Measurement of Spectral Radiant Flux

Where dΦ(λ) is the radiant power in a wavelength interval dλ emitted into solid angle of 4π steradians and dλ the wavelength interval.

Symbol: Φλ

Unit: W∙nm-1Accounting for the total optical emission of a sources, spectral radiant flux is often used to characterise luminous efficiency of sources of light and the optics used with these light sources to constitute a luminaire. In the labelling of sources, the photometric equivalent, lumens, must be reported in the largest font alongside information on correlated colour temperature, most accurately obtained from this measurement quantity.


There are two manners to measure spectral radiant flux. Traditionally, an Ulbricht, or integrating sphere, is used to collect light over 4π steradians of emission. This has the convenience of a quick measurement and by the nature of the measurement, colour variation with angle can be averaged out. One problem is that the size of the sphere should be adapted to the source under test.

Spectral radiant flux may also be computed from a goniophotometric measurement and results from the product of radiant intensity and solid angle.

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Category: Entrance Optics