Illuminance

 ( Physical Quantities ) 

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.International Electrotechnical Commission (IEC): International Electrotechnical Vocabulary. ref. 845-21-060, illuminance Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance. Luminous exitance Drdrbill.com

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

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 (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 .

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Common illuminance levels

100,000
10,000
1,000
100
10
1
0.1
0.01
0.001
0.0001

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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. V-magnitudes can be converted to lux using the formula $$E\_\backslash mathrm\{v\}\; =\; 10^\{(-14.18-m\_\backslash mathrm\{v\})/2.5\},$$ where E_v is the illuminance in lux, and m_v is the apparent magnitude. The reverse conversion is $$m\_\backslash mathrm\{v\}\; =\; -14.18\; -\; 2.5\; \backslash log(E\_\backslash mathrm\{v\}).$$

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Relation to luminance The luminance of a reflecting surface is related to the illuminance it receives: $$\backslash int\_\{\backslash Omega\_\backslash Sigma\}\; L\_\backslash mathrm\{v\}\; \backslash mathrm\{d\}\backslash Omega\_\backslash Sigma\; \backslash cos\; \backslash theta\_\backslash Sigma\; =\; M\_\backslash mathrm\{v\}\; =\; E\_\backslash mathrm\{v\}\; R$$ where the integral covers all the directions of emission , and

• _v is the surface's luminous exitance
• _v is the received illuminance, and
• 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 $$L\_\backslash mathrm\{v\}\; =\; \backslash frac\{E\_\backslash mathrm\{v\}\; R\}\{\backslash pi\}$$

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See also

• Irradiance
• Exposure value
• Luminance

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External links

• 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.

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