Tech Note 1-1: Light Energy/Brightness Units and Conversions
 

Lumitex uses Foot-Lamberts (Ft-L or Fl) as our standard brightness measurement.  This is a unit of Luminance (Visible Flux Density per Solid Angle)

Candela, cd (formerly candle) is the unit of luminous intensity of a light source.  One candela is defined as the luminous intensity in a given direction, of a source that emits monochromatic radiation of frequency 540EE+12 Hz (approximately 555 nm) and of which the radiant intensity in that direction 1/683 W per steradian

Lumen, lm is the unit of luminous flux.  It is equal to the flux on a unit surface all points of which are one unit distance from a uniform point source of one candela.  Such a point source emits 4 pi lumens.

Illuminance is the density of the luminous flux on a surface.  If the foot is taken as the unit of length and the flux is uniformly distributed over the surface, the density in lumens per square foot is called footcandles (fc), in SI units lumens per square meter, lux (lx) is used (One footcandle equals 10.76 lux.) 

Luminance (formerly photometric brightness) is the luminance intensity of any surface in a given direction per unit of projected area of the surface as viewed from that direction.  The unit of luminance is candela/in2, in SI units cd/m2) is used.  (1 cd/in2 = 1,550 cd/m2.)  In general, a luminous surface will have a different luminance when viewed from different angles.  For a perfectly diffuse surface, 1 cd/m2 = 452 lm/ft2 = 452 foot-lamberts.
 
 

Photometric Units:

Luminance: (Visible Flux Density per Solid Angle)
 

1 lumen per meter squared per steradian (lm/m2/sr) =

 
 

1 candela per meter squared (cd/m2)
 

1 nit
 

1EE-04 lm/cm2/sr
 

1EE-04 cd/m2
 

1EE-04 stilb (sb)
 

9.290EE-02 cd/ft2
 

9.290EE-02 lm/ft2/sr
 

3.142 apostilbs (asb)
 

3.142EE-04 lamberts (L)
 

2.919EE-01 foot-lamberts (fL or Ft-L)
   
  Illuminance (Visible Flux Density)
 

1 lumen per meter squared (lm/m2) =

 
 

1 lux (lx)
 

1EE-04 lm/cm2
 

1EE-04 phot (ph)
 

9.290EE-02 lm/ft2
 

9.290EE-02 foot-candles (fc)
   
  Luminous Intensity (Visible Flux per Solid Angle)
 

1 lumen per steradian (lm/sr) =

 
 

1 candela (cd)
 

12.566 lumens (isotropic)
 

1.464EE-03 watts/sr at 555nm
   
  Energy (Visible Flux * Time)
 

1 lumen per second (lm/s) =

 
 

1 talbot (T)
 

1.464EE-03 joules at 555nm
   
  Luminous Flux (Visible Flux)
 

1 lumen (lm) =

  
 

1.464EE-03 watts at 555nm
 

7.958EE-02 candela (isotropic)
 
 

Radiometric Units

Radiant Flux (Flux)
 

1 Watt (W) =

 
 

1 joule/second
 

1EE07 erg/second
 

0.27 lm at 400 nm
 

25.9 lm at 450 nm (blue)
 

220 lum at 500 nm
 

679 lm at 550 nm (green)
 

683 lm at 555 nm (CIE peak)
 

430 lm at 600 nm

 

73 lm at 650 nm (red)
 

2.78 lm at 700 nm
 

14.33 gram*calorie/minute
   
  Energy (Flux * Time)
 

1 joule (J) =

 
 

1EE07 erg
 

1 watt * second
 

0.2388 gram * calories
   
  Radiant Intensity (Flux per Solid Angle)
 

1 W/sr =

 
 

12.566 watts (isotropic)
 

683 candela at 555 nm
   
  Irradiance (Flux Density)
 

1 W/cm2 =

 
 

1EE+04 watts/m2
 

6.83EE+06 lux at 555 nm
 

14.33 gram * calories/cm2/min
 

14.33 langley/min
   
  Radiance (Flux Density per Solid Angle)
 

1 W/cm2/sr =

  
 

6.83EE+06 lm/m2/sr at 555 nm
 

683 cd/cm2 at 555 nm

 
 
 

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