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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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