Re: Practically achievable beam divergence for white, non-coherent light
From: Ian Stirling (root_at_mauve.demon.co.uk)
Date: 02/26/05
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Date: 26 Feb 2005 22:11:09 GMT
Joe D. <joe@nospam.invalid> wrote:
> I have two questions about beam divergence for white, non-coherent light:
>
> (1) What is the lowest practically achievable beam divergence for a
> searchlight. The GE WWII searchlights had about 2 degree divergence. Can you
> build a searchlight with, say, 0.5 degree beam divergence? What are the
> governing principles that bound this?
Fundamental diffraction limit:
1.22 * wavelength/diameter.
This is more-or-less irrelevant, it only means that you cannot have a
1/100th radian searchlight beam that's under about 122* lightwavelength, or
about 60um.
In practice, you'll never get a significant amount of light through a hole
that diameter (barring the use of multicoloured lasers).
> (2) Can you design a Cassegrain optic system where the beam off the
> secondary mirror has near-zero divergence? IOW instead of the secondary
> having a fixed focal length, it has infinite focal length? So that at, say,
> 400 meters the beam width off the secondary is still the diameter of the
> secondary?
There is nothing magic about a Cassegrain (or any other system).
You end up focussing one point on the light source to one angle of output
from the main mirror/optic.
The mirrors are locally flat, and if you reverse the direction of the bounced
ray, you can consider what would happen if you replace a small portion of the
mirror with a pinhole.
You get an image projected of the light source.
Then, it gets into how large a mirror you have, compared to the source.
A mirror 1m away from a .5cm light source will have a divergance of 1/200
radians, or about 2m at 400m.
The bigger the mirror, the more light it catches from the bulb and reflects.
This rapidly leads to wanting bigger and bigger mirrors.
R/G/B lasers are obtainable, that are mixed into one output beam, and
will not diverge much over 400m, with a beam over a few mm.
Expensive though.
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