Effect of increased ambient lighting on detectability - A psychophysical study
Last year in this conference, we presented a theoretical analysis of how ambient lighting in dark reading rooms could be moderately increased without compromising the interpretation of images displayed on LCDs.' Based on that analysis, in this paper we present results of two psychophysical experiments which were designed to verify those theoretical predictions. The first experiment was designed to test how an increase in ambient lighting affects the detection of subtle objects at different luminance levels, particularly at lower luminance levels. Towards that end, images of targets consisting of low-contrast objects were shown to seven observers, first under a dark room illumination condition of 1 lux and then under a higher room illumination condition of 50 lux. The targets had three base luminance values of 1, 12 and 35 cd/m 2 and were embedded in a uniform background. The uniform background was set to 12 cd/m 2 which enabled fixing L adp, the visual adaptation luminance value when looking at the display, to 12 cd/m 2. This value also matched the luminance value of about 12 cd/m 2 reflected off the wall surrounding the LCD at the higher ambient lighting condition. The task of the observers was to detect and classify the displayed objects under the two room lighting conditions. The results indicated that the detection rate in dark area (base luminance of 1 cd/m 2) increased by 15% when the ambient illumination is increased from 1 to 50 lux. The increase was not conclusive for targets embedded in higher luminance regions, but there was no evidence to the contrary either. The second experiment was designed to investigate the adaptation luminance value of the eye when viewing typical mammograms. It was found that, for a typical display luminance calibration, this value might lie between 12 and 20 cd/m 2. Findings from the two experiments provide justification for a controlled increase of ambient lighting to improve ergonomic viewing conditions in darkly lit reading rooms while potentially improving diagnostic performance.
