Illuminance

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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 the intensity of the incident light, 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.

In SI derived units, these are both measured in lux (lx) or lumens per square metre (cd·sr·m⁻²). In the CGS system, the unit of illuminance is the phot. One phot is equal to 10,000 lux. The foot-candle is a non-metric unit of illuminance that is used in photography.

Illuminance was formerly often called brightness, but this leads to confusion with other uses of the word. "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 1,000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting afterimages.^{[citation needed]}

[edit] See also

[edit] References

Knowledgedoor, LLC (2005). Library of Units and Constants: Illuminance Quantity. Retrieved 2006-07-21.
Illuminance Converter. Retrieved 2006-07-21.

[edit] External links

A Kodak guide to Estimating Luminance and Illuminance using a camera's exposure meter. Also available in PDF form.

SI photometry units

Quantity	Symbol	SI unit	Abbr.	Notes
Luminous energy	Q_v	lumen second	lm·s	units are sometimes called talbots
Luminous flux	F	lumen (= cd·sr)	lm	also called luminous power
Luminous intensity	I_v	candela (= lm/sr)	cd	an SI base unit
Luminance	L_v	candela per square metre	cd/m²	units are sometimes called nits
Illuminance	E_v	lux (= lm/m²)	lx	Used for light incident on a surface
Luminous emittance	M_v	lux (= lm/m²)	lx	Used for light emitted from a surface
Luminous efficacy		lumen per watt	lm/W	ratio of luminous flux to radiant flux
SI • Photometry

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Categories: Physical quantities | Photometry