The perception of room brightness over photopic luminances ranging from 30 cd/m2 to 67 cd/m2 was judged by 12 subjects in an almost uniformly white experimental chamber. Two different illuminants were compared which had different spectral compositions, but were color matched. Brightness judgements were often opposite to large differences in photopic luminance. These results are inconsistent with models of brightness perception that depend solely on cone receptors. At the luminance levels considered here subjective evaluation of light intensity depends upon both photopic and scotopic spectral contributions. These results imply that aspects of the visual system operate mesopically under most interior lighting conditions.

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The near visual acuity (400 mm distance) of 27 children aged 10-11 years old was measured by a licensed optometrist under two common fluorescent lamps of CCT 3600 K and 5500 K. Acuities were measured for three lighting conditions, either both lamps providing equal task luminance or a condition where the task and room luminance from the 5500 K lamps was set 50% lower. For the equal luminance condition, the results showed visual acuity was significantly better (P B/0.001) under the higher CCT lamp with 24 of 27 children having better acuity. Paired t-tests comparing the lower luminance condition showed significantly less acuity for the 5500 K lamps at the lower luminance, but no significant difference between the 3600 K lamps at the higher luminance compared with the 5500 K lamps at the lower luminance.

Subjects judged the orientation of a 2 min. gap Landolt C located at a distance of 2.4 m. The stimuli were presented in central vision on a CRT, at low to medium contrast. The effects of varying the spectrum and luminance of surround lighting were assessed on both pupil size (measured using infrared pupillometry during task performance) and task accuracy. The task display was protected from the surround lighting, so that its luminance and contrast could be varied independently of the changes in the surround lighting. Indirect surround illumination was provided by . either two illuminants of very different scotopic spectral content but with the same · photopic luminance (Experiment 1 and 3 each), or by using the same illuminant at two different luminance levels (Experiment 2). In experiment 3, the effect of changing surround spectrum was compared to the effect of varying task background luminance between 12 cd/m2 and 73 cdfm2. In all experiments, scotopically enhanced surround lighting produced pupil areas which were reduced by almost 50% in comparison with surround lighting with relatively less scotopic luminance.Concomitantly there was improvement in Landolt C task performance with the scotopically enhanced surround lighting at all contrast and luminance levels. In these experiments, smaller pupil sizes were associated with significantly better visual-task performance in spite of lower task retinal illuminance when compared to the condition with larger pupils. These results suggest that changes in surround spectrum can compensate for the effect on task performance of a reduction in task luminance and supports the possibility that lighting energy savings can accrue in the workplace by shifting lamp spectra to obtain greater scotopic efficacy.

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