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The lux (symbolized lx) is the unit of illuminance in the International System of Units ( SI ). It is defined in terms of lumen s per meter squared (lm/m 2 ). Reduced to SI base units, one lux is equal to 0.00146 kilogram per second cubed (1.46 x 10 -3 kg / s 3 ).

One lux is the equivalent of 1.46 milliwatt (1.46 x 10 -3 W) of radiant electromagnetic (EM) power at a frequency of 540 terahertz (540 THz or 5.40 x 10 14 Hz ), impinging at a right angle on a surface whose area is one square meter. A frequency of 540 THz corresponds to a wavelength of about 555 nanometer s (nm), which is in the middle of the visible-light spectrum.

The lux is a small unit. An alternative unit is the watt per meter squared (W / m 2 ). To obtain lux when the illuminance in watts per meter squared is known, multiply by 683. To obtain watts per meter squared when the illuminance in lux is known, divide by 683 or multiply by 0.00146.

Illuminance varies inversely with the square of the distance from the source on a free-space line of sight. If the distance is doubled, the illuminance is cut to 1/4; if the distance increases by a factor of 10, the illuminance becomes 1/100 (0.01 times) as great.

Also see candela , electromagnetic field , lumen , steradian , watt per meter squared , and International System of Units ( SI ).

This was last updated in March 2011
Posted by: Margaret Rouse

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