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Mathematically, for the spectral power distribution of a radiant exitance or irradiance one may write: =where M(λ) is the spectral irradiance (or exitance) of the light (SI units: W/m 2 = kg·m −1 ·s −3); Φ is the radiant flux of the source (SI unit: watt, W); A is the area over which the radiant flux is integrated (SI unit: square meter, m 2); and λ is the wavelength (SI unit: meter, m).
In SI units illuminance is measured in lux (lx), or equivalently in lumens per square metre (lm·m −2). [2] Luminous exitance is measured in lm·m −2 only, not lux. [4] In the CGS system, the unit of illuminance is the phot, which is equal to 10 000 lux. The foot-candle is a non-metric unit of illuminance that is used in photography. [5]
In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit.
The lux (symbol: lx) is the unit of illuminance, or luminous flux per unit area, in the International System of Units (SI). [1] [2] It is equal to one lumen per square metre.In photometry, this is used as a measure of the irradiance, as perceived by the spectrally unequally responding human eye, of light that hits or passes through a surface.
The photopic includes the CIE 1931 standard (solid), the Judd-Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). The horizontal axis is wavelength in nm. Photometry is a branch of optics that deals with measuring light in terms of its perceived brightness to the human eye. [1]
Illuminance: E v: lux (= lumen per square metre) lx (= lm/m 2) L −2 ⋅J: Luminous flux incident on a surface Luminous exitance, luminous emittance M v: lumen per square metre lm/m 2: L −2 ⋅J: Luminous flux emitted from a surface Luminous exposure: H v: lux second: lx⋅s L −2 ⋅T⋅J: Time-integrated illuminance Luminous energy ...
Typical photographic scene lit by full moon [7] 10 −2: 5 mcd/m 2: Approximate scotopic/mesopic threshold [8] 10 −1: 10 0: cd/m 2: 2 cd/m 2: Floodlit buildings, monuments, and fountains [9] 10 1: 5 cd/m 2: Approximate mesopic/photopic threshold [8] 25 cd/m 2: Typical photographic scene at sunrise or sunset [7] 30 cd/m 2: Green ...
Luminous efficacy can be normalized by the maximum possible luminous efficacy to a dimensionless quantity called luminous efficiency.The distinction between efficacy and efficiency is not always carefully maintained in published sources, so it is not uncommon to see "efficiencies" expressed in lumens per watt, or "efficacies" expressed as a percentage.