Radiometric and photometric measurements require defining apertures. The accuracy to which these measurements can be accomplished requires minimization of the uncertainty in the aperture area.
The NIST aperture measurement facility is a machine vision-based metrological system. It consists of a high accuracy XY stage which carries the sample aperture and has an interferometer for positioning feedback, and a long working distance microscope mounted on a high resolution vertical stage for edge detection. An automated measurement algorithm finds the XY coordinates of the edges circumscribing the aperture. The edge points are then used in a circle and ellipse fitting routine to determine the area. Details about the design of the instrument can be found at Aperture area facility.
This is a non-contact edge detection method which is preferred, especially for knife-edged defining apertures used in radiometry. This dimensional measurement is SI-traceable to the meter, and therefore has a separate traceability route from the rest of radiometric measurements that involve areal dimensions such as radiance [W m-2 sr-1] and irradiance [W m-2]. While this measurement is used primarily to support internal and external radiometry efforts, this can be used to measure object dimensions requiring no contact provided the sample can be physically accommodated.
NIST participated in an international intercomparison sponsored by the Consultative Committee on Photometry and Radiometry (CCPR) of the methods by which different measurement laboratories measure the areas of apertures. Each of the nine participating laboratories measure eight different apertures varying in diameter size, fabrication method, material, and edge type. For more information about this intercomparison, see Final report on the CCPR-S2 supplementary comparison of area measurements of apertures for radiometry.
We measure circular apertures routinely in the diameter range from 0.2 mm to larger sizes that can be accommodated by our stage, which can accommodate up to 95 mm artifact outer diameter. Other shapes such as slits may also be measured but may require some additional effort and cost. This is a calibration service Special Test 39200S.