A LuminousFlux
value represents the total amount of light emitted by a
light source. You can think of it as roughly "photons per second", although
it's a bit more complicated than that.
Luminous flux is stored in lumens. It's often used to describe the total output of a light bulb; for example, a 50 watt incandescent bulb and a 6 watt LED bulb might each have an output of 400 lumens.
Quantity Basics.Float Lumens
lumens : Basics.Float -> LuminousFlux
Construct a luminous flux value from a number of lumens. See here and here for the number of lumens emitted by some common light sources.
inLumens : LuminousFlux -> Basics.Float
Convert a luminous flux value to a number of lumens.