I think you mean "Conservation of radiance". It corresponds pretty much directly with "Power per unit area". Obviously, mirrors and lenses can reduce the unit a

I think you mean "Conservation of radiance". It corresponds pretty much directly with "Power per unit area". Obviously, mirrors and lenses can reduce the unit area, while keeping power constant (at best). http://en.wikipedia.org/wiki/Radiance#Intensity http://en.wikipedia.org/wiki/Radiance#Intensity