This paper describes a new optical interferometric technique for measuring the gap between a mask and a substrate in proximity printing lithography systems. A uniform-intensity, collimated He–Ne laser beam is focused onto the mask. Light reflected from the mask interferes with the light reflected from the substrate to form a characteristic pattern. To first order, the spatial frequency of this pattern is linearly dependent upon the mask-to-substrate gap. A linear array is used to observe the interference pattern. The resulting signal is processed using correlation techniques to determine the gap. Highly accurate, absolute measurements can be made using this scheme which is quite distinct from the well known interferometric methods which can only measure relative motion. The achievable accuracy and range are functions of illumination, system geometry, signal detection, and processing. An experimental measurement system has demonstrated ±0.25% accuracy over a 25 to 120 μm range.