A typical problem of signal detection is the search for key words. This can be done automatically by means of optical matched filtering. Here we describe an alternative method in which the key word is used directly, not in the form of a matched filter. We compute optically the correlation integral of the intensity distributions of the Fresnel diffraction patterns from the input (printed page) and from the reference signal (key word). In the output plane a detection peak (light point) indicates the position of the key word on the input page. © 1967 Optical Society of America.