Thorsten Froehlich wrote:
> 
> In article <39118632@news.povray.org> , "Rune" <run### [at] inamecom>
> wrote:
> 
> > I sometimes use transmitting surfaces for special effects.
> > For example you can make a surface which inverts colors this way:

> 
> This is simply because what is a surface that inverts light? It does not
> exist, there isn't a surface that increases the strength of light while it
> is passing through it.

Ok, I'm nit-picking here, and it's not really a surface but a media, but there
is such material that increase the strength of light while it is passing 
through it (and/or even change it radically).

That's what the first laser were made with: a cristal was excited by a strong
flash lamp and a resonating cavity made of a 100% reflective mirror and 
a 99.99 % one was used to amplify a spontaneous photon emission.

There is also this strange medical laser media: you have a classical laser
beam as input via optical fiber and the output is also a laser beam, but
the frequency is different (halfed or doubled, I do not remember, but
the effect is that on one side you have a red beam and on the other side a 
blue beam (because the blue is 0.35 whereas red is 0.7 in the relevant unit :-)

There is also some protective material against infra-red laser beam used
in labs and industrial environment, which, when hit by the beam double
the light frequency, turning the invisible beam into an orange spot.
 (the material may be destroyed by high energy laser and need replacement, 
but at least you and your eyes were a minimum protected from the loose beam).

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