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. This is particularly critical, for example, in climate and weather applications on which measurements of the total and spectral irradiance of the Sun are dependent.
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.
This is a non-contact edge detection method which is preferred, especially for knife-edged apertures. The instrument can also be fitted with a touch-probe stylus to detect the edges of an aperture with a land.
Details about the design of the instrument can be found at Aperture measurement facility.
NIST recently 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 has a 100 mm travel in the X and Y. Other shapes may also be measured, but may require some additional effort and cost. Contact Maritoni Litorja for more information.
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