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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.
Luminosity is an absolute measure of radiated electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. [1] [2] In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a star, galaxy, or other astronomical objects. [3] [4]
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 ...
Luminous intensity, a photometric quantity measured in lumens per steradian (lm/sr), or candela (cd) Irradiance, a radiometric quantity, measured in watts per square meter (W/m 2) Intensity (physics), the name for irradiance used in other branches of physics (W/m 2) Radiance, commonly called "intensity" in astronomy and astrophysics (W·sr −1 ...
Luminous flux (in lumens) is a measure of the total amount of light a lamp puts out. The luminous intensity (in candelas) is a measure of how bright the beam in a particular direction is. If a lamp has a 1 lumen bulb and the optics of the lamp are set up to focus the light evenly into a 1 steradian beam, then the beam would have a luminous ...
This is sometimes also confusingly called "intensity". Spectral radiosity: J e,ν [nb 3] watt per square metre per hertz W⋅m −2 ⋅Hz −1: M⋅T −2: Radiosity of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m −2 ⋅nm −1. This is sometimes also confusingly called "spectral intensity". J e,λ [nb 4]
In physics and many other areas of science and engineering the intensity or flux of radiant energy is the power transferred per unit area, where the area is measured on the plane perpendicular to the direction of propagation of the energy.
This is sometimes also confusingly called "intensity". Spectral radiosity: J e,ν [nb 3] watt per square metre per hertz W⋅m −2 ⋅Hz −1: M⋅T −2: Radiosity of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m −2 ⋅nm −1. This is sometimes also confusingly called "spectral intensity". J e,λ [nb 4]