Optical pattern recognition is an activity readily describable in information or entropy terms. In that respect, for example, target discrimination can be done with rather low energy if we had the wisdom to design precisely the proper measurements. We show that the traditional Fourier optical pattern recognition apparatus is capable of achieving such low energy operation automatically. Viewed in another way, we can say that the energy per calculation in this (and all other coherent optical processors) can be significantly less than the "thermal limit" of kT. While digital computer theorists have shown that they can also beat the kT limit, digital computers now in use operate at about 108 kT. A comparison of coherent optical versus digital low energy schemes will show why coherent optics works so well.
H. John Caulfield,
"Thermodynamics Of Optical Pattern Recognition", Proc. SPIE 1134, Optical Pattern Recognition II, (25 October 1989); doi: 10.1117/12.961622; https://doi.org/10.1117/12.961622