High Illuminance Calibration Facility and Procedures

The range of calibration of illuminance meters and luminance meters has been normally limited to levels up to several thousand lx and several thousand cd/m2 using a high-power luminous intensity standard lamp. The calibration of instruments at much higher levels is required in applications such as daylight measurement, evaluation of solar simulators, and testing light sources in imaging devices. A calibration facility and procedures have been developed at NIST utilizing the detector-based method to allow illuminance calibrations at levels up to 100 klx (about the level of direct sun light) and luminance up to 30 kcd/m2, The calibration source is based on a commercial solar simulator source using a 1000 W xenon arc lamp with optical feedback control, and it is combined with a set of color glass filters that corrects the spectral power distribution to be close to CIE Illuminant A. The illuminance level can be varied without changing the color temperature significantly and without changing the distance. The developed source was evaluated for stability, spectral distribution, illuminance uniformity, and divergence of the beam. Experiments were also conducted to study the effect of heat by radiation on the glass filters used with the source and various diffuser materials, such as PTFE, opal glass, and acrylic, used to create luminance standards. The linearity and the effect of heat on standard photometers and commercial illuminance meters were also investigated, and appropriate procedures for high illuminance/luminance calibrations have been established.