Estimating Perceived Brightness in Low Luminance Conditions

Mesopic photometry, which studies visual perception of low-level optical radiation, is of great interest today in lighting engineering. It involves investigating human responses to visual observations in low light conditions in the object space, determining the optimum artificial illumination levels in industrial areas, and solving clinical perimetry problems. The estimation procedure for mesopic photometry recommended by the International Commission on Illumination (CIE) is based on computing a combination of photopic (daylight) and scotopic (nighttime) visual perception levels. This procedure being iterative makes it inconvenient to apply in engineering practice, as the number of iterative steps proves to be several dozens on average, exceeding a hundred in certain cases. As a result, the feasibility of using the CIE procedure instead of a purely photopic perception technique becomes questionable. The discrepancy in the results obtained via these methods informs the selection criterion. The paper compares computation results for perceived brightness in photopic and mesopic vision in low luminance conditions. We also establish whether it is possible to find analytical solutions using the CIE procedure. We show that, for radiation of a colour temperature in the range of 950--12000 K, the maximum computational discrepancy between photopic and mesopic vision scenarios lies in the --200--50 % range, while the minimum discrepancy is approximately 5 % for radiation characterised by a colour temperature of approximately 2000 K. We also present analytical solutions for several specific cases according to the CIE procedure

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