Abstract
The determination of the geometrical and effective area for optical-quality apertures is one of the fundamental sources of uncertainty in many radiometric and photometric measurements. The National Institute of Standards and Technology (NIST) has developed two non-contact instruments for measuring the area of these optical apertures. Some details of the instruments and their capabilities are presented. Both instruments can be used to measure the area of apertures with diameters ranging from 3.5 mm to 25 mm. The measurements using the absolute instrument result in k = 2 relative uncertainties ranging from 3 × 10−5 for 25 mm diameter apertures 5 × 10−5 for 3.5 mm diameter apertures. The uncertainty depends to some extent on the quality of the edge and the diameter of the aperture. The measurements using the relative instrument result in k = 2 relative uncertainties ranging from 2 × 10−4 for 25 mm diameter apertures to 3 × 10−4 for 3.5 mm diameter apertures, assuming adequate optical-edge quality. For apertures with an edge of very poor quality, the difference between geometrical and effective area can be as high as 0.3 %. The increased uncertainty largely arises from scattering from the edge. Both instruments play a major role in the Consultative Committee for Photometry and Radiometry (CCPR) key comparison on aperture area, which began in 1999.