Rubylith

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A two-layer acetate film comprising a transparent base and a red or amber emulsion. Although it has many uses in graphic arts, it was most often attached to paste-up boards in place of halftones which, when the mechanical was photographed as a negative, would leave a blank space into which the halftone negative could be stripped. (It is thus called a red patch or a black patch). It is also used in the making of color separations. See also Amberlith.