4.2.2 Projection screening

Projection screen is comprised of the opaque stripes produced by inking the grooves engraved on a glass plate. The screen giving not linear, but dot structure of printed and blank elements was obtained by gluing two such plates so that their strokes passed at an angle of 90° to each other.

The non uniform light distribution behind the screen cell can be explained by diffraction and shading theories. According to the latter, its formation is illustrated in figure 4.3, which schematically presents: the camera diaphragm 1, one of screen cells 2 and film layer 3. Relationships of these elements sizes and distances there between define the result of halftone photoengraving and are used in its settings computation. There are also shown below two exposure distributions 5 and 6 in the latent image for utmost illuminations, i.e. for the lightest and darkest areas of an original.

Figure 4.3. Range of halftone dot size variation depends on the camera diaphragm dimension D

It’s seen in figure 4.3 (a) that the maximum amount of light, which has passed through the diaphragm 1, falls on the central part of cell 2. With the transition to the edge of the cell, the number of light rays reaching the photo layer 3 decreases linearly and in the limit there can be specified the point where only one beam 4 falls. From a comparison of schemes a), b) and c) it also follows that the form of light distribution depends on the diaphragm size D.

For elaboration of the whole range of tone values, for example in the range of 5 to 95%, the film was exposed in two steps. The first "partial" exposure was made with a relatively large "light" diaphragm to obtain the greatest dots for “white” of an original in the shadows of a negative (Figure 4.3, b). The second exposure with a small "shadow" diaphragm gave the smallest dots in the light areas of a negative (blanks for “black” of an original), without significant increasing the size of the large dots provided by the first exposure (Figure 4.3, c). This mode of exposing allowed for providing the entire effective range of tone variation on a transparency and thus the proper contrast of a halftone.

The dot area, as seen in figure 4.3, depends on the sizes of a screen cell d and diaphragm D, as well on the distances: diaphragm - film (R) and screen – film (r). R and d sizes are specified by the screen ruling and image scale. Therefore, only the diameter D and the distance r can serve as variable shooting factors.

From the ratio of these variables should follow:

4.1

In practical calculations of the aperture value and exposure duration, a number of empirical coefficients were also used.

It is appropriate to note here that in addition to adjusting the copy contrast, there were no other options for gradation curve (TRC) form variation within effective range in the projection screening [2.5]. Therefore, the gradation correction was carried out previous to screening by continuous tone photographing of the original with the appropriate selection of the photo layer characteristics, the use of gradation masks, etc.