Illuminance refers to the luminous flux on a surface per unit of area. Illumination decreases (shows an inverse relation) as the square of the distance from the source.
illuminance (E, in lux) =
= (light source intensity in lumen) / ((distance from source in meters) ^2)
where:
• 1 lux = 1 lumen per square meter = 1 candle steradian per square meter
• 1 lumen = 1 candle steradian = the luminous flux through a unit-solid angle (steradian) from a uniform point source of one candle
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