There are two main families of optical correlator, the Matched Filter (MF) and the Joint Transform Correlator (JTC). Both types achieve largely the same results, but process the information in different ways.

The Matched Filter
The Matched Filter is based on the Vanderlugt Filter, invented by A. Vanderlugt in 1964. This is the most commonly used type of correlator, but also the most complex and sensitive to build, owing to its strict alignment criteria and long optical train.

The process involves taking the individual Fourier Transforms of the “Input” and “reference” images, taking their product and then Fourier Transforming the result. Usually, the reference transform is done off-line in electronics (FFT) to provide the “filter”, which is usually displayed on a transmissive Spatial Light Modulator (SLM). Typically, the input transform is produced optically, using an input SLM. The input FT is then optically multiplied with the reference filter and the result optically Fourier transformed to produce the correlation.

The advantages of this method are a high space-bandwidth product and extremely fast process time.