radiant intensity

[2.8] I = dΦ/dω (in a given direction)
The radiant flux proceeding from a source per unit solid angle in the given direction; e.g., watts per steradian.

Note: Mathematically, a solid angle must have a point at its apex; the definition of radiant intensity, therefore, strictly applies only to a point source. In practice, however, radiant energy emanating from a source whose dimensions are negligible in comparison with the distance from which it is observed may be considered as coming from a point. Specifically, this implies that with change of distance, 1) the variation in solid angle subtended by the source at the receiving point approaches 1/(distance)2, and 2) the average radiance of the projected source area as seen from the receiving point does not vary appreciably.

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