Light StandardsDefinitions and Formulas The intensity of visible light is measured in units of candela or candles. The rate of flow of lightenergy (luminous flux) is measured in lumens. One lumen is the flux on one square foot of a sphere, onefoot in radius with a light source of one candle at the center and radiating uniformly in all directions. Onelux is a unit of illumination equal to one lumen per square meter. The foot-candle is the directmeasurement of visible radiation falling on a surface. The foot-lambert is the unit measure of physicalbrightness on any surface emitting or reflecting visible light.Foot - candl esX10. 764= l umens/ sq. met er = l uxLumens/ sq. f t . X1= f oot - candl esLumens/ sq. f t . X10. 764= l umens/ sq. met erLumensX0. 07958= spher i cal candl epowerLuxX0. 0929= f oot - candl esLamber t X0. 3183= candl es/ sq. cmLamber t X295. 720= candl essq. f t .Lamber t X1= l umens/ sq. cm Luminance is a measure of photometric flux density per unit solid angle. Luminance is typicallyexpressed in candela per square meter (lumens per square meter per steradian), or foot-lamberts (1/lumens per square foot per steradian).Radiance is a measure of radiometric flux density per unit solid angle (all radiant energy, not justvisible). Radiance is typically expressed in W/cm2srIrradianceis a measure of radiometric flux per unit area, or flux density. Irradiance is typicallyexpressed in W/cm2 or W/m2.1 cd/m2 =1 lm/m2/sr =1 nit =10-4 lm/cm2/sr =10-4 cd/cm2 =10-4 stilb (sb) =9.290 x 10-2 cd/ft2 =9.290 x 10-2 lm/ft2/sr =3.142 x 10-4 lamberts (L) =2.919 x 10-1 foot-lamberts (fL)1 W/cm2/sr =6.83 x 106 lm/m2/sr at 555 nm =683 cd/cm2 at 555 nmIlluminance is a measure of photometric flux per unit area. Illuminance is typically expressed in lux(lumens per square meter) or foot-candles (lumens per square foot).Radiant Flux is a measure of radiometric power. Radiometric flux, expressed in watts, includes lightat all wavelengths of the spectrum; ultraviolet (200 to 400 nm), visible (400 to 700 nm), and infrared (700to 30000 nm).Luminous Flux is a measure of photometric power. Photometric flux, expressed in lumens, includesonly visible light, and is weighted to match the responsivity of the human eye, which is most sensitive toLight Definitions http://fiziks.net/webpages/lightdef.htm1 of 2 13/05/2015 9:38 PMyellow-green.RADI ANTFLUX= 1Wat t= 0. 27l mat 400nm= 25. 9l mat 450nm= 220. 0l mat 500nm= 679. 0l mat 550nm= 683. 0l mat 555nm= 430. 0l mat 600nm= 73. 0l mat 650nm= 2. 78l mat 700nm Luminous Efficacyis the total luminous flux divided by the total radiant flux or in the case of alamp bulb, the total light flux in lumens divided by the electrical power input in watts. It is measured inlumens per watt.THE LIGHT HANDBOOKLight Definitions http://fiziks.net/webpages/lightdef.htm2 of 2 13/05/2015 9:38 PM