Mesopic visual efficiency I: detection threshold measurements

Abstract

Achromatic thresholds are measured at three sites to investigate the effects of light spectrum on achromatic detection thresholds in the mesopic region. The results of measurements using quasi-monochromatic targets reveal spectral sensitivity functions with two or three peaks, a so-called `chromatic effect' which is assumed to be due to the influence of the colour-opponent channels. This chromatic contribution seem to be less significant at lower luminances but it is accentuated for peripheral observation. Results obtained with broadband stimuli show that the contrast threshold, defined in terms of V(λ), is markedly higher for red targets than for other colours, particularly at lower light levels. For these broadband targets, contrast threshold values calculated using the new spectral sensitivity curves instead of V(λ) are in better agreement with the experimental results, particularly for the peripheral detection task.