Why does Luminous intensity get it’s own base SI unit instead of something like W/sq-m, or W/Sr?



The intensity of no other kind of wave (e.g.: sound waves, or tidal waves) gets its own base SI unit, and while I’m not expert, is measured by derived units. Why the special treatment for visible light waves?

In: Physics

The lumen is NOT a base SI unit. The lumen describes the quantity of visible light emitted by a source per unit of time, and is a DERIVED unit based on the candela. The candela is the base unit in the SI system for luminous intensity. One lumen = 1 candela * sr (steradians).

From Wikipedia: “A full sphere has a solid angle of 4π steradians, so a light source that uniformly radiates one candela in all directions has a total luminous flux of 1 cd × 4π sr = 4π cd⋅sr ≈ 12.57 lumens.”

EDIT: Oh, you meant the candela itself! Sorry, I’m removing myself now…