Measurement of aspects of surface form using an optical differential height measurement technique

We describe a fibre interferometer for making non-contact differential height measurements of machined metal surfaces without removal from the machine tool. The system comprises a probe which produces two small laser spots (similar to 7 mu m diameter, 3 mm apart) on the surface, an optical fibre addressing lead and signal processing unit incorporating an optical source and detector. The probe is an extrinsic fibre optic Michelson interferometer whose arms terminate in the laser spots on the test surface, and the signal processing unit calculates the difference in local surface height between the two spots. Differential height profiles of machined aluminium surfaces and optical mirror surfaces are presented.