High Accuracy Form Measurement Of Specularly Reflecting Surfaces By Laser Autocollimation

A non-contacting method is being employed to measure the form of optical surfaces to nanometre accuracy. Slope values obtained by laser autocollimation are integrated with respect to distance to obtain profile height. A single-axis laser profilometer based on this principle was initially developed to monitor profiles along axial generators of conicoid mirrors for x-ray microscopes. The instrument was further modified to enhance the spatial resolution in order to detect surface waviness caused by imperfections in the lapping processes and in the slideways of diamond turning machines. This paper also includes measurement results obtained from a diamond machined optical flat and from silica flats produced by teflon and pitch polishing methods. The instrument has been used further to measure large radii of curvature of spherical mirrors and to investigate the damage caused in a ceramic material when bombarded with a high energy ion beam. A second profilometer is being developed to measure optical flatness over an extended area in two dimensions. The profiles obtained using both instruments along a 50 mm diameter of an optical flat are compared.