INFORMATION PROCESSING WITH COHERENT LIGHT

In any plane of an optical tract coherent light is fully described by the complex amplitude, that is to say by a two-valued function. Nevertheless, it can be also described by a one-valued or real function, such as photographic density plus a known, coherent reference wave. This is the basis of wavefront reconstruction or holography. The history of holography is presented, and some of its applications, such as holographic interferometry, contour mapping and three-dimensional pictures in monochrome or in natural colours are discussed. Some prospective applications are holographic panoramas and three-dimensional projection of moving pictures. Information coding and storage are very promising applications, but held up by the phenomenon of ‘laser speckle’. Finally it is pointed out that holographic coding, which is a very complicated and multi-dimensional type of coding, has greater potentiality than expected on the basis of Shannon’s communication theory. With a code of blocks consisting of N data only N data can be transmitted exactly, but a much greater number can be transmitted with a tolerable error. J. P. Wild’s Culgoora radioheliograph, which maps 3000 points with only 96 aerials by a method which could be (though for economy reasons is not) simultaneous, can be considered as the first practical application of this principle. © 1969 Taylor & Francis Group, LLC.