Absolute measurement of surface roughness

Jn an interferometer which uses a reference surface, the measured surface heights correspond to the difference between the test and reference surfaces. To determine the rms roughness of supersmooth surfaces accurately, the effects of the reference surface roughness need to be removed. One technique for doing this involves averaging a number of uncorrelated measurements of a mirror in order to generate a reference surface profile which can then be subtracted from subsequent measurements so that they do not contain errors due to the reference surface. Th & other technique provides an accurate rms roughness of the surface by taking two uncorrelated measurements of the surface. These two techniques for measurement of supersmooth surfaces are described, and results of the measurement of a 0.7 A rms roughness mirror are presented. The expected error in the rms roughness measurement of a supersmooth mirror due to instrument noise is 0.02 A. Subtraction of a Generated Reference Surface A profile of the reference surface can be generated by averaging a number of measurements N of a smooth mirror. The mirror surface used to do the averaging does not need to be supersmooth, but the smoother it is, the fewer measurements will need to be averaged. Between measurements, the mirror is moved by a distance greater than the correlation length of the surface.1 Once the reference surface profile is generated, it ca