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| [[File:Flashlight effect Sumo Jan08.jpg|thumb|upright|Glare from a [[flash (photography)|camera flash]] during a [[Sumo]] fight]]
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| '''Glare''' is difficulty seeing in the presence of bright [[light]] such as direct or reflected [[sunlight]] or artificial light such as car [[headlamp]]s at night. Because of this, some cars include mirrors with automatic anti-glare functions.
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| Glare is caused by a significant ratio of [[luminance]] between the task (that which is being looked at) and the glare source. Factors such as the angle between the task and the glare source and [[eye adaptation]] have significant impacts on the experience of glare.
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| ==Discomfort and disability==
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| Glare can be generally divided into two types, discomfort glare and disability glare.
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| Discomfort glare results in an instinctive desire to look away from a bright light source or difficulty in seeing a task. Disability glare impairs the vision of objects without necessarily causing discomfort.<ref>{{cite web | url=http://eilv.cie.co.at/term/330 | title=CIE e-ILV: 17-330 disability glare | publisher=CIE}}</ref> This could arise for instance when driving westward at sunset. Disability glare is often caused by the inter-reflection of light within the [[human eye|eyeball]], reducing the contrast between task and glare source to the point where the task cannot be distinguished. When glare is so intense that vision is completely impaired, it is sometimes called '''dazzle'''.<ref>{{cite book |title= Road Lighting for Safety|last= Schreuder|first= D. A.|year= 1998|publisher= Thomas Telford Publishing|location= London|isbn= 0-7277-2616-1|page= 107|url= http://books.google.com/books?id=C9UxoH5ehwYC&printsec=frontcover&source=gbs_v2_summary_r&cad=0#v=onepage&q=&f=false|accessdate=September 25, 2009}}</ref>
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| ==Reducing factors==
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| [[File:Cowes Park Road traffic on Firework Night 2010 2.JPG|thumb|Example of a situation where glare can be problematic, if, for instance, the ability to determine the distance and speed of passing cars is reduced.]]
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| Glare can reduce visibility by:
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| * reduction of brightness of the rest of the scene by constriction of the [[pupil]]s
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| * reduction in [[Contrast (vision)|contrast]] of the rest of the scene by [[scattering]] of the bright light within the [[human eye|eye]].
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| * reduction in [[Contrast (vision)|contrast]] by [[scattering]] light in particles in the air, as when the headlights of a car illuminate the fog close to the vehicle, impeding vision at larger distance.
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| * reduction in [[Contrast (vision)|contrast]] between print and paper by reflection of the light source in the printed matter (veiling glare).
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| * reduction in [[Contrast (vision)|contrast]] by reflection of bright areas on the surface of a transparent medium as glass, plastic or water; for example when the sky is reflected in a lake, so that the bottom below or objects in the water cannot be seen (veiling glare).
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| * [[bloom (shader effect)|bloom]] surrounding objects in front of glare
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| [[Sunglasses]] are often worn to reduce glare; [[polarizer|polarized]] sunglasses are designed to reduce glare caused by light reflected from non-metallic surfaces such as water, glossy printed matter or painted surfaces. An anti-reflective treatment on eyeglasses reduces the glare at night and glare from inside lights and computer screens that is caused by light bouncing off the lens. Some types of eyeglasses can reduce glare that occurs because of the imperfections on the surface of the eye.
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| [[Light field]] measurements can be taken to reduce glare with digital post-processing.
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| ==Measurement==
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| Glare is typically measured with [[luminance meter]]s or luminance cameras, 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.
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| The [[International Commission on Illumination]] (CIE) defines glare as:
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| <blockquote>''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.''<ref>{{cite web | url=http://www.opticianonline.net/Articles/2007/10/12/19552/Glare+%28C7654%29.htm | title=Glare (C7654) | publisher=http://www.opticianonline.net | date=12 October 2007 | accessdate=October 29, 2012 | author=Murray Ian}}</ref><ref>{{cite web | url=http://www.cwct.co.uk/ibcwindow/adaptive/glare.html | title=Glare | publisher=http://www.cwct.co.uk | accessdate=October 29, 2012}}</ref> </blockquote>
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| The CIE recommends the ''Unified glare rating'' (UGR) as a quantitative measure of glare.<ref>{{cite book | title=CIE 117-1995 Discomfort Glare in Interior Lighting | publisher=CIE | year=1995 | isbn=978 3 900734 70 1}}</ref><ref>{{cite book | title=CIE 190:2010 Calculation and Presentation of Unified Glare Rating Tables for Indoor Lighting Luminaires | publisher=CIE | year=2010 | isbn=9783901906879}}</ref>
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| Other glare calculation methods include ''CIBSE Glare Index'', ''IES Glare Index'' and the ''Daylight Glare Index'' (DGI).<ref>{{cite web | url=http://www.new-learn.info/packages/clear/visual/people/performance/glare/index.html | title=Glare | publisher=LEARN, Low Energy Architecture Research Unit | accessdate=October 29, 2012}}</ref>
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| ===Unified glare rating===
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| The unified glare rating (UGR) is a measure of the glare in a given environment, recommended by the [[International Commission on Illumination]]. It is basically the logarithm of the glare of all visible lamps, divide by the background lumination <math>L_{b}</math>.
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| <math>U
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| GR=8\log{0.25\over L_{b}}\Sigma_{n}({L_{n}^2\omega_{n}\over p_{n}^2}),\!</math>
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| where <math>\log</math> is the [[common logarithm]], <math>L_{n}</math> is the luminance of each light source numbered <math>n</math>, <math>\omega_{n}</math> is the solid angle of the light source seen from the observer and <math>p_{n}</math> is the [[Guth position index]], which depends on the distance from the line of sight of the viewer.
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| == See also ==
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| * [[Afterimage]]
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| * [[Lens flare]]
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| * [[Lyot stop]]
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| * [[Over-illumination]]
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| * [[Specular reflection]]
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| * [[Visual comfort probability]]
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| * [[Selective yellow]]
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| ==References==
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| {{reflist}}
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| {{Artificial light sources}}
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| {{DEFAULTSORT:Glare (Vision)}}
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| [[Category:Vision]]
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| {{Link GA|ja}}
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