Jun 27 2008
Candela
Lumen
Illuminance
Luminance
SI Units for Photometry
Unit Conversions
Candela
Candela (cd) has replaced a standard lamp or candle as a unit of luminous intensity. One candela is defined as the luminous intensity of a light source in a given direction emitting monochromatic radiation in the frequency of 540x1012 Hz and has a radiation intensity of 1/683 Watt per steradian (sr) in that direction.
Lumen
Lumen (lm) is the unit used to describe the amount of light or luminous flux. Lumen is defined as the amount of light streaming outward through one steradian from a uniform point source with an intensity of one candela.
Illuminance
Illuminance is defined as the amount of light or density of luminous flux striking a unit area. Footcandle (fc) means the unit of illuminance on a one square foot area on which there is a uniformly distributed flux of one lumen. On the other hand, the unit of illuminance on a one square meter area on which there is a uniformly distributed flux of one lumen is referred to as lux. One footcandle is equal to 10.76 lux (lx).
Luminance
Luminance is a measure of the density of luminous intensity emitted by a light source in a given direction. Normally, a luminous surface will have a different luminance when observed from different directions.
SI Units for Photometry
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Quantity
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Symbol
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Units
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Equivalent
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Illuminance
|
Ev
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lux (lx)
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lm/m2
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Luminous Flux / Luminous Power
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F / Φv
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lumen (lm)
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cd•sr
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Luminous Emittance / Luminous Exitance
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Mv
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lux (lx)
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lm/m2
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Luminous Intensity
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Iv
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candela (cd)
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lm/sr
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Luminous Energy
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Qv
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talbot
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lm•s
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Luminous Efficacy
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lumen per watt
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lm/W
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Luminance
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Lv
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candela per square metre or nit
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candela /m2 = lm /m2•sr
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Unit Conversions
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Quantity
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Equivalent
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|
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Illuminance
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1 lux (lm/m2) =
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1x10-4 lm/cm2
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9.290x10-2 lm/ft2
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9.290x10-2 foot-candles (fc)
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Luminous Flux
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1 lumen (lm) =
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7.958x10-2 candela (isotropic)
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1.464x10-3 watts at 555nm
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Luminous Intensity
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1 candela (lm/sr) =
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1.464x10-3 watts/sr at 555nm
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12.566 lumens (isotropic)
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Luminous Energy
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1 talbot (lm•s) =
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1.464x10-3 joules at 555nm
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Luminance
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1 candela per square metre or nit (candela /m2 or lm /m2•sr) =
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1x10-4 lm /cm2•sr
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1x10-4 cd/cm2
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9.290x10-2 cd/ft2
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9.290x10-2 lm/ft2•sr
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2.919x10-1 foot-lamberts
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Source: AZoOptics