Luminance meter for photopic and scotopic measurements in the mesopic range

This paper presents a design and realization of a dual-channel luminance meter for simultaneous measurement of luminance with photopic and scotopic weightings. Such measurements are useful in mesopic conditions, i.e. when the luminance is in the range of 0.005–5 cd m−2. The instrument is a spot luminance meter with two spectrally weighted channels. The collected light is detected with silicon detectors and a computer-controlled dual-channel switched-integration amplifier. The instrument is characterized for relative spectral responsivity against a calibrated spectroradiometer using a radiance source based on an integrating sphere with input from a monochromator. An absolute luminance responsivity calibration is made against a sphere-based luminance standard at a level close to the high end of the mesopic range. The standard uncertainty in luminance responsivity calibration is 0.3% for the photopic channel and 0.6% for the scotopic channel. In addition, characterization measurements were carried out for the instrument's linearity, stray light sensitivity and polarization dependence. The results show very good noise performance, allowing fast measurements over the whole mesopic range. The noise equivalent power was measured to be approximately 20 fW Hz−1/2, equal to a noise equivalent luminance of 30 µcd m−2 Hz−1/2. Estimated uncertainty of measurements for typical light sources is 2.2% (k = 2) at the lowest luminance levels of the mesopic range.