Non-contact aperture area measurement by occlusion of a laser beam

NMI has established a non-contact aperture area measurement facility based on occlusion of a focused laser beam transmitted through the aperture being measured. The method is fast, accurate and insensitive to edge-defect-scatter. The aperture edge position is determined by local radial scans over 30 mu m at 256 points around the circumference, a sufficient number of positions to make aliasing errors negligible. An uncertainty analysis identifies 14 possible sources of uncertainty in the measured area including aliasing (circumference under-sampling), laser beam diffraction, edge-signal fitting, edge-scattered light and area integration choice. Excluding translation stage uncertainty, for an aperture radius of R = 3 mm the standard area uncertainty is 0.009%. When translation stage error is corrected using a calibrated line grid with standard uncertainty +/- 0.5 mu m, and the uncertainty included, this yields a total standard uncertainty of 0.034% for an aperture with R = 3 mm.