Glare (vision) | Wikipedia audio article


Glare is difficulty of seeing in the presence
of bright light such as direct or reflected sunlight or artificial light such as car headlamps
at night. Because of this, some cars include mirrors with automatic anti-glare functions.
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.==Discomfort and disability==
Glare can be generally divided into two types, discomfort glare and disability glare.
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. This could arise for instance when driving westward at
sunset. Disability glare is often caused by the inter-reflection
of light within the 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.==Reducing factors==Glare can reduce visibility by: reduction of brightness of the rest of the
scene by constriction of the pupils reduction in contrast of the rest of the scene
by scattering of the bright light within the eye.
reduction in 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.
reduction in contrast between print and paper by reflection of the light source in the printed
matter (veiling glare). reduction in 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). bloom surrounding objects in front of glareSunglasses
are often worn to reduce glare; 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. Light field measurements can be taken to reduce
glare with digital post-processing.==Measurement==
Glare is typically measured with luminance meters 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.
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.
The CIE recommends the Unified glare rating (UGR) as a quantitative measure of glare.
Other glare calculation methods include CIBSE Glare Index, IES Glare Index and the Daylight
Glare Index (DGI).===Unified glare rating===
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 L b {\displaystyle L_{b}}
: U
G R=
8 log
⁡ 0.25 L b ∑ n ( L n 2 ω n p n 2 ) , {\displaystyle \mathrm {UGR}=8\log {\frac
{0.25}{L_{b}}}\sum _{n}\left(L_{n}^{2}{\frac {\omega _{n}}{p_{n}^{2}}}\right),} where log {\displaystyle \log }
is the common logarithm (base 10), L n {\displaystyle L_{n}}
is the luminance of each light source numbered n {\displaystyle n}
, ω n {\displaystyle \omega _{n}}
is the solid angle of the light source seen from the observer and p n {\displaystyle p_{n}}
is the Guth position index, which depends on the distance from the line of sight of
the viewer.==See also==
Afterimage Lens flare
Lyot stop Over-illumination
Specular reflection Visual comfort probability
Selective yellow

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