Illuminance

Not to be confused withLuminance.

Illuminance
Common symbols	Ev
SI unit	lux
Other units	phot,foot-candle
InSI base units	cd·sr·m−2
Dimension	${\mathsf {L}}^{-2}{\mathsf {J}}$

Illuminance diagram with units and terminology

Inphotometry,illuminanceis the totalluminous fluxincident on a surface, per unitarea.It is a measure of how much the incidentlightilluminates the surface, wavelength-weighted by theluminosity functionto correlate with humanbrightnessperception.Similarly,luminous emittanceis the luminous flux per unit area emitted from a surface. Luminous emittance is also known asluminous exitance.

InSI unitsilluminance is measured inlux(lx), or equivalently inlumenspersquare metre(lm·m⁻²).Luminous exitance is measured in lm·m⁻²only, not lux.In theCGSsystem, the unit of illuminance is thephot,which is equal to10000lux.Thefoot-candleis a non-metric unit of illuminance that is used inphotography.

Illuminance was formerly often calledbrightness,but this leads to confusion with other uses of the word, such as to meanluminance."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, at5×10⁻⁵lux(50 μlx), while at the bright end, it is possible to read large text at 10⁸lux (100 Mlx), or about 1000 times that of directsunlight,although this can be very uncomfortable and cause long-lastingafterimages.

Common illuminance levels

Alux meterfor measuring illuminances in work environments

Lighting condition	Foot-candles	Lux
Sunlight	10,000	100,000
Shade on a sunny day	01,000	010,000
Overcast day	00100	001,000
Very dark day	00010	000100
Twilight	00001	000010
Deep twilight	00000.1	000001
Full moon	00000.01	000000.1
Quarter moon	00000.001	000000.01
Starlight	00000.0001	000000.001
Overcast night	00000.00001	000000.0001

Astronomy

Inastronomy,the illuminance stars cast on the Earth's atmosphere is used as a measure of their brightness. The usual units areapparent magnitudesin the visible band.V-magnitudes can be converted to lux using the formula $$E_{\mathrm {v} }=10^{(-14.18-m_{\mathrm {v} })/2.5},$$ whereE_vis the illuminance in lux, andm_vis the apparent magnitude. The reverse conversion is $$m_{\mathrm {v} }=-14.18-2.5\log(E_{\mathrm {v} }).$$

Relation to luminance

Comparison of photometric and radiometric quantities

The luminance of a reflecting surface is related to the illuminance it receives: $$\int _{\Omega _{\Sigma }}L_{\mathrm {v} }\mathrm {d} \Omega _{\Sigma }\cos \theta _{\Sigma }=M_{\mathrm {v} }=E_{\mathrm {v} }R$$ where the integral covers all the directions of emissionΩ_Σ,and

• M_vis the surface'sluminous exitance
• E_vis the received illuminance, and
• Ris thereflectance.

In the case of a perfectlydiffuse reflector(also called aLambertian reflector), the luminance is isotropic, perLambert's cosine law.Then the relationship is simply $$L_{\mathrm {v} }={\frac {E_{\mathrm {v} }R}{\pi }}$$

See also

• Irradiance
• Exposure value
• Luminance

External links

• Illuminance Converter
• Knowledgedoor, LLC (2005)Library of Units and Constants: Illuminance Quantity
• Kodak's guide toEstimating Luminance and Illuminanceusing a camera's exposure meter. Also available inPDF form.

SI photometry quantities

Quantity		Unit		Dimension	Notes
Name	Symbol	Name	Symbol
Luminous energy	Qv	lumen second	lm⋅s	T⋅J	The lumen second is sometimes called thetalbot.
Luminous flux,luminous power	Φv	lumen(= candelasteradian)	lm (= cd⋅sr)	J	Luminous energy per unit time
Luminous intensity	Iv	candela(= lumen per steradian)	cd(= lm/sr)	J	Luminous flux per unitsolid angle
Luminance	Lv	candela per square metre	cd/m2(= lm/(sr⋅m2))	L−2⋅J	Luminous flux per unit solid angle per unitprojectedsource area. The candela per square metre is sometimes called thenit.
Illuminance	Ev	lux(= lumen per square metre)	lx(= lm/m2)	L−2⋅J	Luminous fluxincidenton a surface
Luminous exitance,luminous emittance	Mv	lumen per square metre	lm/m2	L−2⋅J	Luminous fluxemittedfrom a surface
Luminous exposure	Hv	lux second	lx⋅s	L−2⋅T⋅J	Time-integrated illuminance
Luminous energy density	ωv	lumen second per cubic metre	lm⋅s/m3	L−3⋅T⋅J
Luminous efficacy(of radiation)	K	lumen perwatt	lm/W	M−1⋅L−2⋅T3⋅J	Ratio of luminous flux toradiant flux
Luminous efficacy(of a source)	η	lumen perwatt	lm/W	M−1⋅L−2⋅T3⋅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