Optics 101

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

## 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