amberlith area appears black to the camera, and prints clear on the resulting film, red-orange acetate is used for masking mechanicals to be photographed for plates.
An orange plastic sheet, placed over a pasted-up page, that contains shapes that the printer needs to screen, overprint or print in another color.
(Brand Name) - A red or orange material for preparing art and dummies. Transparent to the eye but appears opaque to light sensitive materials.