There is an increased demand for high quality optical components with minimal surface roughness for precision optical systems. As a result, it has become necessary to determine the surface quality using an accurate, quantitative and non-destructive measurement. This has led to considerable efforts to replace stylus methods with non-contact optical methods for surface texture measurements. Several non-contact optical methods have been tried and some of these have been implemented in commercial instruments. One of these methods utilizes the principle of differential interference con-trast. The system theory and implementation are described; results and calculations are presented for polished surfaces.