High-accuracy aperture-area measurement facilities at the National Institute of Standards and Technology

The uncertainty in the measurement of aperture area can limit high-accuracy radiometric and photometric measurements. Relative total uncertainties in some measurements have now been determined at or below the 0.1% level, making substantially smaller aperture-area measurement uncertainties necessary. The National Institute of Standards and Technology (NIST) has recently implemented an absolute aperture-area measurement facility and a relative aperture-area measurement facility; the two facilities together are designed to determine aperture areas with low uncertainty. The absolute instrument measures the aperture-area using optical edge detection, along with high-precision positioning of the optical edge relative to the sensor, resulting in an expected relative combined standard uncertainty of less than 10(-4). The relative instrument uses optical flux transfer to compare aperture areas and also has an expected relative combined standard uncertainty of less than 10(-4). The absolute instrument will be used to measure the area of standard apertures for use with the relative instrument.