Illuminance
Common symbols	E v
SI unit	lux
Other units	phot , foot-candle
In SI base units	cd · sr · m ?2
Dimension	${\displaystyle {\mathsf {L}}^{-2}{\mathsf {J}}}$

In photometry , illuminance is the total luminous flux incident on a surface, per unit area . ^[1] It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. ^[2] Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance . ^{[3] [4]}

In SI units illuminance is measured in lux (lx), or equivalently in lumens per square metre ( lm · m ^?2 ). ^[2] Luminous exitance is measured in lm·m ^?2 only, not lux. ^[4] In the CGS system, the unit of illuminance is the phot , which is equal to 10 000  lux . The foot-candle is a non-metric unit of illuminance that is used in photography . ^[5]

Illuminance was formerly often called brightness , but this leads to confusion with other uses of the word, such as to mean luminance . "Brightness" should never be used for quantitative description, but only for nonquantitative references to physiological sensations and perceptions of light.

The human eye is capable of seeing somewhat more than a 2 trillion-fold range. The presence of white objects is somewhat discernible under starlight, at 5 × 10 ^?5  lux (50 μlx), while at the bright end, it is possible to read large text at 10 ⁸ lux (100 Mlx), or about 1000 times that of direct sunlight , although this can be very uncomfortable and cause long-lasting afterimages . ^{[ citation needed ]}

Common illuminance levels [ edit ]

Lighting condition	Foot-candles	Lux
Sunlight	10,000 [6]	100,000
Shade on a sunny day	0 1,000	0 10,000
Overcast day	00 100	00 1,000
Very dark day	000 10	000 100
Twilight	0000 1	0000 10
Deep twilight	0000 0.1	00000 1
Full moon	0000 0.01	00000 0.1
Quarter moon	0000 0.001	00000 0.01
Starlight	0000 0.0001	00000 0.001
Overcast night	0000 0.00001	00000 0.0001

Astronomy [ edit ]

In astronomy , the illuminance stars cast on the Earth's atmosphere is used as a measure of their brightness. The usual units are apparent magnitudes in the visible band. ^[7] V-magnitudes can be converted to lux using the formula ^[8]

where E _v is the illuminance in lux, and m _v is the apparent magnitude. The reverse conversion is

Relation to luminance [ edit ]

The luminance of a reflecting surface is related to the illuminance it receives:

where the integral covers all the directions of emission Ω _Σ , and

• M _v is the surface's luminous exitance
• E _v is the received illuminance, and
• R is the reflectance .

In the case of a perfectly diffuse reflector (also called a Lambertian reflector ), the luminance is isotropic, per Lambert's cosine law . Then the relationship is simply

See also [ edit ]

• Irradiance
• Exposure value
• Luminance

References [ edit ]

External links [ edit ]

• Illuminance Converter
• Knowledgedoor, LLC (2005) Library of Units and Constants: Illuminance Quantity
• Kodak's guide to Estimating Luminance and Illuminance using a camera's exposure meter. Also available in PDF form .

SI photometry quantities

• v
• t
• e

Quantity		Unit		Dimension [nb 1]	Notes
Name	Symbol [nb 2]	Name	Symbol
Luminous energy	Q v [nb 3]	lumen second	lm ?s	T ? J	The lumen second is sometimes called the talbot .
Luminous flux , luminous power	Φ v [nb 3]	lumen (= candela steradian )	lm (= cd?sr)	J	Luminous energy per unit time
Luminous intensity	I v	candela (= lumen per steradian)	cd (= lm/sr)	J	Luminous flux per unit solid angle
Luminance	L v	candela per square metre	cd/m 2 (= lm/(sr?m 2 ))	L ?2 ? J	Luminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit .
Illuminance	E v	lux (= lumen per square metre)	lx (= lm/m 2 )	L ?2 ? J	Luminous flux incident on a surface
Luminous exitance , luminous emittance	M v	lumen per square metre	lm/m 2	L ?2 ? J	Luminous flux emitted from a surface
Luminous exposure	H v	lux second	lx?s	L ?2 ? T ? J	Time-integrated illuminance
Luminous energy density	ω v	lumen second per cubic metre	lm?s/m 3	L ?3 ? T ? J
Luminous efficacy (of radiation)	K	lumen per watt	lm/ W	M ?1 ? L ?2 ? T 3 ? J	Ratio of luminous flux to radiant flux
Luminous efficacy (of a source)	η [nb 3]	lumen per watt	lm/ W	M ?1 ? L ?2 ? T 3 ? J	Ratio of luminous flux to power consumption
Luminous efficiency , luminous coefficient	V			1	Luminous efficacy normalized by the maximum possible efficacy
See also: SI Photometry Radiometry