Abstract: An apparatus and method for detecting deviations from a normal ocean wave pattern which may be caused by interference with artificially created wave patterns, such as from an underwater moving vehicle. A camera mounted in an aircraft has its optic axis vertically disposed and is provided in its image plane with an opaque plate having an arcuate aperture slit concentric with the vertical optic axis and angularly displaced an amount where the ratio of change in reflectance to the change of incidence angle for a light reflected from a smooth surface is a maximum.