Luminous efficiency and the measurement of daytime displays, signals, and visors

PURPOSE: Measurements concerning the usability or safety of optical equipment are based on assumptions regarding luminous efficiency. The current luminous efficiency functions are derived from human sensitivity experiments taken at low light levels compared with the outdoor daytime environment. The amount of error induced by extrapolating from low light level data to high light level applications is not known. We sought to determine whether standard luminous efficiency curves CIE V(lambda) and CIE Heterochromatic Brightness Matching are appropriate for measuring day-use optical equipment such as display phosphors, lasers, LEDs, and laser eye protection, which are becoming more common in aviation. METHODS: Flicker photometry and successive heterochromatic brightness matching were used to measure changes in luminance efficiency functions with increasing levels (1, 10, 100, and 1000 fL) of light adaptation. RESULTS: Luminous efficiency was found to depend on both the method and the reference intensity with which the measurements were taken. For heterochromatic brightness matching, luminous efficiency increased for longer wavelengths as reference intensity increased. Peak luminous efficiency shifted from approximately 540 nm to greater than 600 nm with increasing intensity. Peak luminous efficiency was constant for flicker photometry across all intensities, but the function narrowed slightly at 100 fL. CONCLUSION: Luminous efficiency curves measured at high reference intensities are substantially different from the standard luminous efficiency functions. Caution should be used when measuring spectrally narrow and bright sources such as lasers and LEDs with a V(lambda) corrected photometer because the measured luminance may correlate poorly with perceived brightness.