They seem to be measuring basically the same thing (luminous flux is restricted to visible light), but luminous flux is measured in lumens and radiant flux is measured in Watts. Is a lumen just a fancy Watt?
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Radiant flux measures how much total electromagnetic energy something emits. Basically, if you recaptured every photon that was coming out, how much energy would you have over time? So it’s measured in watts.
Luminous flux basically adjusts that measurement to account for how the human eye actually perceives light.
So, for example, say you have two laser pointers, a red one and a green one. Both of them emit 1W of power in their respective colors, so they’d have the same radiant flux. But, because the human eye is much more sensitive to green than it is to red, the green laser pointer would have a higher luminous flux and would look brighter.
And if you had a lightbulb that emitted infrared light only, it would still have radiant flux but no luminous flux because our eyes don’t perceive infrared light at all.
So they are related, and calling a lumen a fancy watt is pretty close.
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