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Printing from Screen Plates
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to 800 black grains. A comparatively simple means of avoiding this trouble seems to be in sacrificing some sharpness of the second image, and by suitable choice of the unsharpness, the colors of the adjacent elements run into one another.
Let R, Fig. 138, be a transparent plate containing black areas s, which are projected by a small source of light on to a surface F. The elements in this case will be reproduced quite sharp, and so will the transparent spaces e. If, however, the source of light is not a point, but an extended surface, such as L, in Fig. 139, the transparent elements are enlarged to e', and these are not of uniform illumination, the latter falling off towards the edges of each unit, as indicated by the shaded areas in Fig. 140. This will be readily understood as the center of c' receives the light from the whole of the light source, whilst towards the edges the illumination gradually falls off. The transparent element c is thus reproduced as an image
e'y which is larger and of vignetted outlines, its size depending on the size of the light-source, other things being equal. In other words, by suitably choosing the dimensions of the light-source, a color-screen picture may be reproduced with the introduction of unsharpness and enlargement of its color elements, and the disturbing want of homogeneity in the colors thus avoided. In Fig. 141 R shows a portion of a screen-plate, in which the blue and green elements are covered by silver and the red grains free, and the plate represents a red. If this portion of the picture is projected on to the surface F, under the conditions shown in the diagram, the red elements are enlarged three times to r7, r9 \ so that instead of appearing sharply defined against a black ground they are so enlarged that they join up. As a result of uneven illumination the whole surface is not even now of perfectly homogeneous character. In order to obtain this, the lightsource must be larger still, and by so choosing it that the degree of en