Brightness Unit - Definitions, Units and Conversions of Commonly Used Brightness Units

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

Quantity

Symbol

Units

Equivalent

Illuminance

Ev

lux (lx)

lm/m2

Luminous Flux / Luminous Power

F / Φv

lumen (lm)

cd•sr

Luminous Emittance / Luminous Exitance

Mv

lux (lx)

lm/m2

Luminous Intensity

Iv

candela (cd)

lm/sr

Luminous Energy

Qv

talbot

lm•s

Luminous Efficacy

 

lumen per watt

lm/W

Luminance

Lv

candela per square metre or nit

candela /m2 = lm /m2•sr

Unit Conversions

Quantity

Equivalent

 

Illuminance

1 lux (lm/m2) =

1x10-4 lm/cm2

 

9.290x10-2 lm/ft2

 

9.290x10-2 foot-candles (fc)

 

Luminous Flux

1 lumen (lm) =

7.958x10-2 candela (isotropic)

 

1.464x10-3 watts at 555nm

 

Luminous Intensity

1 candela (lm/sr) =

1.464x10-3 watts/sr at 555nm

 

12.566 lumens (isotropic)

 

Luminous Energy

1 talbot (lm•s) =

1.464x10-3 joules at 555nm

 

Luminance

1 candela per square metre or nit (candela /m2 or lm /m2•sr) =

1x10-4 lm /cm2•sr

 

1x10-4 cd/cm2

 

9.290x10-2 cd/ft2

 

9.290x10-2 lm/ft2•sr

 

2.919x10-1 foot-lamberts

Source: AZoOptics

Date Added: Jun 27, 2008
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