Spectral luminous efficiency function for a 2-deg field at a photopic level (100 Td) was measured for 91 observers by flicker photometry (FP) and for 97 observers by direct brightness matching (DBM), to find age-related change in the efficiency function as well as to obtain a reliable data set to be used in photometry. Observers ranged in age from 11 to 78 years. A gradual reduction of luminous efficiency with age was observed for both functions by FP and by DBM in the short-wave region, which was expected because of the age-related increase of optical density of eye lens. A similar age-related reduction of efficiency was observed in the long-wave region for the function obtained by DBM; this reduction was regarded as being due to reduced chromatic contribution to brightness with age. Principal components analysis on the spectral efficiency data and an analysis of the efficiency difference between the data obtained by DBM and FP confirmed this conclusion. Assuming a log-linear change in efficiency with age for any wavelength throughout the life span, spectral luminous efficiency function at any age was derived for photometric use.
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