Little is known about how color signals and cone- and rod-based luminance signals contribute to perceived contrast in the mesopic range. In this study the perceived contrast of colored, mesopic stimuli was matched with that of spatially equivalent achromatic stimuli. The objective was to develop a metric for perceived contrast in the mesopic range in terms of an equivalent achromatic luminance contrast, referred to here as effective contrast. Stimulus photopic luminance contrast, scotopic luminance contrast, and chromatic difference from the background all contributed to effective contrast over the mid-mesopic range, but their contributions were not independent and varied markedly with background luminance. Surprisingly, color made a significant contribution to effective contrast from 10 to approximately 0.003 cd m(-2). A model describing this relationship is introduced (R2 = 0.89) and compared with predictions of mesopic luminance contrast obtained from a number of models proposed as systems of mesopic photometry.
A project to measure visual performance at mesopic light levels has recently been completed. An empirical relationship between visual search time and the photometric characteristics of a target and background has been developed on the basis of experiments with 15 subjects. This search time model has been submitted for consideration by the CIE as a new metric for predicting visual performance. The photometric characteristics of the target and background require measurement using standard CIE photopic and scotopic responses. Examples are presented of potential applications of this measurement technique.
Adequate lighting is important in many practical situations. Roads must be lit well enough to minimize the number of accidents, while safety regulations cover many industrial procedures. This means that it is vital to measure light levels accurately, which is possible in daytime and night-time conditions by using photometers calibrated to our visual response. However, the complex nature of the human visual system means that standards have not yet been defined for the light levels that are typical of dusk or twilight. A team of UK scientists has now taken a fresh approach to the problem. We have developed a model that predicts visual performance in the range of lighting conditions at dusk or twilight.
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