Glare (vision)

Discomfort glare has often been studied using psychophysics experiments, where the common methods have been the luminance adjustment and category rating procedures.[6] Studies conducted by Petherbridge and Hopkinson[7] and Luckiesh and Guth.[8] were amongst the first to compared subjective assessments given by observers against physical measurements produced by a glare source.

A comprehensive review of the methods used to measure glare showed that there are biases associated with its measurement.[9] Luminance adjustments are sensitive to anchoring (cognitive bias) effects caused when the initial starting luminance viewed influences the final assessment of visual discomfort.[10] Glare is also subject to stimulus range bias effects.[11][12] This occurs when the luminance range influences the final evaluation of glare given by the observer. A larger range, often results in higher glare evaluations given.

Glare from artificial lights is typically measured with luminance meters. From daylit windows, cameras are used to convert the pixels into luminance. Both of which are able to determine the luminance of objects within small solid angles. The glare of a scene i.e. visual field of view, is then calculated from the luminance data of that scene.

The International Commission on Illumination (CIE) defines glare as:

"Visual conditions in which there is excessive contrast or an inappropriate distribution of light sources that disturbs the observer or limits the ability to distinguish details and objects".[13][14]

The CIE recommends the Unified glare rating (UGR) as a quantitative measure of glare.[15][16] Other glare calculation methods include CIBSE Glare Index, IES Glare Index and the Daylight Glare Index (DGI).[17]

The unified glare rating (UGR) is a measure of the glare in a given environment, proposed by Sorensen in 1987 and adopted by the International Commission on Illumination (CIE). It is basically the logarithm of the glare of all visible lamps, divided by the background lumination ${\displaystyle L_{b}}$:[18]

${\displaystyle \mathrm {UGR} =8\log {\frac {0.25}{L_{b}}}\sum _{n}\left(L_{n}^{2}{\frac {\omega _{n}}{p_{n}^{2}}}\right),}$

Where ${\displaystyle \log }$ is the common logarithm (base 10), ${\displaystyle L_{n}}$ is the luminance of each light source numbered ${\displaystyle n}$, ${\displaystyle \omega _{n}}$ is the solid angle of the light source seen from the observer and ${\displaystyle p_{n}}$ is the Guth position index, which depends on the distance from the line of sight of the viewer.