Distance measurements by differential confocal optical ranging.

A new technique is described for measuring the distance between a lens and reflecting surface extremely accurately. It is based on the sharply peaked depth response of type II confocal systems. By dithering either the sample or the optical system, a differential measurement is generated, placing a zero-crossing at the peak of the depth response and improving the ranging accuracy. The technique is independent of surface roughness or tilt and hence is useful for robotics or machining applications. Sensitivities to surface vibrations of 0.01 nm and thin film measurements to 0.04 microm demonstrated. Signal-to-noise calculations are presented, and the procedure for measuring the thickness of transparent films is outlined.