Definitions

• Radiant flux (W)

  • the rate at which light energy is emitted

• Steradian (sr)

  • a unit of solid (3D) angle (there are 4 p steradians in a sphere)

• Radiant Intensity I (W/sr)

  • the rate that light energy is radiated through a given solid angle

• Radiance L(W/(sr m2))

  • the rate of energy radiated through a given solid angle as seen reflected from a surface (i.e. the hemisphere is projected onto the surface)

• Irradiance (W/m2)

  • the rate of incident or incoming energy at a surface point per unit surface area.

Hyperphysics : http://hyperphysics.phy-astr.gsu.edu/hbase/vision/photomcon.html