[gbhn]

You can't do that (at least not with passive components). There's a famous optics result called the 'brightness theorem' that explains why not.

[anthonyb]

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