What Is LUX?

The practical unit of illumination, using the metric system of measurement, the illumination of a surface one square  meter in area receiving an evenly distributed flux of one Lumex
 or

When luminous flux strikes a surface, the surface is said to be illuminated. The intensity of illumination, analogous to the intensity of electromagnetic radiation (which is power per unit area) is the luminous flux per unit area, called the illuminance, denoted by E. The unit of illuminance is the lumen per square meter, also called the lux:

1 lux = 1 lm/m²

We have defined the intensity of light and other electromagnetic radiation as power per unit area, measured in watts per square meter. Similarly, the total rate of radiation of energy from any of the sources of light discussed in Sec. 38-2 is called the radiant power or radiant flux, measured in watts. These quantities are not adequate to measure the visual sensation of brightness, however, for two reasons: First, not all the radiation from a source lies in the visible spectrum; and ordinary incandescent light bulb radiates more energy in the infrared than in the visible spectrum. Second, the eye is not equally sensitive to all wavelengths; a bulb emitting 1 watt of yellow light appears brighter than one emitting one watt of blue light.

The quantity analogous to radiant power, but compensated to include the above effects, is called luminous flux denoted by F. The unit of luminous flux is the lumen, abbreviated lm, defined as that quantity of light emitted by 1/60 cm² surface area of pure platinum at its melting temperature (about 1770°C), within a solid angle of 1 steradian (1 sr). As an example, the total light output (luminous flux) of a 40-watt incandescent light bulb is about 500 lm, while that of a 40-watt fluorescent tube is about 2300 lm.