# Luminous intensity

luminous intensity
Photopic (black) and scotopic (green) luminosity 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.
Common symbols
Iv
SI unitcandela
Other units
In SI base unitscd
DimensionJ

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.

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 ${\displaystyle \textstyle {\overline {y}}(\lambda )}$, 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[citation needed] .

## Relationship to other measures

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. 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 intensity of 1 candela. If the optics were changed to concentrate the beam into 1/2 steradian then the source would have a luminous intensity of 2 candela. The resulting beam is narrower and brighter, though its luminous flux remains unchanged.

Luminous intensity is also not the same as the radiant intensity, the corresponding objective physical quantity used in the measurement science of radiometry.

Other Languages
беларуская (тарашкевіца)‎: Сіла сьвятла
Boarisch: Liachtstärkn
čeština: Svítivost
dansk: Lysstyrke
Ελληνικά: Φωτεινή ένταση
Esperanto: Lumintenso
فارسی: شدت نور
한국어: 광도
հայերեն: Լույսի ուժ
Bahasa Indonesia: Intensitas cahaya
қазақша: Жарық күші
lietuvių: Šviesos stipris
македонски: Светлосна јачина
Bahasa Melayu: Keamatan berluminositi

Nordfriisk: Laachtmäächt
norsk: Lysstyrke
norsk nynorsk: Lysstyrke
Plattdüütsch: Lichtstärk (Fotometrie)
русиньскый: Світивость
русский: Сила света
slovenščina: Svetilnost
suomi: Valovoima
svenska: Ljusstyrka
Türkçe: Işık şiddeti
українська: Сила світла