Glare (vision)

Methods

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.

Biases

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.

Prediction models

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]

Unified glare rating

The Unified Glare Rating (UGR) is a measure of the glare in a given environment, accounting only interior artificial lights, 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.

Daylight Glare Probability

The Daylight Glare Probability (DGP) is a measure glare from real daylight condition in side-lit room within the field of view from curtain position, not considering for artificial light. It considers illuminance and luminance from glare sources to estimate the level of dissatisfaction. DGP was proposed by Wienold and Christoffersen in 2006^[19] and adopted as first ever day-lighting standard by the European Standard's EN 17037 (2018) Daylight in Buildings. The EN 17037 glare assessment helps designer to determine shading need, transmission of glazing if the DGP is higher than 0.4.^[20] However, data from tropical climate countries suggests the DGP threshold is expected to be 0.24, significant lower than Wienold and Christoffersen's Copenhagen and Freiburg data.^[21]

${\displaystyle \mathrm {DGP} =5.87\times 10^{-5}\times E_{V}+9.18\times 10^{-2}\times log\left(1+\sum _{i}{\frac {L_{s,i}^{2}\times \omega _{s,i}}{E_{V}^{1.87}\times P_{i}^{2}}}\right)+0.16,}$

Where ${\displaystyle \log }$ is the common logarithm (base 10), ${\displaystyle E_{V}}$ is the illuminance at eye level (lx), ${\displaystyle L_{s}}$ is the luminance of glare source (cd/${\displaystyle m^{2}}$), ${\displaystyle i}$ is the number of glare sources, ${\displaystyle \omega _{s}}$ is the solid angle of the glare sources seen from the observer and ${\displaystyle p_{i}}$ is the Guth Position Index, which depends on the distance from the line of sight of the viewer.