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Unformatted text preview: ize of the sphere. The intensity is the same
at every sheet in the pic below. In a sphere 7 m in radius, I is the flux that falls
on a 49 m2 surface on the sphere. The SI unit for intensity is the candela, cd.
1 cd = 1 lumen per steradian. A footcandle is the
illuminance one foot away from a 1 candela source. P 1 m 2 m 3 m 4 m Efficiency of light sources
Light sources, like light bulbs, vary in efficiency. This means
that some bulbs, e.g. fluorescent bulbs, will produce more light
while using less energy. (They can do this by producing less
waste heat.) The efficiency of a simple machine is the work
done by the machine divided by the work put into it. In this
context, efficiency is the rate at which light is produced by the
bulb divided by the rate at which energy is used to produce that
light: eff = Ø / P, where P is power. Note that both flux and
power are rates, so eff is really “light over energy.” It is
measured in lumens per watt. A typical candle has an efficiency
of about 0.1 lumen /...
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- Fall '11