luminous intensity
n.
The luminous flux density per solid angle as measured in a given direction relative to the emitting source.
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The luminous flux density per solid angle as measured in a given direction relative to the emitting source.
The solid angular luminous flux density in a given direction from a light source. It may be considered as the luminous flux on a small surface normal to the given direction, divided by the solid angle (in steradians) which the surface subtends at the source of light. Since the apex of a solid angle is a point, this concept applies exactly only to a point source. The size of the source, however, is often extremely small when compared with the distance from whichit is observed, so in practice the luminous flux coming from such a source may be taken as coming from a point. See also Candlepower; Photometry.
The luminous flux per unit solid angle in a specific direction from a point source of light; in practice, an interior source may be considered a point source if the distance exceeds 5 to 10 times the maximum source dimension of the luminaire; in US Customary units, expressed in candlepower; in SI units, expressed in candelas.
In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction, 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.
Photometry deals with the measurement of visible light as perceived by human eyes. The human eye
can only see light in the visible spectrum and has different sensitivities to
light of different wavelengths within the spectrum. When adapted for bright conditions
(photopic vision), the eye is most sensitive to greenish-yellow light at 555 nm.
Light with the same radiant intensity at other wavelengths has a lower luminous
intensity. The curve which measures the response of the human eye to light is a defined standard, known as the luminosity function. This curve, denoted V(λ) or
, is based on an average
of widely differing experimental data from scientists using different measurement techniques. For instance, the measured
responses of the eye to violet light varied by a factor of ten.
Luminous intensity should not be confused with another photometric unit, luminous flux, which is the total perceived power emitted in all directions. Luminous intensity is the perceived power per unit solid angle. Luminous intensity is also not the same as the radiant intensity, the corresponding objective physical quantity used in the measurement science of radiometry.
One candela is defined as the luminous intensity of a monochromatic 540 THz light source that has a radiant intensity of 1/683 watts per steradian, or about 1.464 mW/sr. The 540 THz frequency corresponds to a wavelength of about 555 nm, which is green light near the peak of the eye's response. Since there are about 12.6 steradians in a sphere, the total radiant flux would be about 18.40 mW, if the source emitted uniformly in all directions. A typical candle produces very roughly one candela of luminous intensity.
In 1881, Jules Violle proposed the Violle as a unit of luminous intensity, and it was notable as the first unit of light intensity that did not depend on the properties of a particular lamp. It was superseded by the candela in 1946.
The luminous intensity for monochromatic light of a particular wavelength λ is given by

where
is the
standard luminosity function.If more than one wavelength is present (as is usually the case), one must sum or integrate over the spectrum of wavelengths present to get the luminous intensity:

| Quantity | Symbol | SI unit | Abbr. | Notes | ||||
|---|---|---|---|---|---|---|---|---|
| Luminous energy | Qv | lumen second | lm·s | units are sometimes called talbots | ||||
| Luminous flux | F | lumen (= cd·sr) | lm | also called luminous power | ||||
| Luminous intensity | Iv | candela (= lm/sr) | cd | an SI base unit | ||||
| Luminance | Lv | candela per square metre | cd·m–2 | units are sometimes called nits | ||||
| Illuminance | Ev | lux (= lm·m–2) | lx | Used for light incident on a surface | ||||
| Luminous emittance | Mv | lux (= lm·m–2) | lx | Used for light emitted from a surface | ||||
| Luminous efficacy | - - - - | lumen per watt | lm/W | ratio of luminous flux to radiant flux; maximum possible is 683.002 | ||||
| Units-interrelation diagram (full page) | ||||||||
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