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One lux corresponds to the luminous intensity, which a candle produces at a distance of one meter. If a light-current (1 lumen) evenly meets a surface of 1m² , then this corresponds to a illuminanc of 1Lux. 1 lx = 1 lm/m2
The illuminance is a pure receiver size and serves the unit for brightness.
Midday sun light in summer
Covered skies in summer
Rain weather with dark thunderclouds
Twilight after sunset
Midnight with full moon
Moonless starry sky at night
In the site link luminous intensity we describe the strength of a light source. If we now want to know with effect of a light source has, the illuminance gives the answer.
At home or in the job it interests less, how brightly the lamp or the fluorescent tube radiates, but if one wants to know, has I sufficient light around to read, is my working area sufficient illuminated , which mood I want to produce on the illuminated objects? The term "mood" was treated already in the chapter color temperature and one can easily imagine that the brightness of an object is related with the color temperature, but it is a big difference whether I use a 10W or 100W bulb.
We find the brightness of a candle as a very pleasant light and want to find out whether this brightness would be sufficient e.g.. to be able to read a book. Our cosy reading chair is 2m away from the candle. The light-current of a candle is approx.. 12 lm. In the distance from 2 m the Illuminance is („brightness“)= 12 lm/(4p · (2 m · 2 m)) = 0,24 lm/m² = 0,24 lx A candle is an isotropic source of light, therefore we have here a spatial light propagation (Sphare surface = SR = 12.566)
The result: Our white book sheet, illuminated by a candle in the distance from 2 m, appear approximately as bright as in the light of the full moon at midnight, thus for reading a little too weak.
A further example with an LED (anisotrope light source) with a luminous intensity of 6cd within a light cone of 30° and a distance r = 2m. 30° = SR of 0,2. (see table).
Luminus flux = 6 x 0,2 = 1,2 lm
The area, which a LED with 0,2 sr in a distance of r illuminates, amounts to:
A = 0,2sr x 4m² = 0,8m².
We know 1 lx = 1 lm/m2
Illuminanc = 1,2 lm : 0,8m² = 1,5 Lux.
Who has good eyes, could read there.