[3.4]
(of a point source of light in a given direction)
$I = d\Phi /d\omega$

The luminous flux per unit solid angle in the direction in question.Hence, it is the luminous flux on a small surface centered on and normal to that direction, divided by the solid angle (in steradians) that the surface subtends at the source. Luminous intensity may be expressed in candelas (cd) or in lumens per steradian (lm/sr).

Note: Mathematically, a solid angle must have a point as its apex; the definition of luminous intensity, therefore, applies strictly only to a point source. In practice, however, light 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 luminance of the projected source area as seen from the receiving point does not vary appreciably. (For extended sources, see equivalent luminous intensity of an extended source at a specified distance.)

The word intensity as defined above is used to designate luminous intensity (or candlepower). It is also widely used in other ways, either informally or formally, in other disciplines. Stimulus intensity may be used to designate the retinal illuminance of a proximal stimulus (see proximal stimuli) or the luminance of a distal stimulus (see distal stimuli). Intensity is used in the same sense with other modulates, such as audition. Intensity has been used to designate the level of illuminance on a surface, or the flux density in the cross-section of a beam of light. In physical optics, intensity usually refers to the square of the wave amplitude.

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