An optical technique of pseudo colouring a black and white image

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Abstract
Describes an optical analogue method for transforming density variations of a black and white image into colour variations. No photographic process is involved in the technique which consists in modulating the input transparency with a high frequency fringe system and forming its image in the volume of a photo-refractive crystal. A white light source of limited dimensions illuminates the volume grating so recorded and interference is observed between a diffracted and a directly transmitted beam. The interferogram is a colour image of the input transparency. The colour pattern can be continuously varied by simply varying the direction of polarisation of the linearly polarised reading beam.

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