[Laser] Non-Imaging Optics

J. Forster [email protected]
Mon, 13 Jan 2003 00:25:02 -0500


Thomas Upton wrote:

> Mentioning again that there is no power loss when the optical filter is a diffraction
> grating tilted to the frequency desired.  "You want red, you get red", but the rest
> of the visible or invisible spectrum is still there in a spectrograph within the
> collector.

True, but the unwanted wavelengths are blocked by the exit slit.

> So other wavelengths should be recovered by putting their particular  receiving
> element at that portion of the "rainbow" created by the diffraction grating.

True, but you are not proposing wavelength division multiplex. This is a simple link, and
any wavelengths that are not transmitted are noise. That noise can be eliminated by
proper selection of  receive filter bandwidth and center frequency.

> When the collecting element is a concave mirror, there is no difference in the focal
> distance of wavelengths.  Unlike lenses, which bend different wavelengths in
> proportion to the interaction of the lens and the medium in which it resides.

The wavelength dependence of lenses is usually to small to matter.

> Incidentally, when the collecting element is a convex mirror, the light can become
> polarized when the mirror is convex in one direction and not in another.  For
> example, note the light off of a windshield; the sunlight is polarized because it is
> convex in one plane and not in the other.

Convex mirrors do not 'collect' light. Light can become partly polarized by a perfectly
flat mirror.

> When that windshield light comes down a dark corridor onto a wall at the end of the
> corridor, you can see the heat rising off of your hand or a cup of coffee, as well as
> the cold rising from other sources.  You can even see your own breath, or the
> moisture rising from your hand.  Very Interesting!

This has nothing to do with polarization. Warm air has a different index of refraction
than cool air. The effect is called 'seeing' and has to to do with refraction by pockets
of air of different indexes.-John

> .
> Tom Upton  AD6N
>
> J. Forster wrote:
>
> > Bob Williams wrote:
> >
> > >Has anybody tried using just a light-bucket approach for receiving diffuse light
> > from atmospheric scattering - forward scatter, cloudbounce, etc...?
> >
> > >With images on the horizon, I think that light interference might make it
> > pointless.
> >
> > You can eliminate much of the interference with a narrow band optical filter on
> > your receiver, tuned to your transmitter's wavelength.
> >
> > -John
> >
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