Measuring the Effects of Light Distribution on Spatial Brightness

Abstract

<p><b>By changing the light distribution it is possible to double the apparent amount of light in a space without any increase in its overall luminance. If one simply assumes that the apparent amount of light in a space — its spatial brightness — is described by its mean luminance (or similar measures) then substantial errors may be made.</b></p> <p>We carried out two experiments, measuring the brightness of 19 different model spaces. Our results demonstrate that making light distributions more non-uniform can make spaces appear both significantly brighter and significantly darker, depending on how the light distribution is changed. This challenges most existing studies in the field that argue that non-uniformity of the luminance distribution simply makes spaces look darker. Indeed, the observed pattern in brightness between our conditions cannot be consistently explained by a simple measure of the uniformity of the luminance distribution. We thus reject all previously proposed models of light distribution and spatial brightness.</p> <p>The best explanation of this and the apparent disagreements in the literature over the effects of non-uniformity appears to be that spatial brightness is affected by the qualitative appearance of the luminances in the space. Light sources and non-luminous surfaces have different effects. We propose a ‘duel’-process model of spatial brightness in which it is the sum of two opposed processes: the effects of the surfaces, and the effects of the light source(s). Non-uniform patterns of surface reflectance and illumination tend to make a space appear brighter. Non-uniformity as a result of a large difference between luminance of the light source(s) and the surfaces makes a space appear darker. If the light source is hidden from direct view its darkening effect is removed, which can make the space appear significantly brighter. Depending on the relative strength of these two processes, a non-uniform luminance distribution may thus appear either brighter or darker than a more uniform distribution.</p> <p>Additionally, we highlight issues demonstrated in both the failure of models previously proposed by the literature, and our exploration of potential implementations of the ‘duel’-process model. It is very easy to produce a good correlation with a defensible metric that will not generalise to other data sets. A metric having a good correlation in a study provides very little reason to actually believe it. If we wish to develop a model of the effects of light distribution that we can trust, we need to demonstrate its robustness by testing its underlying assumptions and showing them to be well supported. As we show, there is a large variety of these that need to be worked through.</p>

The effect of correlated colour temperature and wall luminance on spatial brightness and scene preference in a windowless office setup

“…While the literature has shown that non-uniformity could affect brightness, [19][20][21][22] this was not investigated, and the uniformity was kept as high as possible and constant throughout the lighting scenes. It is also possible that the adaptation state of an observer can affect the perceived brightness.…”

“…In addition to luminance, uniformity of the lighting environment is a determining factor for the perceived room brightness. Although there is some disagreement in the literature, Sullivan et al [19][20][21] reported that most studies suggest that more uniform lighting appears brighter than less uniform lighting with equal luminances, while others suggest the opposite. A pilot experiment by Sullivan et al 19 was also conducted, which indicated that less uniformly lit spaces were evaluated as brighter.…”

Exploring How Interior Electric Lighting Systems meet the Needs of Autistic People

“…This study did expand an understanding of how autistic adults perceive and describe brightness of electric lighting systems. Understanding of brightness perception even within the general population is complex, however there is research into how to better define the concept and provide metrics for its measurement (P. R. Boyce, 2014;Cuttle, 2010;Houser et al, 2002;Sullivan, 2021). This research helps to recognise the more subjective understanding within a minority population.…”