Abstract: An electro-optical sensor for measuring distance to a test surface, such as a road surface beneath a vehicle frame on which the sensor is mounted. The sensor includes a modulated light source for directing a light beam onto the test surface, and a photosensitive detector for receiving light diffusely deflected from the test surface and providing a pair of output signals indicative of orthogonal position coordinates of reflected light incident on the detector. The detector outputs are fed to amplifiers for obtaining sum and difference signals which are demodulated synchronously with energization frequency of the light source. Distance to the test surface is determined as a function of a ratio of the detector output differential to the detector output sum. In the preferred application of the invention for road profile measurement, a signal indicative of road profile is stored at increments of vehicle motion along the road surface.