The procedure and the results of realizing a low-level photometric scale down to 1 × 10−6 lx with standardized detectors are described. The detectors are two photomultipliers (one of them operated in the photon counting mode) and two operational amplifier/photodiode combinations. The relative spectral sensitivity of these detectors was modified to closely approximate the V(λ) curve, and the absolute monochromatic irradiance sensitivity at 633 nm was determined using self-calibrated Si photodiodes. The estimated uncertainty of the measurement of illuminance using these standardized detectors is about ±1%. A low-level photometric source that can provide illuminance of 1.5 × 10−1 lx−5 × 10−7 lx or luminance of 3 × 102 cd/m2−2 × 10−3 cd/m2 has been developed. The repeatability of calibrations of the low-level photometers was ±0.3%.
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