Extension of NIM illuminance scale to very low levels (1 × 10−10 lx to 2 lx)

Abstract

The realization of the low light level illuminance scale of the National Institute of Metrology (NIM, China) is presented. Two methods were used to achieve the photometric scale covering (1 × 10−10–2 × 100) lx, the first method based on the luminance adjustable two-stage integrating sphere source was used to realize the illuminance scale down to 1 × 10−6 lx, while the other method based on a twin-source superposition linearity measurement facility was applied to extend the scale from 1 × 10−6 lx to 1 × 10−10 lx. The expanded relative uncertainty of the (1 × 10−10–2 × 100) lx illuminance scale was evaluated to be from 1.0 % to 0.8 % varying with range (coverage factor k = 2). As part of our research work, a photon counting type photometer was developed and calibrated in the (2 × 10−10–1 × 10−6) lx range using this low light level illuminance scale with expanded relative uncertainty 13 % to 1.4 % (k = 2). This low light level photometric scale can meet the calibration requirement of high sensitivity photometers such as the micro-lux photometers and photon counting type photometers.