Luminance contrast is a photometric term determined by the difference in the colour and brightness of an object compared to its background within the same field of view. A lighting design that brightens some objects while others stay shaded can provide improved visibility.
Many lighting professionals recommend a maximum ratio of 5 : 1 or lower between general and task lighting, and a maximum ratio of 10 : 1 between hallways and other “traffic” areas
The luminance distribution in a room is determined by the windows, the position of the luminaires, the light distribution and the reflective surfaces in the room. A well balanced luminance distribution increases visual acuity and contrast sensitivity.
Generally, good contrast and colour selection is preferable to a very high illuminance because the human visual system is more sensitive to contrast than high illuminance. Luminaires can be directed towards working areas, the lecturer, a sign or other important subjects without increasing the general lighting. The benefits of good contrast are: increased visual impression (provided no glare) and low energy consumption.
Lighting can also cause interfering reflections on computer screens. The standard EN12464-1 specifies average luminance limits for luminaires, to avoid reflection onto flat screens. The limits are specified for elevation angles of 65° and above and are typically <1000 cd/m² or 1500 cd/m². For critical computer screen activities this starts from 55° all around.
Glare appears in the case of very high luminance or large luminance contrast often due to lack of shielding from a window, reflection or from direct sight of the LED light sources in the luminaire. Normally, the eye adapts to any lighting situation, but if the lighting of the object or its background is too bright or the contrast is too great, vision suffers by glare.
Disability glare, which affects the ability to see and is produced by high luminances in a low luminance scene, typically by point light sources such as bright spotlights aiming at the observers eye, or by large area sources such as a bright LED panel.
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