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.
