Glare is difficulty 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 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. 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.
bloom surrounding objects in front of glare
Sunglasses 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 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 as a quantitative measure of
glare. Other glare calculation methods include CIBSE Glare Index, IES Glare
Index and the Daylight Glare Index.=Unified glare rating=
The unified glare rating is a measure of the glare in a given environment,
proposed by Sorensen in 1987 and adopted by the International Commission on
Illumination. It is basically the logarithm of the glare of all visible
lamps, divided by the background lumination :
where is the common logarithm, is the luminance of each light source numbered
, is the solid angle of the light source seen from the observer and 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 References