[Laser] a simple test

[TWOSIG at aol.com]

The next time you setup your light receiver, here is a simple test  that
may give you some insight into how well your system is  adjusted.

Point the receiver at a moderate to weak, isolated signal  source.  It
should be strong enough to be clearly above the noise level,  at least 6
dB if you can measure it.  The source should be isolated that  you can
swing the receiver so that the source is out of the receiver field  of
view without picking up another source of similar or greater  strength.
Then block off half the aperture of the receiver.  A sheet of  black
construction paper should work by holding it in front of the lens  so
that only half the lens can "see" the source.  The signal should drop  by
3 dB.  Then shift the paper so that the other half of the lens can  "see"
the source.  It should sound the same, that is 3 dB down from the  clear
aperture.

Another way to run the test, that may give clearer  results, would be to
cut a hole in the black paper that is half the diameter  of the the lens.
The area of the hole is one fourth the area of the lens  (assuming that
it is a round lens) so it should result in 6 dB drop in  signal.  You may
need a stronger  source.  If you move the  paper so that different areas
of the lens are exposed, there should be no  change in the signal
strength as the paper moves.

The purpose of the  test is to determine if parts of your lens are not
providing light to your  detector as effectively as other parts.  If
there is an inconsistency,  particularly if you find parts of your lens
are "dead", then it is likely  that your sensor is not placed at the
proper distance from the lens, and  therefore light is spilling past your
sensor.  It is also likely that  you are more prone to get other noise
sources rather than the source you  want.

The situation may not be all bad.  If you are getting enough  light to be
able to communicate, and if there is not interference from  other
sources, it may mean that you have a wider field of view.  That in  turn,
may make it easier for you to find your desired source.  On the  other
hand, if you are just a few millimeters off the correct distance  from
the lens, you can be losing 20 to 40 dB of signal.

The test is  based on a technique, called a Hartman Mask, used by
astronomers, both  amateur and professional, to adjust the focus of their
telescopes.  I  believe that the technique is used more often for
astrophotograph than visual  observing.  I think of light communication
receivers as single pixel  telephoto cameras.  There seems to be a lot of
problems that telescope  makers (and users) have had and worked on
solving, that can be transferred to  light communications.

James
N5GUI