The NIST detector-based luminous intensity scale

Abstract

The Système International des Unités (SI) base unit for photometry, the candela, has been realized by using absolute detectors rather than absolute sources. This change in method permits luminous intensity calibrations of standard lamps to be carried out with a relative expanded uncertainty (coverage factor k = 2, and thus a 2 standard deviation estimate) of 0.46 %, almost a factor-of-two improvement. A group of eight reference photometers has been constructed with silicon photodiodes, matched with filters to mimic the spectral luminous efficiency function for photopic vision. The wide dynamic range of the photometers aid in their calibration. The components of the photometers were carefully measured and selected to reduce the sources of error and to provide baseline data for aging studies. Periodic remeasurement of the photometers indicate that a yearly recalibration is required. The design, characterization, calibration, evaluation, and application of the photometers are discussed.

NIST reference spectral responsivity scales for improving the SI candela and kelvin

“…The present NIST illuminance unit, maintained on a group of eight photometers, has a relative expanded uncertainty of 0.39 % (k=2) [1]. The dominant uncertainties (originating from the converging beam geometry, the small spot size on the photometers, wavelength errors, aperture area measurements error, and the spatial non-uniformity of detector and filter combination) are related to the traditional lamp/monochromator spectral comparator facility (SCF) used to calibrate the eight photometers [1]. Calibrating these photometers with the tunable laser sources of the SIRCUS would increase the calibration uncertainty because of the interference fringes in their output signals.…”

The candela and photometric and radiometric measurements

“…At NIST, the calibration of s v,f (or s v,i ) is traced to the HACR. The current realization of the NIST candela has a relative expanded uncertainty of 0.41 % (coverage factor k = 2) with the major component of the uncertainty due to the uncertainty in the illuminance responsivity [ 38 , 39 ]. This is roughly a factor-of-ten increase in uncertainty over the uncertainty of a HACR measurement at the beginning of the measurement chain.…”

Responsivity calibration methods for 365-nm irradiance meters

“…This method has been used by NIST since 1991 [5]. It has been shown [6] that the effective aperture area is the ratio of the total signal summed over the scanned area (total irradiance responsivity) to the product of the total beam power and the average spectral power responsivity , within the active area of the aperture (2) Fig. 4.…”