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Convert between lux and other major units of illuminance.

In SI, the unit of illuminance – how brightly a surface is illuminated. One lux is 1 lumen per square meter. (One lux is about 0.09 foot-candles.) The plural is also “lux”, not “luxes”.

On the Earth’s surface, the illumination from the sun is about 100,000 lux; from a full moon at the zenith on a clear night, about 0.27 lux; and from all the stars about 0.00022 lux.

The lux is a derived unit. In terms of base units, it is

A fraction. The numerator is candela and the denominator is steradian times meters squared



The word lux was originally suggested by Sir W. Preece, at the 1899 Paris Congress of Electricians, as the name for a unit of illumination, and applied by him to express an illumination equal to a Carcel-metre, nearly equal to 1 candle-foot in magnitude.

J. A. Fleming.
The Photometry of Electric Lamps.
The Electrician, vol 50, no 15 (issue No. 1289) January 30, 1903.
Page 599, footnote.


The report of the committee, as adopted by the congress [International Congress of Electricians, Geneva, 1896], included a suggestion that, as the bougie unit of luminous intensity differed from the Hefner unit by only from two to six per cent, the two might be used interchangeably for many purposes. More recent determinations indicate this difference to be in the neighborhood of eleven per cent, so that this loose statement as to the relation between these two units has occasioned much confusion, as a result of which we find the lux, the unit of illumination intensity, used in connection with the Hefner, in Germany, and in connection with the bougie in France. There is, perhaps, no one unit which has been so variously defined as the lux. If one searches far enough he can find some more or less authoritative definitions of the lux based upon every unit of luminous intensity that has ever been recognized. It has even been defined as the illumination produced upon a normal surface at a distance of one meter by a Carcel. It is well to bear in mind that this unit, as proposed and adopted at the 1896 congress, at Geneva, was based upon the bougie as a unit of luminous intensity.

Preston S. Millar.
Photometric Units.
Electrical Review, vol 51, no. 11 (14 September 1907).
page 427.

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