| Illuminance | |
|---|---|
Common symbols | Ev |
| SI unit | lux |
Other units | phot,foot-candle |
| InSI base units | cd·sr·m−2 |
| Dimension | |
Inphotometry,illuminance is the totalluminous flux incident on a surface, per unitarea.[1] It is a measure of how much the incidentlight illuminates the surface, wavelength-weighted by theluminosity function to correlate with humanbrightness perception.[2] Similarly,luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known asluminous exitance.[3][4]
InSI units illuminance is measured inlux (lx), or equivalently inlumens persquare metre (lm·m−2).[2] Luminous exitance is measured in lm·m−2 only, not lux.[4] In theCGS system, the unit of illuminance is thephot, which is equal to10000 lux. Thefoot-candle is a non-metric unit of illuminance that is used inphotography.[5]
Illuminance was formerly often calledbrightness, but this leads to confusion with other uses of the word, such as to meanluminance. "Brightness" should never be used for quantitative description, but only for nonquantitative references to physiological sensations and perceptions of light.
The human eye is capable of seeing somewhat more than a 2 trillion-fold range. The presence of white objects is somewhat discernible under starlight, at5×10−5 lux (50 μlx), while at the bright end, it is possible to read large text at 108 lux (100 Mlx), or about 1000 times that of directsunlight, although this can be very uncomfortable and cause long-lastingafterimages.[citation needed]
| Lighting condition | Foot-candles | Lux |
|---|---|---|
| Sunlight | 10,000[6] | 100,000 |
| Shade on a sunny day | 1,000 | 10,000 |
| Overcast day | 100 | 1,000 |
| Very dark day | 10 | 100 |
| Twilight | 1 | 10 |
| Deep twilight | 0.1 | 1 |
| Full moon | 0.01 | 0.1 |
| Quarter moon | 0.001 | 0.01 |
| Starlight | 0.0001 | 0.001 |
| Overcast night | 0.00001 | 0.0001 |
Inastronomy, the illuminance stars cast on the Earth's atmosphere is used as a measure of their brightness. The usual units areapparent magnitudes in the visible band.[7] V-magnitudes can be converted to lux using the formula[8]whereEv is the illuminance in lux, andmv is the apparent magnitude. The reverse conversion is
The luminance of a reflecting surface is related to the illuminance it receives:where the integral covers all the directions of emissionΩΣ, and
In the case of a perfectlydiffuse reflector (also called aLambertian reflector), the luminance is isotropic, perLambert's cosine law. Then the relationship is simply
| 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: | |||||
Authority control databases: National |
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