Light Level or Illuminance is the total amount of visible light incident upon a surface, per unit area from all directions above the surface. This “surface” can be a physical surface or an imaginary plane.
The measure of brightness from a light source which is taken through a light meter’s sensor at a horizontal position on a horizontal surface is called horizontal illuminance and a vertical position on a vertical surface is called vertical illuminance.
Object recognition depends not on how bright the scene is, but on the ability of the eye to discriminate differences in illumination within the object and against its environment, At high light levels contrast threshold increases, absolute sensitivity decreases and the eyes switch to photopic (cone vision) vision.
Light level standards are affected by light quantity and quality desired, fixture efficiency and other applicable factors
Units and Measures
Light levels are generally measured by using a light meter.
Following are the units for measuring light levels:
- Lux (SI): One lumen per square meter (lux = 10.752 fc).
- Foot Candles (Imperial): One lumen of light density per square foot (fc = lux / 10.752).
Following are some common and recommended light levels:
- Sunlight - 10,000 (ft.cd) / 107,527 (lux)
- Full Daylight - 1,000 (ft.cd) / 10,752 (lux)
- Warehouses, Homes, Theaters, Archives – 150 (lux)
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