The use of light scattering or diffraction for the determination of the surface roughness of machined metal components has been investigated by many researchers. As a result, a number of theoretical formulations relating the r.m.s. height Rq to the scattered light intensity distribution have been derived for periodic and random surfaces. Experimental results for rougher surfaces have been reported and from them the individual researchers have derived empirical relationships between the standard deviation of the angularly scattered light distribution and the r.m.s. surface slopes. Each of these relationships are claimed to be valid for a certain class of surfaces. As a result of this, commercial optical surface finish sensors, based upon light scattering have been produced. The manufacturers of these instruments have defined some new surface finish parameters like Sn and Rop, thereby contributing to the so-called "parameter rash". Based upon scalar diffraction theory, formal relationships are derived, between a The specular reflectance and the r.m.s. height parameter Rq, and b The standard deviation of the scattered light distribution andthe r.m.s. slope parameter Δq.
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