Radiometric and photometric quantities

Physics has a perfectly good, colour-blind way to measure light: count the energy arriving per second, in watts, exactly as you would for any other form of electromagnetic radiation. That's the radiometric approach, and it's what a power meter or a solar cell responds to — it doesn't care whether the light is red, green, ultraviolet, or infrared, only how much energy is in it.

Human vision doesn't work that way. The eye's sensitivity to light varies enormously with wavelength: it's most sensitive to green light around 555 nanometres, falls off toward both red and blue, and drops to essentially nothing past the edges of the visible spectrum — infrared and ultraviolet carry real radiant power but produce no sensation of brightness at all. Photometric quantities exist specifically to capture what a source looks like to a human observer rather than what a power meter reads. The photometric counterpart to radiant flux (watts) is luminous flux, measured in lumens, and it's built by weighting each wavelength's power by the eye's sensitivity curve before adding it up.

Because that weighting varies so much across the spectrum, there is no single fixed conversion factor between watts and lumens — it depends entirely on the light's colour. The one number worth remembering is the theoretical ceiling: at the eye's peak sensitivity wavelength, one watt of radiant power converts to 683 lumens, which is in fact how the lumen itself is defined. Any other colour converts less efficiently than that, and a source radiating substantial power entirely outside the visible range — a heat lamp, an infrared remote control — can have a large radiant flux and essentially zero luminous flux, because none of that power falls anywhere the eye can register it.