[Catieliecutty]

Luminance is an interesting property. It is defined as the luminous intensity of light travelling in a particular direction, divided by the area.

Two things interest me about it.

1/ it is an intrinsic rather than extrinsic property. If a flat screen TV has a luminance of 80 candelas per square metre, then two flat screen TVs stuck together have a luminance of 80 candelas per square metre.

2/ It is invariant under conservative optics. In particular, there is no way to _increase_ the luminance via passive optics. No matter what kind of prisms, lenses or mirrors you use, you can't increase the luminance from a particular source. The luminance of the sun is 1.5 gigacandelas per square metre. If you go to Mercury and use a lens a kilometre wide to concentrate the sun's rays down to a small point on Mercury's surface, the luminance at that point will be 1.5 gigacandelas per square metre, the same as it is under normal conditions on the surface of Pluto. (You can of course _reduce_ this luminance through absorption etc.)