scene. Depth information is lost and objects frequently overlap.

Vision requires a large amount of memory and many computations. For

an image of 1000 X 1000 picture elements, even the simplest operation

may require 108 operations. The human retina, with 108 cells

operating at roughly 100 hertz, performs at least 10 billion

operations a second. Thus, to recognize objects at a rate even

closely resembling human vision, very special processor technologies

must be considered. One promising technology has emerged in the form

of optical computing.

 Optical Computers

 Optical computers permit the manipulation of every element of an

image at the same time. This parallel processing technique involves

many additions and multiplications occurring simultaneously. Most

digital processors must perform one operation at a time. Even though

the digital processors are very fast, the number of total operations

required to recognize patterns in an image is very large. Using

optical Fourier transformers, an optical processor can operate on the

image and its Fourier transform simultaneously. This permits many

standard image processing techniques, such as spatial filtering and

correlation, to be performed at tremendous rates.

 The Fourier transform is formed optically by use of a lens. The

usual case that is considered in optical computing is when the

illuminating source is located at infinity (by use of an auxiliary

collimating lens) and the image transparency is located at a distance

equal to focal length from the transforming lens. The distribution in

the output plane located a focal length behind the transforming lens

is the exact Fourier transform of the input distribution. The Fourier