Abstract: A technique for identifying and measuring a first surface area and apparatus for aiding such identification are disclosed. Each incremental portion of said first surface area and at least adjacent portions of surrounding surface areas are illuminated with diffuse light incident through a predetermined sectorial angle related to an anticipated waviness of the surface areas. The first surface area so illuminated is then measured by opto-electronic measuring techniques whereby measuring errors caused by shadow effects due to waviness in the surfaces are minimized by substantially uniform light dispersion within the sectorial angle.