Screen processes are basic to our perception of the world around us in a variety of communications media, from the dots on your monitor to the pictures you see in magazines. Screens are basically just dots arranged either in a pattern or stochastically (random). A long time ago, photography too, used a variety of screen processes from colored glass beads melted into a glass base to dyed potato starch, as in the autochrome process. All these processes work because the human eye at a certain distance doesn't see the individual dots but rather perceives their cumulative effect.

The most common of the color screen processes is that used for photomechanical reproduction by offset and web presses. Color separation negatives form a rosette pattern, using subtractive color, in addition to black, in a repetitive order to synthesize reality. As you can see from the illustration, the color halftone screens are rotated to form the pattern. These negatives are used to form a image that is "burned" onto printing plates to carry the ink on the press.

By varying the density of the dots color the cumulative effect of say lack of cyan density, gives the appearance of red, by you visually assimilating the combination of the yellow and magenta inks, which in conjunction with the dots small size makes you perceive a continuous tone image with a red color. Modern color photographic film works by using dyes in the form of clouds (grain) as a stochastic dot reproduction of exposed reality. Old photographic film methods used a variety of screens. One of the earliest was that created by Louis Dufay in France 1910 Called the Dioptichrome plate (aka Dufaycolor,later) which consisted of a mosaic of alternating green and blue dye squares crossed at right angles by a pattern of parallel red dye lines, each element measuring only 0.0002 in (0.05mm) in width. This screen was coated with a panchromatic emulsion, the material was exposed through the base, the screen, and exposing the emulsion from the back. Processed to reversal, the result was a positive transparency. (1) Other methods involved the use of woven fabrics, ruled lines and resists, among others. I'm always interested in discussion, scientific or otherwise. You can Email me at jscruggs@bway.net ( 1 ) ICP Encyclopedia of Photography. ( additive )( autochrome )( color )( film )( grainmaker specifics )( subtractive )