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RE: starship-design: Casimir-Foreward balloon

> > From owner-starship-design@lists.uoregon.edu Thu Jan 13
> 20:05:32 2000
> > From: "N. Lindberg" <nlindber@u.washington.edu>
> >
> > 	I have a question about the 'phosphor' inside the balloon, and
> > about phosphorescent materiels in general.
> >
> As far as I know:
> > Apparently, a phosphor absorbs
> > a high-energy photon, and re-emits low energy photons.
> >
> Sometimes other way around, but rarely.
> Also, there is phosphorescence and luminescence.
> The difference is in the delay between absorbing
> (some energy, e.g., a photon) and emitting a photon.
> Phosphorescent materials have long delay (even days),
> so they shine long after the excitation ended
> (e.g., materials glowing in the dark) while luminescent
> materials stop to radiate almost immediately after
> the excitation stopped.

First, lets straighten out the terminology...

"Luminescence" is an emission of light such as "fluorescence" or
"phosphorescence" which is not directly attributable to heat as is for
instance "incandescence". "Fluorescence" is the property possessed by some
materials of absorbing radiation at a _particular_ wavelength and then
emitting it as _white_ light while the stimulus, the radiation source, is
still active. Fluorescence is distinct from "phosphorescence" which merely
means the emission of light without any perceptible heat. Phosphorescence
may be caused by many different stimuli such as chemical reactions or
radiation. Luminescence includes both of these phenomena and fluorescence
may include phosphorescence or may not.


> > What is the typical energy difference/proportion between
> > absorbed & emitted light.
> >
> Ehem, what means "typical" here?
> There is the whole range o proportions, byt of course,
> always there is less energy emitted than absorbed
> (usually much less - the efficiency of the process is rather low).
> > Finally, is any energy left over?
> >
> Yes, usually a significant percentage (as stated above).
> The sources of loss:
> - not all exciting photons are absorbed (some go through,
>   some are reflected);
> - still some incoming photons are absorbed in other ways
>   (eg., in inner electron layers) and are converted to heat;
> - the emitted photons are of lower energy (longer wavelength),
>   or - where it does not apply - are of lower energy than the sum
>   of energies of those photons which were used in the process
>   of generating them;
> - falling down of electrons not always occurs along the standard
>   route, so that they may emit other photons than desired - often
>   those that are then easily (re)absorbed and converted to heat;
> - the "desired" photons may get absorbed and converted to other ones
>   or to heat before they manage to leave the material.

As the question is stated, yes there may be a difference between "absorbed
radiation" and "emitted light". However, there is never any difference in
the energy of the system, as in "left over energy". Energy is always
conserved. Some may be converted to other forms of energy besides "light"
but all the energy absorbed by the material is eventually emitted in some

I suppose you could say that in an extreme case, say a particle accelerator,
you could theoretically insert enough energy to permanently raise the energy
of the material, by transmuting it into a different material. But the total
energy of the system still remains the same even then.