Vision impairment glare
Glare occurs when a part of the interior is significantly lighter than the room’s luminosity in general. In this position the eyes are exposed to a stronger light than what they are normally adapted to. The most common causes of glare are that the luminaires and window are seen directly or via reflections within the normal field of vision.
Older people are usually more irritated by glare than younger people. This is mainly due to the increased clouding of the lens experienced with age as well as a reduction in the ability to adapt.
Glare can be divided into two different types of glare: vision impairment glare and discomfort glare. Vision impairment glare and discomfort glare can occur simultaneously or separately.
Vision impairment glare
Vision impairment glare usually occurs when an object in the vicinity of the normal line of sight has a significantly higher luminosity or luminance than what normally occurs in the field of vision. If the eye is exposed to a irritating strong light the eye’s adaptation can be affected negatively and cause deterioration in visibility, also known as contrast reduction.
This contrast reduction can be sufficient to make important details invisible and in doing so, make it difficult to carry out vision tasks. Alternatively, if the light source causing the glare is directly in the line of sight, this can cause noticeable after-images.
The most common source of vision impairment glare indoors is the sun and sky seen from a window or a badly screened source, viewed directly or via reflection. Vision impairment glare shall therefore be prevented by using luminaires that effectively shield the light source (see the table).
Lowest screening angle for the light source’s luminance
| Luminance of light source [kcd/m²] |
Minimum screening angle |
| 20 to < 50 | 15° |
| 50 to <500 | 20° |
| ≥ 500 | 30° |
The values in the table do not include luminaires with only indirect light or luminaires that are installed below normal eye height.
For luminaires where the light source is instead screened via some form of optics, the standard instead sets values for maximum average luminance from 60–90 degrees.