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Radiant Intensity

Radiant intensity is a measure of light energy, both visible and invisible to the human eye, emitted over a set period of time at a specific angle measured in watts per steradians (W/sr). The angle of the steradian are measured from a single imaginary point that is at the center of the radiant source. A steradian is a cone-shaped area, and should be treated as solid. All the light at the end of that cone is measured to determine radiant intensity, and assuming that there is nothing between the light source and the measuring instrument, the amount of radiant intensity measured at any "slice" ending of the cone should be equal to each of the others.

Radiant intensity is used to determine how efficient a lensed source like headlights are, and it can also be used to measure how bright any object appears to be.

Radiant intensity differs from luminous intensity in that radiant intensity measures all the light emitting from an object, while luminous intensity only measures the human-visible light with a benchmark set at 555 nm, the most visible wavelength.

Web Resources On Radiant Intensity

Radiant Intensity Measurements
Notes on LEDs

Book Resources On Radiant Intensity

Theory of Non-classical States of Light by V.V. Dodonov (Ed)
Measurement of radiant energy by William Elmer Forsythe

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