RANDOMISED MASK FOR A DIFFUSING SCREEN

摘要

A mask through which a sheet or layer of a photopolymerisable monomer can be exposed to polymerising light is manufactured by placing a fibre optic plate (14), or a ground glass plate or the equivalent over the emulsion layer (12) of a photographic plate (10) and exposing the photographic emulsion, through the fibre optic plate or ground glass plate, by a laser beam (22) scanned across the plate in a raster. By this technique, there is produced, over the photographic plate, a random speckle pattern of substantially the same density and intensity over the whole plate. The photographic plate is then developed and used as a master to produce "daughter" masks from glass plates having an opaque thin layer of metallic chromium on one side, by coating such thin layer with photoresist, using the master to record, by direct contact printing, the speckle pattern on the photoresist, treating the photoresist to remove it from the regions corresponding to the speckles and etching the plate bearing the chromium layer and resist coating to remove the chromium from the regions from which the resist has been removed to form light transmitting apertures corresponding to these regions. Diffusion screens produced using a mask formed in this way incorporate sufficient randomness in the distribution of the light-modifying features thereon to avoid diffraction effects and to avoid Moiré effects when used in conjunction with video displays which comprise regular arrays of phosphor dots or of pixels.