Luminous intensity, the quantity of visible light that is emitted in unit time per unit solid angle. The unit for the quantity of light flowing from a source in any one second (the luminous power, or luminous flux) is called the lumen. The lumen is evaluated with reference to visual sensation.

Several measures of light are commonly known as intensity:

Radiant intensity, a radiometric quantity measured in watts per steradian (W/sr)
Luminous intensity, a photometric quantity measured in lumens per steradian (lm/sr), or candela (cd)
Irradiance, a radiometric quantity, measured in watts per square meter (W/m2)
Intensity (physics), the name for irradiance used in other branches of physics (W/m2)
Radiance, commonly called "intensity" in astronomy and astrophysics (W·sr−1·m−2)