GB1459558A - Compound screen for object screening - Google Patents

Abstract

1459558 Half-tone screen XEROX CORP 22 Oct 1974 [23 Oct 1973 (2)] 45582/74 Heading G2M A half-tone screen for use in reflex exposures during electro-photographic processes comprises a clear transparent substrate bearing opaque areas consisting of a repetitive pattern of opaque mixed dots comprising light-absorbing like dots (e.g. black dots) and light-reflecting like dots (e.g. white dots), the average like dot inch frequency being within 50 to 400 dots. The substrate may be a single sheet of transparent material carrying black dots on one side and white dots on the other or may comprise two sheets superimposed one carrying white dots and the other black dots. The black and white dots may each be in rectilinear arrays which may be angled to avoid MoirÚ patterns. In colour processes the dots may be coloured and white instead of black and white. In use, the screen is placed in substantial contact with the original document to be copied and illuminated and the light reflected is passed through a lens system to fall on a photosensitive member, the average like dot inch frequency being such that the lens system employed passes the fundamental spatial frequencies and attenuates the harmonic spatial frequencies.