Coherent optical analog-to-digital conversion using a single halftone photograph.

Analog-to-digital (A-D) conversion of intensity transmittance (or reflectance) can be accomplished through nonlinear coherent optical image processing. Theoretically speaking, for any given positive integer N, 2(N) discrete levels of transmittance (or reflectance) can be digitized into binary form simultaneously via pure optical means, involving the proper design and fabrication of a specific halftone screen. The digital outputs of the N bit-planes are selected at the Fourier plane by spatial filtering the (2(n) - 1)th diffraction order, where n = 1, 2, ..., and N, respectively. The specific halftone screen must have (2(N) - 1) gray levels, and it should be emphasized that only a single halftone photograph is required for the optical A-D conversion. The general principle of this new method and a preliminary experimental result of an eight-level A-D conversion are described in this paper.