Lux Lux Lumen Category="Photometry"Category="SI derived units"Category="Units of illuminance"

: For other meanings, see Lux (disambiguation).

The lux (symbol: lx) is the SI unit of Illuminance.

Definition

1 lx = 1 lm/m² = 1 m²·m^–4·cd

SI multiples

Multiple	Name	Symbol	Multiple	Name	Symbol
10⁰	lux	lx
10¹	decalux	dalx	10^–1	decilux	dlx
10²	hectolux	hlx	10^–2	centilux	clx
10³	kilolux	klx	10^–3	millilux	mlx
10⁶	megalux	Mlx	10^–6	microlux	µlx
10⁹	gigalux	Glx	10^–9	nanolux	nlx
10¹²	teralux	Tlx	10^–12	picolux	plx
10¹⁵	petalux	Plx	10^–15	femtolux	flx
10¹⁸	exalux	Elx	10^–18	attolux	alx
10²¹	zettalux	Zlx	10^–21	zeptolux	zlx
10²⁴	yottalux	Ylx	10^–24	yoctolux	ylx

Explanation

It is equal to one lumen per Square metre.

sunlight on an average day ranges from 32 000 (32 klx) to 100 000 lux (100 klx)
TV studios are lit at about 1 000 lux (1 klx)
a bright office has about 400 lux of illumination
At Sunset and Sunrise, ambient outdoor light is also about 400 lux (if the sky is clear).
moonlight represents about 1 lux
starlight measures a mere 0.000 05 lux (= 50 μlx)

Trivia: Unicode has a symbol for "lx": (㏓), but this is just a legacy code to accommodate old code pages in certain Asian languages, and it is not recommended for use in any language today.

Lux versus lumen

The difference between the lux and the lumen is that the lux takes into account the area over which the luminous flux is spread. 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux.

Achieving an illuminance of 500 lux might be possible in a home kitchen with a single fluorescent light fixture with an output of 12000 lumens. To light a factory floor with dozens of times the area of the kitchen would require dozens of such fixtures. Thus, lighting a larger area with the same number of lux requires a larger number of lumens.

Relationship between illuminance and power

Illuminance is not a direct measure of the energy of light, but rather of the illumination it provides as perceived by the human Eye. Thus, the conversion factor varies with the Wavelength composition or Color temperature of light. At 555 nm, the middle of the visible spectrum, one lux is equal to 1.46 mW/m².

SI photometry units

SI photometry units
Quantity SI unit Symbol Notes
Luminous energy lumen seconds lm · s lumen seconds are sometimes called Talbots
Luminous flux lumen or (candela · Steradian) lm also called luminous power
Luminous intensity Candela or (lumen / steradian) cd
Luminance candela / Square metre cd/m² also called luminosity
Illuminance lux or (lumen / square metre) lx
Luminous efficacy lumens per Watt lm/W maximum possible (@ 555 nm) is 683