Glare There is a distinction between physiological and psychological glare as well as direct and reflected glare. Physiological glare objectively and measurably leads to decreased visual performance immediately upon exposure. This glare effect is described through equivalent veiling luminance, which covers the visual detail reproduced on the retina like a veil and thus debases contrast and acuity. Equivalent veiling luminance leads to a limit value (threshold value) increment for the adaptation luminance necessary to recognize a visual detail. This threshold increment (TI) is used, for example, in street lighting as a measure for physiological glare (TI method) and is defined in numbers in the relevant standards, e.g. TI ≤ 15%. Another method used for evaluating glare in outdoor lighting, the glare rating (GR) method, is also based on a glare formula using veiling luminance. Psychological glare usually only leads to a subjective perception of interference (discomfort) after some time has elapsed. Psychological glare caused by luminaires with elevated luminance in indoor lighting is rated via the unified glare rating (UGR) method.
Direct glare impacts the eye directly and immediately through elevated luminance levels (e.g. in luminaires) and can cause physiological or merely psychological effects.
Reflected glare has the same effects as direct glare. It is caused by the reflection of high luminance levels from shiny surfaces. Moreover, it causes an additional decrease in contrast since the directly reflected light outshines the visual information of the light reflected from the object in a scattered way.
The following rule applies: The lower the gloss level of a surface – e.g. the better the antireflection coating on screens or glass surfaces – the higher the admissible luminance for luminaires and luminous surfaces can be without causing disturbing effects. Moreover, reflected glare can be effectively avoided by choosing a suitable geometric alignment of the visual task in relation to the potential source of glare. An elevated level of ambient luminance also reduces both direct and reflected glare of a light source (see also chapter “Direct glare” and “Reflected glare on horizontal visual tasks”).
