In article .net,
Jim Wilson wrote:
Peter Reilley wrote...
You cannot get better than the 1 to 2 mrad divergence
with a lens or a slit or the expensive lasers would do it.
I understand, and you are correct, of course. I did not mean to suggest
anything else. I only intended to advise the OP that she could get a
finer line with a slit, as she in fact may well be able to do.
To be sure, beam divergence is always increased by any close aperture,
including a slit. I hope I didn't lead anyone to think otherwise.
When a slit does effectively narrow the beam, I suspect it accomplishes
this by blocking part of the diffraction pattern produced by whatever is
already in the optical path. If this is the case, my additional
suggestion that the slit should be very narrow was bad; it should be no
narrower than the "main" beam width at the point of the aperture. This
should minimize the beam divergence caused by the slit.
What I *think* is happening is that the divergence follows a
curve -- I believe it is a "sinc" function -- with the most energy per
unit area in the center, and decreasing amounts as the divergence
increases -- up to the point at which it drops to zero and starts back
up -- to produce rings for a single beam, or parallel lines for a beam
which would otherwise produce a line.
What adding the slit will do is decrease intensity at *all*
points in that function, and reduce in a narrower part of it being above
the threshold of detectability -- which is probably a function of the
ambient illumination. So -- it gives the *user* a narrower apparent
beam.
I have observed this effect with a laser pointer and a set of
pinholes from an IR blackbody illuminator, designed to control the total
illumination received by the device under test.
I have not had a laser set up to produce a line, so I have not
experimented with the same effect there. I would probably attempt it
using a pair of double-edge razor blades -- or perhaps even blades from
injector type razors -- if such are still made with a single blade. (I
haven't shaved since about 1977 or so. :-)
Enjoy,
DoN.
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