Extension of NIM illuminance scale to very low levels (1 x 10(-10) lx to 2 lx)

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 x 10(-10)-2 x 10(0)) lx, the first method based on the luminance adjustable two-stage integrating sphere source was used to realize the illuminance scale down to 1 x 10(-6) lx, while the other method based on a twin-source superposition linearity measurement facility was applied to extend the scale from 1 x 10(-6) lx to 1 x 10(-10) lx. The expanded relative uncertainty of the (1 x 10(-10)-2 x 10(0)) 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 x 10(-10)-1 x 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.