Illuminance

A lux meter for measuring illuminances in work environments.

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. 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. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.^[1]

Illuminance

In SI derived units these are measured in lux (lx) or lumens per square metre (cd·sr·m⁻²). In the CGS system, the unit of illuminance is the phot, which is equal to 10000 lux. The foot-candle is a non-metric unit of illuminance that is used in photography.^[2]

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, while at the bright end, it is possible to read large text at 10⁸ lux, or about 1000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting afterimages.

Astronomy

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.^[3] V-magnitudes can be converted to lux using the formula^[4]

$E_\mathrm{v} = 10^{(-14.18-M_\mathrm{v})/2.5}$,

where E_v is the illuminance in lux, and M_v is the apparent magnitude. The reverse conversion is

$M_\mathrm{v} = -14.18-2.5 \log(E_\mathrm{v})$.

See also

• Irradiance
• Exposure value
• Luminance

References

1. Luminous exitance Drdrbill.com
2. One phot = 929.030400001 foot-candles, according to http://www.unitconversion.org/unit_converter/illumination.html
3. Schlyter, Paul. "Radiometry and photmetry in astronomy FAQ, section 7".
4. "Formulae for converting to and from astronomy-relevant units" (PDF). Retrieved Nov 23, 2013.

• Knowledgedoor, LLC (2005) Library of Units and Constants: Illuminance Quantity
• Illuminance Converter

External links

• A Kodak guide to Estimating Luminance and Illuminance using a camera's exposure meter. Also available in PDF form.
• Autodesk Design Academy - Measuring Light Levels

SI photometry quantities

Quantity		Unit		Dimension	Notes
Name	Symbol[nb 1]	Name	Symbol	Symbol
Luminous energy	Qv [nb 2]	lumen second	lm⋅s	T⋅J [nb 3]	Units are sometimes called talbots.
Luminous flux / luminous power	Φv [nb 2]	lumen (= cd⋅sr)	lm	J [nb 3]	Luminous energy per unit time.
Luminous intensity	Iv	candela (= lm/sr)	cd	J [nb 3]	Luminous power per unit solid angle.
Luminance	Lv	candela per square metre	cd/m^2	L^−2⋅J	Luminous power per unit solid angle per unit projected source area. Units are sometimes called nits.
Illuminance	Ev	lux (= lm/m^2)	lx	L^−2⋅J	Luminous power incident on a surface.
Luminous exitance / luminous emittance	Mv	lux	lx	L^−2⋅J	Luminous power emitted from a surface.
Luminous exposure	Hv	lux second	lx⋅s	L^−2⋅T⋅J
Luminous energy density	ωv	lumen second per cubic metre	lm⋅s⋅m^−3	L^−3⋅T⋅J
Luminous efficacy	η [nb 2]	lumen per watt	lm/W	M^−1⋅L^−2⋅T^3⋅J	Ratio of luminous flux to radiant flux or power consumption, depending on context.
Luminous efficiency / luminous coefficient	V			1
See also: SI · Photometry · Radiometry

1. Standards organizations recommend that photometric quantities be denoted with a suffix "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
2. Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
3. "J" here is the symbol for the dimension of luminous intensity, not the symbol for the unit joules.