High beam of head-light of a on-coming car has blinding effect and decreases visibility dangerously. A fielder in the outfield of a cricket match misses a crucial catch because of glare from the flood lights or the sun. Every one knows what is glare and every one has experienced it too, but when asked to describe or define most are confused.
Glare can be defined as the contrast lowering effect of stray light in a visual scene. Glare forms a veil of luminance which reduces the contrast and thus the visibility of a target is decreased. We cannot see intensity differences efficiently in the presence of a high background of light intensity. And the sensitivity to glare is amplified as scattering in cornea or lens increases.
Cornea | 30 % |
Lens | 70 % |
Aqueous & vitreous | < 1 % |
Glare can be divided into two types:
Discomfort Glare refers to the sensation one
experiences when the overall illumination is too bright e.g. on a
snow field under bright sun.
Disability Glare refers to reduced visibility of
a target due to the presence of a light source elsewhere in the
field. It occurs when light from glare source is scattered by the
ocular media. This scattered light forms a veil of luminance
which reduces the contrast and thus the visibility of the target.
Contrast can be defined as the difference
between the luminance of the target and the background relative
to the average luminance of the scene.
Contrast = | Target illuminance-Background illuminance |
Target illuminance+Background illuminance |
usually expressed as percentage (%) or as a fraction.
Scattered light raises the luminance of both the target and the
background to same extent thereby reducing the contrast.
I. Discomfort Glare
II. Disability Glare
|
I. Discomfort Glare
It can be measured under experimental conditions by having a patient adjust brightness of a light source until it reaches an ill-defined threshold of unpleasantness. However, it does not seem to have any clinical relevance or applicability.
II. Disability Glare
A conventional visual function test, usually acuity or contrast-sensitivity, is administered in the presence of a glare source. Following tests are available:
To measure visual disability in mild cataract especially:
To distinguish decreased vision because of anterior segment pathology (media opacity) from retinal pathology. Glare sensitivity is markedly increased in anterior segment disease but is unaffected in retinal disorders. Glare sensitivity tests are more sensitive and more specific for anterior segment pathology.
It is a measure of the speed with which the visual system regains function following exposure to bright light. Factors and diseases affecting light and dark adaptation will affect glare recovery. This is used to predict predisposition of a patient to retinopathy.
To conclude, glare is an important factor in visual disability with anterior segment disease and sensitivity tests are useful in evaluating the visual problems of a patient with early cataract. It also forms a convenient and non-invasive method of differentiating anterior segment pathology from retinal disease. Glare recovery time is useful in predicting the possibility of development of retinopathy in diabetics.
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