Brightness distribution in the visual field – the distribution of luminance – has a major impact on visual performance and visual comfort. Luminance (symbol: L) is the measure of the brightness of a luminous or illuminated surface as perceived by the human eye.

Marked differences in luminance in the visual field impair visual performance and affect our sense of wellbeing, so they should be avoided. This applies to both indoor lighting and lighting for outdoor areas, e.g. sports facility or road lighting.

Factors that determine luminance include the reflectance of surfaces and the illuminance on them. Luminance is measured in candelas per unit area (cd/m²); the unit of measurement used for lamps is normally cd/cm². Luminance indicates the physiological impact of light on the eye and is used as a planning variable in outdoor lighting.

Avoidance of extreme differences in brightness

We find extreme differences in brightness in the visual field unpleasant. They should be avoided because they constantly compel our eyes to readjust (adaptation). This quickly leads to visual fatigue.

Visual comfort is also affected by

• excessively low luminance and uniform lighting, which make for a monotonous atmosphere that we find dull and unattractive.
• excessively high luminance, which can cause troublesome glare.