Welcome to question of the day #53

Eyetools question of the day #53

Glare is where light causes a problem for the person. There are two main types: discomfort glare and disability glare.

Discomfort glare occurs in fully sighted people when the person is suddenly exposed to a much higher level of illuminance than that to which they have adapted to. The discomfort is usually transient but can be long-lasting. Discomfort glare annoying and tiring but producing no measurable drop in vision. It is recommended that the ratio between task illuminance (the light on the task) and the illuminance of the surrounding area should not be greater than 3 to 1.

Discomfort from glare can be removed by reducing the amount of light with a tinted lens. The colour and amount of tint are often selected on the basis of subjective reports and cosmetic acceptability.

Disability glare may occur at the same time as discomfort glare but is distinguished by the change in the contrast of the image of the object of interest on the retina. This causes a reduction in vision. Disability glare prevents a person seeing and reduces vision in the area of the glare source.

In the presence of high surrounding illumination such as a very white background light going into the eye is scattered especially if there is any cataract in the eye. The glare source has a greater effect on image contrast the brighter it is and the closer it is to the main line of sight. Scattering of light within the eye creates a ‘veil’ of luminance across the image on the retina which reduces the contrast of the image and reduces the vision.

Another type of disability glare occurs when there is a reflection of light from a shiny surface and this reflection obscures the view of the underlying object. For example, this may occur when trying to read from a glossy magazine page or when using a computer screen and a nearby localised light source. This type of disability glare is sometimes called reflection glare. Avoiding this type of glare would require a change in the angle of illumination (changing the angle of the local light source) to remove or change the position of the reflection. Glare from glossy paper can be eliminated by photocopying on to matt paper. If this is not possible the use of polarizing filters would selectively reduce the intensity of the reflected light. Any measures which can remove the glare source out of the person’s field of view will be beneficial. Moving the position of a person’s chair may be enough to avoid this type of glare.