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Technical environment
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Glare – types and definition

Glare not wanted: store window with optimal display of mannequin and shiny objects.

An essential characteristic for the quality of a lighting system is the limitation of glare. Glare is understood to be both the objective reduction in visual performance and the subjective disturbance caused by the occurrence of high luminances or high luminance contrasts in the field of vision. The opposite of glare is visual comfort.

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Glare: an overview

What is glare?

Glare is a general term for the reduction of visual performance or the disturbance of perception, as caused by high luminances or contrasts in luminance within a visual environment. A distinction is made between physiological glare, in which there is an objective reduction of visual performance, and psychological glare, where perception is subjectively disturbed due to a disparity between the luminance and the information content of the area viewed.

Glare can be caused by the light source itself (direct glare) or by reflection of the light source (reflected glare).

What types of glare can occur?

Both physiological and psychological glare occur in the two forms of direct glare and reflected glare. With lighting design, this is relevant for e.g. the illumination of workplaces or for street lighting. With glare for office workplaces, a differentiation is made between:

  • Direct glare, primarily caused by luminaires (1),

  • Reflected glare on horizontal visual tasks (2), and

  • Reflected glare on vertical visual tasks, e.g. screens (3).

Direct glare considers the ceiling area in front of the viewer viewed at angles shallower than 45°. Reflected glare is glare which causes an unpleasant visual disturbance directly in front of the viewer, mainly caused by luminaires in the ceiling area. A special case is reflected glare on screens. Here, glare is mainly caused by sources of glare light in the ceiling area behind the viewer. With office workplaces, the UGR method (Unified Glare Rating) can be applied to determine the glare for a specific viewer position.

What are the possible consequences of glare?

In the case of objective reduction in visual performance, the term physiological glare is used. Here, in the eye the light from a glare light source is superimposed on the luminance pattern of the actual visual task, thus impairing that eye's ability to perceive. However, the superimposition of spill light, which is caused by the scattering of glare light in the eye, is sufficient to reduce visual performance. Increasing clouding of the eyes with age is responsible for the higher glare sensitivity of older people.

The extreme case of physiological glare is absolute glare. This happens when luminances of more than 104 cd/m2 are present in the field of vision, e.g. when looking at the sun or looking directly into artificial light sources. Absolute glare is independent of the luminance contrast to the surroundings: it cannot be avoided by a brighter environment. To prevent the eye from being exposed to danger, a protective reflex is triggered which causes the eyes to close or even the head to be turned away.

The constant, repeated adjustment to different brightness levels and the different distance between the visual task and the source of glare light causes strain on the eye, which is felt to be uncomfortable or even painful. Despite objectively constant visual performance, psychological glare causes considerable discomfort, and the ability to perform, for example at the workplace, is reduced.

Furthermore, unlike physiological glare, psychological glare is a phenomenon relating to information processing that cannot be described in detachment from the specific context. For example, brilliance on crystal chandeliers is an expected glare effect and is thus not perceived as physiological glare. The situation is different with reflections on glossy paper. The reflection itself contains no information and superimposes itself on the printed information.

Glare

Absolute glare though, in the context of architectural lighting, is rarely a problem. In such cases relative glare is much more frequent, in which the reduction in visual performance is not caused by extreme luminances but by excessive contrasts of luminance in the field of vision.

If the source of glare light does not cause an objective reduction in visual performance but only a subjective sensation of disturbance, this is termed psychological glare. The view of the observer is repeatedly drawn from the visual task to the source of glare light, without this area of increased brightness offering the expected information. Like an irritating noise, the source of glare light creates optical noise that attracts attention and disturbs the perception.

How can glare be avoided?

A reduction of glare effects can initially be achieved by reducing the contrast in luminance between the surroundings and the glare light source. Therefore, either the brightness of the surroundings has to be increased or the luminances of the glare light source reduced. However, it is more pre-emptive to avoid glare by selecting luminaires with good shielding and by correctly arranging the luminaires.

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Further topics on seeing and perceiving

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