. Overview

The feature of continuation of manuscripts that differ from line manuscripts is that the shading change of the image of successive manuscripts is continuous, gapless, and gradeless. When printing, to faithfully reproduce the shades of an image on a continuous tone, there are two methods. One is to use a dot to obtain a halftone image to represent the image density of the continuous tone, such as letterpress, lithography, and stencil. Printing and other; the other is the use of different depth of the cavern on the plate cylinder, the ink is transferred to the substrate, the depth of the density of the ink layer formed by the ink layer is different, such as gravure printing. Of course, sometimes gravure printing still uses dots. The purpose of the gravure printing screen is to make the printing plate cylinder store the necessary ink in the printing, and the produced network cable plays the role of supporting the squeegee in printing, as shown in FIG. 1 .

Figure 1

Print duplication of continuous tune originals includes color reproduction, gradation reproduction, and sharpness reproduction. All three are achieved through the transfer of outlets.

Fig. 2a is a monochromatic continual tune original. Without the screening technique, there are no continuous dots in the negative dot pattern. When the printing plate is printed, the middle density part also has light through the film and reaches the printing plate. The photosensitive film on the see see photolysis to form a hydrophilic blank area, the resulting plate is only black and white, and no intermediate tone level, after printing can only be obtained as Figure 2b line manuscript, also known as block surface Image originals that have lost the shade of the original. Only when the manuscript is screened, the continuous manuscript will be turned into a halftone film. When the plate is printed, the negative part of the film will be opaque, and the transparent part will be completely transparent, so that the photosensitive layer on the plate will be fully decomposed. On the printing plate, a blank area of ​​hydrophilic ink repellent and an ink-and-water repellent image area are formed. Depending on the size of the dot area, the gradation change on the original is reproduced, as shown in Figs. 2c and d. For the color copy of the original copy of the original copy, but also through the screening process so that each color version has become a screen printing version, get a halftone print.

Figure 2 (a-d)


Therefore, dots are tools for copying continuous images; dots are the carriers that make up changes in shades of printed matter; dots are used to dilute ink thinners in printing, as if using oil to match colors.

Second, the formation mechanism of outlets

In 1852 the British physicist W. H. Fox. TalBot uses a gauze-like object as a screen to decompose the continuous tone image into dots with different sizes and the same density at each dot. These different sizes of dots express different levels of shades, thus creating a single-color printing using dot duplication or printing. Color continuous tone image is the first of its kind.

Since then, the screening technology has gone through three stages of glass screen screening, contact screen screening and electronic laser screening.

1. Glass screen screening:

The glass screen (Fig. 3) is marked with opaque vertical crosshairs, each of which has a clear edge. The screen is placed on the screen shelf of the camera black box, parallel to the wet plate coated with silver iodide but at a distance. When the screen is added, light is projected onto the photosensitive wet plate through the screen mesh screen. Due to the limitation of the sensitivity performance of the photosensitive wet plate, light is sensitized to form different sizes of dots on the wet plate so as to continuously change the brightness of the image. Converted into light dots of different sizes and equal spacing on the photosensitive wet plate. After printing, these dots showed the light and dark levels of the images on the prints.



Figure 3

Due to glass screen screening, there are many deficiencies, such as: high cost of glass screens, fragility, poor light transmission, poor photosensitivity of wetted photosensitive screens, etc., which were later replaced by contact screens made of photosensitive films, as shown in the figure. 4 shows.

Figure 4
(to be continued)