Surface roughness measurement using an optical system

Abstract

In recent years there have been a number of research studies carried out into optical surface roughness measurement, since the optical method has several advantages over the mechanical measuring system. In the present study, an optical method relying on the reflected beam intensity profile is introduced. An He–Ne laser beam is used to scan the surface and a fibre optic probe is employed to collect the reflected beam from the workpiece surface. The reflected beam intensity distribution is approximated by a Gaussian function. Since the data collection is sampled every 1 ms, a considerable number of profiles corresponding to each point at the workpiece surface are collected. However, for a known surface, there exists a unique average surface roughness value. Therefore, the data collected for each profile should be combined to produce a profile that represents the average surface roughness value for that particular surface. Consequently, the reflected beam intensity profile is processed using two different methods, which are: (i) averaging the data points in the intensity profile before introducing the curve-fitting technique; and (ii) introducing curve fitting to obtain the constants for the Gaussian function for each intensity profile, then averaging the constants over the number of profiles. The surface roughness profiles and average surface roughness value (Ra) is measured initially using a Bendix surface proficoder. The relationship between Ra values and the standard deviation (S.D.) of Gaussian function (SDGF) is developed. The study is extended to include uncertainty analysis and standard estimate of errors in the measurement system. It is found that the first method gives an improved standard estimate of error. Moreover, a linear relationship exists between the Ra values and the SDGF of the reflected beam intensity, the greater are the SDGF values the greater is the surface roughness. However, the measurement is limited to a certain range of Ra values; in this case, the accuracy of the measurement drops considerably as the Ra value increases beyond 1 μm.