[MAP] Light recycling; Piccione lip

[Kirk T McDonald]

Folks,
I have become fortuitously aware of an old trick in the lamp industry that is now sometimes called “light recycling” – with the goal of enhancing the optical brightness of light sources.
Remember, brightness = power / area in transverse phase space
(although the opticians don’t generally say it this way, perhaps using the buzzword “etendue” instead of “area in transverse phase space”)

In our project, we try to increase the brightness by “cooling”/shrinking the area in transverse phase space.
The opticians’ trick is to “recycle” the light so that one photon gets counted many times in the same area in phase space, effectively increasing the power, while leave the emittance the same.
The historical way of doing this (dating back at least to 1936) involves a cylindrical cavity lined with a phosphor (i.e., a fluorescent lamp) with a small slit in the phosphor to let light out.
A photon has only a small probability P to escape out the slit directly after being emitted by the phosphor.
Generally, the photon hits another region of the phosphor, is absorbed, and then re-emitted.  [The cavity can be lined with a reflector to assist in this process.]
On average, the photon bounces around N = 1 / P times before it escapes through the slit.
Hence, the steady-state emission of photons by the phosphor surface is N times greater than if the photons flew away on their first emission – as holds for an ordinary fluorescent lamp.
The net effect is that the light coming out of the slit is N times brighter than the light from an ordinary fluorescent bulb of the same output power.
The brightness has been enhanced N-fold (with no emittance reduction) to the extent that the absorption and re-emission involves no losses.
[I think the lamps in Xerox machines and scanners are of this type.]
I’ve written up a pedagogic note on this:
http://puhep1.princeton.edu/~mcdonald/examples/lamp.pdf
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This trick seems different from what we do to enhance the brightness of particle beams.
However, a comment by Fred Mills, dated 9/98, near the bottom of my web page
http://puhep1.princeton.edu/~mcdonald/mumu/physics/
has me wondering if part of the effect of the “Piccione lip” seen on p. 4 of
http://puhep1.princeton.edu/~mcdonald/mumu/physics/lichtenberg_mura-110.pdf 
was to use multiple scattering in the “lip” to kick some particles into a desired area of phase space.
That is, perhaps we can say that ionization cooling also includes a small effect equivalent to the opticians’ trick of “light recycling”.
What do you think?
--Kirk
PS  The opticians are after big game = use of such tricks to make better solar energy concentrators for photovoltaic energy generation (or even just heating water).
In the past, such efforts have not involved brightness enhancement, but only clever rearrangement of light in phase space (as in Winston cones).
The next generation of brightness enhancement schemes uses materials with differing absorption and emission spectra to play additional “tricks”.   New engineered optical materials, called photonic band gap materials, could play a key role here.
If these solar brightness-enhancement schemes pay off, they will be able to fund all of high energy physics....
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