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Photopic (black) and scotopic (green) luminous efficiency functions. [c 1] The photopic includes the CIE 1931 standard [c 2] (solid), the Judd–Vos 1978 modified data [c 3] (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data [c 4] (dotted). The horizontal axis is wavelength in nm. A luminous efficiency function or luminosity function ...
The luminous efficacy of the source is a measure of the efficiency of the device with the output adjusted to account for the spectral response curve (the luminosity function). When expressed in dimensionless form (for example, as a fraction of the maximum possible luminous efficacy), this value may be called luminous efficiency of a source ...
Within the visible spectrum, wavelengths of light are weighted according to a function called the "photopic spectral luminous efficiency." According to this function, 700 nm red light is only about 0.4% as efficient as 555 nm green light.
The 26th General Conference on Weights and Measures (CGPM) redefined the candela in 2018. [10] [11] The new definition, which took effect on 20 May 2019, is: The candela [...] is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10 12 Hz, [a] K cd, to be 683 when expressed in the unit lm W −1, which is equal to cd sr W −1 ...
When not specified, the luminous efficiency function generally refers to the photopic luminous efficiency function. Photopic (black) and scotopic (green) luminous efficiency functions. [c 1] The photopic includes the CIE 1931 standard [c 2] (solid), the Judd–Vos 1978 modified data [c 3] (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 ...
In photometry, luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux , the measure of the total power of electromagnetic radiation (including infrared , ultraviolet , and visible light), in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different ...
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.
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).