The eye is capable of adopting to a large range of luminance, spanning about 14 decades. The adaptation, however, takes time, and once the eye is adapted, this range is reduced to four to five orders of magnitude. Objects that are brighter result in glare.

useful region

If the object luminance is too high or low compared to the adaptation luminance, the visual performance is impared.

Glare can not always completely avoided, neither is this necessary. In places where the visual performace is of secondary importance such as shopping centres, some controlled glare can actually be desirable to produce a visually stimulating and pleasent environment.

glare deliberately created

Sometimes, glare is a desired effect and
adds some sparkle and interest to the lighting

(Image courtesy of Marlin Lighting)

Based on the effect on people, glare may be put into either of two categories:

Discomfort Glare
If the glare sources are not too bright, they are merely a nuisance and do not directly interfere with vision. This condition is called discomfort glare.
Disability Glare
If the luminance of the glare source is much higher, disability glare arises. This causes the adaptation level of the eye to increase resulting in in a reduced contrast of the object.

Another approach to glare classification is based on the cause of the glare sensation:

Direct glare
This is caused by light sources such as candles, artificial light fittings, or windows.
Indirect glare
Even if the light source itself can not be seen by the observer, very reflective or glossy objects can cause bright splotches acting as indirect sources. In this case, a further differentiation may be made into
  • Reflected glare: The patches are so bright that they create discomfort;
  • Veiling reflection: The patches only reduce the contrast of the objectl
Waterstones1 Waterstones1 Waterstones1

This curved show window reduces reflected glare. Waterstones in Piccadilly, London

There are a number of different approaches to controlling and reducing glare. They are applicable to both, glare from artifical light sources, and daylight glare.

  1. Limit the luminance of the source in the direction of the eye;
  2. Screen the source from view;
  3. Re-position the work station so that the glare source is not in the field of view of the worker;
  4. Raise the background luminance against which the glare source is seen.
Imagine that surfaces on and around the task are mirror-like. If the person at work would see a light source reflected then in practice there may be reflected glare.