Research on the interference fringe contrast in ground glass shape measurement

To measure the ground glass surface shape, the oblique incidence method applied using a Fizeau interferometer. Interferograms with sufficient contrast values are needed during the measurement. The formula to calculate the fringe contrast in ground glass shape measurement is derived to give an optimum incident angle for the ground glass with different surface roughness. Four specimens grinded by abrasives numbered #400, #700, #1500 and #2000 are measured, using aluminum coated mirror as the reflective flat in the experiment. The minimum incident angles of the four specimens are 83°, 82°, 76° and 75° respectively. The contrast values of the interferograms with the specimens at different incident angles are measured using phase shifting interferometry and the results match the theoretical ones. The oblique incidence method can be used to measure ground glasses grinded by #400 abrasive and finer ones. A sufficient contrast can be achieved with the incident angle slightly greater than the minimum angle.