Optical Correlators compare two-dimensional image data at very high speeds. They were invented in the mid-1960s and have traditionally been used in high cost military applications, such as satellite photograph analysis.
With recent advances in liquid crystal technology, optical correlators have become more commercially viable. This, coupled with the revolutionary CCL patented designs, allow optical correlators for the first time to realise their potential and enter the marketplace – at a fraction of the high costs previously associated with such high performance systems.
Image data that is entered into the optical system is compared during the correlation process in terms of two criteria, similarity and relative position. Typically, the comparison is done between a reference image (eg from a database) and an input image (eg from an external camera/sensor), though multiple images may be compared during the same process at no extra cost, limited only by the input resolution of the system.
The optical output consists of highly localised intensities, known as correlation spots or “peaks”, relating to wherever a strong match has been found between the images. The intensity, I, of the spots provides a measure of the similarity of the images being compared, whilst the position, (x,y), of the spots in the output denotes how the images are relatively aligned in the input scene.
If two identical images are compared, the output contains a high intensity peak in the output plane of the optical system. This is known as an autocorrelation.
Entering two dissimilar images results in an output containing no peaks, signalling no match has been found.
If one image is partially obscured or not an exact match, then the intensity of the resulting peak will drop proportionally. This is known as a cross-correlation.
If one image moves relative to the other, the resulting correlation spot will have shifted accordingly, but its intensity will have remained the same.
Entering multiple images results in multiple peaks, each denoting where a match has been found and how strong the match is.