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 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.
The Joint Transform Correlator was invented by Weaver and Goodman in 1966 and is traditionally the less commonly used of the two designs. The process differs significantly from the Matched Filter process, since no reference filter is required. Instead, the “Input” and “Reference” images are aligned and displayed alongside each other on the SLM. The two images are then Fourier Transformed together. A non-linearity (typically a camera or photodiode array) then captures the intensity distribution of transform, producing the Joint Power Spectrum (JPS). The JPS is then Fourier Transformed itself, producing an output of symmetrically identical pairs of correlation peaks, per match.
The CCL correlator designs are a type of JTC, known as a 1/f Phase-Only Joint Transform Correlator. The 1/f term applies to the fact that a single Optical Fourier Transform stage is used for both stages of the JTC process – i.e. the same optics are used twice to produce the correlation. The Phase-Only term relates to the images being displayed on the SLM in phase (i.e. +1/-1 rather than 1/0 in the case of a binary device) which significantly improves system performance.
The advantages of this method are the simplified optical train (and therefore much cheaper to produce) and no strict alignment criteria.