The Detector-Based Candela Scale and Related Photometric Calibration Procedures at NIST

The candela, one of the SI base units, has been realized by using absolutely calibrated detectors rather than sources. A group of eight photometers was constructed using silicon photodiodes, precision apertures, and glass filters for V (λ) match. Their absolute spectral responsivities were calibrated against the NIST absolute spectral responsivity scale. The measurement chain has been significantly shortened compared with the old scale based on a blackbody. This resulted in improving the calibration uncertainty to 0.46% (2σ), a factor-of-2 improvement. This revision has made various photometric calibrations at NIST more versatile and flexible. Luminous intensities of light sources ranging from 10 -3 to 10 4 candelas are directly calibrated with the standard photometers, which have a linear response over that range. Illuminance meters are calibrated directly against the standard photometers. A luminance scale has also been realized on the detector base using an integrating sphere source. Total flux ranging from 10 -2 to 10 5 lumens can be measured in a 2 m integrating sphere using a photometer with a wide dynamic range. The revisions of the calibration procedures significantly improved the calibration uncertainty.