Illuminance

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Illuminance diagram with units and terminology

In photometry, illuminance is the total luminous flux incident on a surface, per unit area.<ref>Template:Cite encyclopedia</ref> 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.<ref name="IEC_845-21-060">International Electrotechnical Commission (IEC): International Electrotechnical Vocabulary. ref. 845-21-060, illuminance</ref> Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.<ref>Luminous exitance Drdrbill.com</ref><ref name="IEC_845-21-081" />

In SI units illuminance is measured in lux (lx), or equivalently in lumens per square metre (lm·m⁻²).<ref name="IEC_845-21-060" /> Luminous exitance is measured in lm·m⁻² only, not lux.<ref name="IEC_845-21-081"> International Electrotechnical Commission (IEC): International Electrotechnical Vocabulary. ref. 845-21-081, luminous exitance</ref> In the CGS system, the unit of illuminance is the phot, which is equal to Template:Gaps. The foot-candle is a non-metric unit of illuminance that is used in photography.<ref>One phot = Template:Gaps, according to http://www.unitconversion.org/unit_converter/illumination.html</ref>

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 Template:Val (50 μlx), while at the bright end, it is possible to read large text at 10⁸ lux (100 Mlx), or about 1000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting afterimages.Template:Citation needed

Common illuminance levelsEdit

File:Lux meter.jpg

A lux meter for measuring illuminances in work environments

Lighting condition	Foot-candles	Lux
Sunlight	citation	CitationClass=web }}</ref> \|\| 100,000
Shade on a sunny day	Template:01,000	Template:010,000
Overcast day	Template:0100	Template:01,000
Very dark day	Template:010	Template:0100
Twilight	Template:01	Template:010
Deep twilight	Template:00.1	Template:01
Full moon	Template:00.01	Template:00.1
Quarter moon	Template:00.001	Template:00.01
Starlight	Template:00.0001	Template:00.001
Overcast night	Template:00.00001	Template:00.0001

AstronomyEdit

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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> V-magnitudes can be converted to lux using the formula<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> <math display="block">E_\mathrm{v} = 10^{(-14.18-m_\mathrm{v})/2.5},</math> where E_v is the illuminance in lux, and m_v is the apparent magnitude. The reverse conversion is <math display="block">m_\mathrm{v} = -14.18 - 2.5 \log(E_\mathrm{v}).</math>

Relation to luminanceEdit

File:Photometry radiometry units.svg

Comparison of photometric and radiometric quantities

The luminance of a reflecting surface is related to the illuminance it receives: <math display="block">\int_{\Omega_\Sigma} L_\mathrm{v} \mathrm{d}\Omega_\Sigma \cos \theta_\Sigma = M_\mathrm{v} = E_\mathrm{v} R</math> where the integral covers all the directions of emission Template:Math, and

Template:Var_v is the surface's luminous exitance
Template:Var_v is the received illuminance, and
Template:Var 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 <math display="block">L_\mathrm{v} = \frac{E_\mathrm{v} R}{\pi}</math>

ReferencesEdit

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External linksEdit

Illuminance Converter Template:Webarchive
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.

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Quantity		Unit		Dimension <ref group="{{#if:\|{{{3}}}\|nb}}" name="note-dimension-symbol">The symbols in this column denote dimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule.</ref>	Notes
Name	Symbol<ref group="{{#if:\|{{{3}}}\|nb}}" name="note-suffix-v">Standards organizations recommend that photometric quantities be denoted with a subscript "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967</ref>	Name	Symbol		Notes
Luminous energy	Template:Math<ref group="{{#if:\|{{{3}}}\|nb}}" name="note-alternative-symbol-photometric">Alternative symbols sometimes seen: Template:Mvar for luminous energy, Template:Mvar or Template:Mvar for luminous flux, and Template:Mvar for luminous efficacy of a source.</ref>	lumen second	lm⋅s	T⋅J	The lumen second is sometimes called the talbot.
Luminous flux, luminous power	Template:Math<ref group="{{#if:\|{{{3}}}\|nb}}" name="note-alternative-symbol-photometric"></ref>	lumen (= candela steradian)	lm (= cd⋅sr)	J	Luminous energy per unit time
Luminous intensity	Template:Math	candela (= lumen per steradian)	cd (= lm/sr)	J	Luminous flux per unit solid angle
Luminance	Template:Math	candela per square metre	cd/m² (= lm/(sr⋅m²))	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	Template:Math	lux (= lumen per square metre)	lx (= lm/m²)	L⁻²⋅J	Luminous flux incident on a surface
Luminous exitance, luminous emittance	Template:Math	lumen per square metre	lm/m²	L⁻²⋅J	Luminous flux emitted from a surface
Luminous exposure	Template:Math	lux second	lx⋅s	L⁻²⋅T⋅J	Time-integrated illuminance
Luminous energy density	Template:Math	lumen second per cubic metre	lm⋅s/m³	L⁻³⋅T⋅J
Luminous efficacy (of radiation)	Template:Math	lumen per watt	lm/W	M⁻¹⋅L⁻²⋅T³⋅J	Ratio of luminous flux to radiant flux
Luminous efficacy (of a source)	Template:Mvar<ref group="{{#if:\|{{{3}}}\|nb}}" name="note-alternative-symbol-photometric"></ref>	lumen per watt	lm/W	M⁻¹⋅L⁻²⋅T³⋅J	Ratio of luminous flux to power consumption
Luminous efficiency, luminous coefficient	Template:Mvar			1	Luminous efficacy normalized by the maximum possible efficacy
See also: Template:Hlist

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