[Leica] IR filters

[reid at mejac.palo-alto.ca.us (Brian Reid)]

The job of a filter is to let certain light pass through, and deny passage 
to other light.

Think about what happens to the light that is denied passage. Usually it is 
absorbed (which turns it into heat, albeit not very much heat). A green 
filter looks green because it absorbs magenta and transmits green.

Sometimes it is easier to build a filter that, instead of absorbing the 
unwanted light, reflects it back out. A typical IR filter does not absorb 
the IR but instead reflects it. Hence the nickname "hot mirror".

IR filters reflect the IR rather than absorbing it because it's easier to 
do, not because there's some deep reason for not wanting to absorb it. The 
black surface of the camera body absorbs plenty of IR, and the camera does 
not typically burst into flames...

Brian Reid