Spatial-frequency filter for a kell-type color camera.

There has been interest recently in a color television system using a single camera tube in a modified Kell system in which the color information is modulated to high frequencies by dichroic spatial modulators placed in the image plane. Because the total allowable bandwidth for the signal is restricted, the overlap of the luminance and chrominance bands produces interference in the form of moiré patterns and color misinformation. This problem can be eliminated by including in the camera optics a low-pass filter that rejects the spatial frequencies causing the interference. In this paper, we discuss several types of phase filters and show that the rectangular wave phase grating, with a grating period of about 1 mm, offers the best performance. The optical transfer function and the filter characteristics of this optical element are discussed in terms of the grating parameters, and subjective tests are made on a prototype television system.