Basic Signal Parameters
The roots of optical signal processing date back to the work of Fresnel and Fraunhofer nearly 200 years ago. But the connection between optics and information theory did not take shape until the 1950’s. In 1953, Norbert Wiener published a paper in the Journal of the Optical Society of America entitled “Optics and the Theory of Stochastic Processes” (1). That same issue contained articles by Elias on “Optics and Communication Theory” (2), and by Fellgett on “Concerning Photographic Grain, Signal-to-Noise Ratio, and Information” (3). Other interesting papers of that decade include those written by Linfoot on “Information Theory and Optical Imagery” (4), by Toraldo on “The Capacity of Optical Channels in the Presence of Noise” (5), and the seminal paper by O’Neill on “Spatial Filtering in Optics” (6). These papers represent the early infusion of information theory into classical optics.
A powerful feature of a coherently illuminated optical system is that the Fourier transform of a signal exists in space. As a result, we can implement filtering operations directly in the Fourier domain. This feature was anticipated in papers by Fresnel, Fraunhofer, and Kirchhoff, and had been demonstrated, before the turn of this century, by Abbe in connection with his work on images produced by microscopes.
It is one thing, of course, to recognize that images can be changed by modifying their spectral content; it is another matter to implement the change. ...