An incident electromagnetic wave-field, preferably in the ultraviolet, visible or infrared spectral range, that is spatially varying in one dimension can be correlated with a fixed, one-dimensional test function, by means of a photo detector consisting of a multitude of rectangular photosensitive elements whose relative lateral widths vary in proportion to the local amplitude of the test function. Several such spatial correlation tracks for different test functions can be arranged in the direction perpendicular to the width modulation direction. Each correlation track provides the degree of correlation of the incident wave-field with a particular test function, by determining the correlation of these two signals in the optical domain, without the need for an additional analog or digital processor. This invention is particularly advantageous for application in optical encoders for the determination of lateral or rotational spatial displacement.