• 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 (W/sr) - the rate that light energy is radiated through a given solid angle
• Radiance (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.