Ute C Besenecker

Influence of Spectral Power Distribution on Scene Brightness at Different Light Levels

ABSTRACT Exterior lighting has multiple objectives. Brightness perception is a relevant parameter for outdoor lighting because it is correlated with perceptions of safety and security. Understanding the influence of the spectral characteristics of lighting to scene brightness perception is important in order to devise exterior lighting specifications that support perceptions of safety and security in exterior lighted environments, as well as to optimize light source technologies to account for these factors. A study of scene brightness perception under different light levels and spectral power distributions was conducted to assess whether scene brightness perception exhibited increased short-wavelength spectral sensitivity as a function of increasing light level. The results confirm that a successful model of spectral sensitivity for scene brightness perception should incorporate a shift in short-wavelength sensitivity like the one investigated in the present study.

Investigating visual mechanisms underlying scene brightness

Short-wavelength (<500 nm) output of light sources enhances scene brightness perception in the low-to-moderate photopic range. This appears to be partially explained by a contribution from short-wavelength cones. Recent evidence from experiments on humans suggests that intrinsically photosensitive retinal ganglion cells (ipRGCs) containing the photopigment melanopsin might also contribute to spectral sensitivity for scene brightness perception. An experiment was conducted to investigate this possibility at two different light levels, near 10 lx and near 100 lx. Subjects provided forced-choice brightness judgments and relative brightness magnitude judgments when comparing two different amber-coloured stimuli with similar chromaticities. A provisional brightness metric including an ipRGC contribution was able to predict the data with substantially smaller errors than a metric based on cone input only.

Spectral sensitivity and scene brightness at low to moderate photopic light levels

A study of scene brightness perception was conducted to assess whether spectral sensitivity for scene brightness perception at low to moderate light levels (∼3–110 lux) could be partially explained by a contribution of intrinsically photosensitive retinal ganglion cells (ipRGC). An experiment was conducted at two different light level ranges and using two different spectral power distributions. The results suggest an increase in short-wavelength spectral sensitivity as a function of increasing light level. The results also confirm that including ipRGC as well as cone photoreceptor input in scene brightness spectral sensitivity resulted in improved predictions compared to including either cone input only or rod as well as cone input.

Work Zone Lighting and Visual Performance: Analysis and Demonstration

In part because of the potential for high levels of glare from work zone illumination, recommendations for light levels from work zone illumination systems are substantially higher than for levels used along roadways in non–work zone locations. In a two-part study, requirements for work zone illumination light levels were assessed. First, levels for workers varying in age from 20 to 60 years were evaluated with the relative visual performance model, with and without the presence of visibility-reducing glare. Except for the smallest, lowest-contrast tasks performed by the older workers, an illuminance of 10 lx resulted in visibility well above the threshold even in the presence of glare, and an illuminance of 30 lx resulted in suprathreshold visibility for these conditions as well. The results of these computational analyses were largely confirmed in a full-scale, outdoor field demonstration attended by transportation agency engineers and highway contractors. Together, the findings suggest that when lighting systems provide sufficient glare control, light levels do not always need to be especially high to ensure adequate visibility for workers.