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A sphere subtends 4 pi square radians ( steradians ) about the origin.
The solid angle in question is only p / 4 steradian , one-sixteenth of the entire sky.
This is the power of the light that is emitted per projected surface area into a given solid angle, and it is measured in watts per square metre per steradian .
There are also two supplementary units, the radian, and the steradian .
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