| Luminous flux | |
|---|---|
Common symbols | Φv |
| SI unit | lumen |
| InSI base units | cd⋅sr |
| Dimension | |
Inphotometry,luminous flux[1] orluminous power[2] is the measure of the perceived power oflight. It differs fromradiant flux, the measure of the total power ofelectromagnetic radiation (includinginfrared,ultraviolet, and visible light), in that luminous flux is adjusted to reflect the varying sensitivity of thehuman eye to differentwavelengths of light.
TheSI unit of luminous flux is thelumen (lm). One lumen is defined as the luminous flux of light produced by a light source that emits onecandela ofluminous intensity over a solid angle of onesteradian.
In other systems of units, luminous flux may have units ofpower.
The luminous flux accounts for the sensitivity of the eye byweighting the power at each wavelength with theluminosity function, which represents the eye's response to different wavelengths. The luminous flux is aweighted sum of the power at all wavelengths in the visible band. Light outside the visible band does not contribute. The ratio of the total luminous flux to the radiant flux is called theluminous efficacy. This model of thehuman visual brightness perception, is standardized by the CIE andISO.[7]
Luminous flux is often used as an objective measure of the useful light emitted by alight source, and is typically reported on the packaging forlight bulbs, although it is not always prominent. Consumers commonly compare the luminous flux of different light bulbs since it provides an estimate of the apparent amount of light the bulb will produce, and a lightbulb with a higher ratio of luminous flux to consumed power is more efficient.
Luminous flux is not used to comparebrightness, as this is a subjective perception which varies according to the distance from the light source and the angular spread of the light from the source.
Luminous flux of artificial light sources is typically measured using anintegrating sphere, or agoniophotometer outfitted with aphotometer or a spectroradiometer.[8]
| Quantity | Unit | Dimension [nb 1] | Notes | ||
|---|---|---|---|---|---|
| Name | Symbol[nb 2] | Name | Symbol | ||
| Luminous energy | Qv[nb 3] | lumen second | lm⋅s | T⋅J | The lumen second is sometimes called thetalbot. |
| Luminous flux, luminous power | Φv[nb 3] | lumen (= candelasteradian) | lm (= cd⋅sr) | J | Luminous energy per unit time |
| Luminous intensity | Iv | candela (= lumen per steradian) | cd (= lm/sr) | J | Luminous flux per unitsolid angle |
| Luminance | Lv | candela per square metre | cd/m2 (= lm/(sr⋅m2)) | L−2⋅J | Luminous flux per unit solid angle per unitprojected source area. The candela per square metre is sometimes called thenit. |
| Illuminance | Ev | lux (= lumen per square metre) | lx (= lm/m2) | L−2⋅J | Luminous fluxincident on a surface |
| Luminous exitance, luminous emittance | Mv | lumen per square metre | lm/m2 | L−2⋅J | Luminous fluxemitted from a surface |
| Luminous exposure | Hv | lux second | lx⋅s | L−2⋅T⋅J | Time-integrated illuminance |
| Luminous energy density | ωv | lumen second per cubic metre | lm⋅s/m3 | L−3⋅T⋅J | |
| Luminous efficacy (of radiation) | K | lumen perwatt | lm/W | M−1⋅L−2⋅T3⋅J | Ratio of luminous flux toradiant flux |
| Luminous efficacy (of a source) | η[nb 3] | lumen perwatt | lm/W | M−1⋅L−2⋅T3⋅J | Ratio of luminous flux to power consumption |
| Luminous efficiency, luminous coefficient | V | 1 | Luminous efficacy normalized by the maximum possible efficacy | ||
| See also: | |||||
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 1steradian 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, however the luminous flux remains the same.
| Source | Luminous flux (lumens) |
|---|---|
| 37 mW "Superbright" whiteLED | 0.20 |
| 15 mW greenlaser (532 nm wavelength) | 8.4 |
| 1 W high-output white LED | 25–120 |
| Kerosene lantern | 100 |
| 40 Wincandescent lamp at 230 volts | 325 |
| 7 W high-output white LED | 450 |
| 6 WCOB filament LED lamp | 600 |
| 18 Wfluorescent lamp | 1250 |
| 100 W incandescent lamp | 1750 |
| 40 W fluorescent lamp | 2800 |
| 35 W xenon bulb | 2200–3200 |
| 100 W fluorescent lamp | 8000 |
| 127 Wlow pressure sodium vapor lamp | 25,000 |
| 400 Wmetal-halide lamp | 40,000 |
| Values are given for newly manufactured sources. The output from many sources decreases significantly over their lifetime. | |
