A non-contact technique providing improved accuracy in area measurements of radiometric apertures

Beam-limiting apertures with accurately known areas are required when measuring the radiance and irradiance of sources. We have already described a non-contact method for measuring aperture areas using a laser beam focused on the aperture surface. By shifting the aperture relative to the laser beam, and collecting and monitoring the reflected light, the onset of reflection was used to indicate the position of the aperture edges. We have now improved this approach by evaluating the position-dependence of the reflection signal recorded across the aperture edge. Whereas the former approach required a determination of the laser focus diameter to correct the measured aperture diameter, the quantity that makes the main contribution to the overall uncertainty of the aperture area, the new approach implicitly determines the laser focus diameter, so reducing the corresponding uncertainty and making it possible to determine aperture diameters with standard uncertainties of less than 0.2 mum. Measurements made in 1998 for aluminium apertures are compared with results obtained in 1995 using the same apertures.