HALFTONE SCREEN — KAINEN 423 



continuous tone of ink, running from transparency to opacity de- 

 pending upon the amount of ink in the wells, except for the fine lines 

 of the screen, which remain without ink, having provided a surface 

 for the "doctor" blade to wipe. 



During the first quarter of the twentieth century numerous screens 

 were suggested and developed, and many were designed to produce 

 unusual line patterns and textural effects, but the overwhelming ma- 

 jority were the cross-lined halftone and rotogravure screens. Another 

 type of screen that was popular for some years after the turn of the 

 century was the mezzograph screen, patented by James "^Vlieeler in 

 England in 1897. This screen contained no pigment and resembled 

 a sheet of frosted glass. It was made by coating a glass plate with 

 minute drops of liquid resin, which acted as an acid resist, and etch- 

 ing with hydrofluoric acid. When the resin was removed the glass 

 was uniformly pitted and composed of levels of transparency. Used 

 as a screen in j^hotoengraving, this glass produced a granular struc- 

 ture in the final printing, which, while sometimes too evident, was 

 well suited to certain types of subjects. 



In 1928 A. Ronald Trist, of England, inventor of the bimetallic 

 Pantone process of lithographic printing, patented a screen which in 

 principle anticipated the present variable-opacity or contact screen 

 developed by Kodak. Trist used an electrically rotated disk in con- 

 junction with standard screens. By this means he could produce 

 vignetted densities in transparent celluloid-process films. The re- 

 sulting screens could then be used for contact printing, being particu- 

 larly serviceable in his lithographic process. 



The variable-opacity or contact screen was developed to correct 

 some of the shortcomings of the conventional cross-line screen. Con- 

 trast is often difficult to control, and tone reproduction, consequently, 

 is often uncertain. Variable-opacity screens are intended to correct 

 this uncertainty and in addition to take less camera time. Problems 

 of focal length are overcome since the screens are placed in contact 

 with the sensitized film. Gradations of light are managed by vari- 

 ations in dot intensity, the most opaque points being the centers of the 

 dots, which fade out gradually to translucency at the peripheries. 



Several proposed variable-opacity screens led up to the Kodagraph 

 screens, the first of which was patented in 194'2 and 1943. This earlier 

 Kodagraph screen, orange in color, was used with a continuous-tone 

 magenta negative placed in contact with Kodalith film. The positives 

 obtained were composed of dots — like those produced by the Levy 

 screen — and contrast control was secured by variation in relative 

 amounts of orange and red light during exposure. The later improved 

 screen, magenta in color, produced negatives rather than positives. 

 The screen was used primarily in lithography, but screens for photo- 

 engraving are now used commercially. 



