Radiant & Luminous Flux
Watts measure energy; lumens measure how bright that energy looks.
Lesson at a glanceShow the key points
- Total radiant flux: energy emitted per unit time at all wavelengths (watts).
- Equal watts of different colours don't look equally bright: the eye is most sensitive near 555 nm (yellow-green) and the sensitivity falls off gradually on both sides.
- Luminous flux measures the brightness-producing capacity; unit lumen. 1 W of 555 nm light = 685 lumen.
- Relative luminosity = luminous flux of a source of a given wavelength ÷ that of a 555 nm source of the same power.
Two lamps both radiate 10 watts of light, one yellow and one red. The yellow one looks far brighter. Photometry is the measurement of light as the human eye judges it.
Total radiant flux
The eye’s sensitivity
How bright light looks depends on its wavelength as well as its power. Under normal lighting, an average eye is most sensitive near 555 nm (yellow-green), and the sensitivity falls off gradually toward violet and red. There is no sharp edge to the visible range: radiation is visible wherever this luminosity isn’t zero.
Luminous flux and the lumen
Luminous flux measures a source’s capacity to produce the sensation of brightness. Its unit is the lumen (lm):
A 5.0 W source emits monochromatic light of 600 nm, where the relative luminosity is 0.60. Find its luminous flux.
A source emits 10 W at 630 nm (relative luminosity 0.27) and 6.0 W at 570 nm (0.95). Find its radiant flux and luminous flux.
- 1Total radiant flux: energy per second at all wavelengths (W).
- 2The eye is most sensitive near 555 nm; relative luminosity falls off on both sides.
- 3Luminous flux in lumens: 1 W at 555 nm = 685 lm; multiply by the relative luminosity for other wavelengths.
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