Operation Manual

10/ 2010 BENNING LUXMETER B
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3.4 Photometric units conversion table
Candela [cd], Lumen [lm], Lux [lux]
lm/m2=lux(lx)
lm/cm2=phot(ph)
lm/ft2=footcandle(fc)
lm/sr=cd
Units fc and lux are dominating in the field.
Conversionbetweenfcandluxismadeaccordingtothefollowingequation:
1fc=10.764lux
3.5 FAQ: Illuminance vs Luminance
Luminanceis ameasureoftheperceivedbrightnessof thesourcewhenyou
look at it. The definition implies a small source, because the energy stream
fromitisdefinedasenergywithinagivensolidangle,independentofdistance
totheobserver.Ifthesourceisverysmall,atinyquartzhalogentorchbulbfor
example,the brightnesswill appearto beintense, evenif itsemission isone
candela. If the source is, like a candle, small but not really a point, you will get
animpressionofasmallareaoflightofmoderatebrightness,eventhoughthe
light intensity is also one candela.
For measuring Luminance, the sensors must be equipped with special optics to
providethedesiredeffectiveviewingangle.
Illuminance is the light flux of one or more different light sources that are strik-
ing a certain area. Only the flux portion orthogonal to the surface is considered.
The viewingangleis 180% (plane) and weightedwiththecosinus correction
function.
Example:
Themoonmaybeconsideredverymuchasapointsource.Ontheotherhand
moonlight, refracted and reflected by the atmosphere, contributes significantly
totheoverallamountoflightreachingtheearth’ssurface.Asaresultmoonlight
is a combination of a point source and a 2 π steradian area source. If you aimed
the detector directly at the moon and tracked the moon‘s path, you would be
measuring the maximum illuminance. Luminance measurements require a
narrow viewing angle (< 4º) in order to satisfy the conditions underlying the
measurement units.