From: dp@cec1.wustl.edu (David Prutchi)
Subject: Re: Long distance IR detection
Nntp-Posting-Host: cec1
Organization: Washington University, St. Louis MO
Lines: 88

In article <1993Apr20.183416.18744@dres.dnd.ca> sburton@dres.dnd.ca (Stan Burton) writes:
>
>--
>
>I would like to be able to detect the angular position (low accuracy) of an
>IR emitting source at a distance of about 100 meters (more is better) in
>daylight.  The IR source could be emitting a signature; I'm leaning toward
>30 KHz square wave with 50% duty cycle.
>
>I am considering the use of a quadrant detector from Centronic Inc. to give
>information to a pan/tilt head to point the sensor and thus determine the
>angles.  For the source I am considering wazing the heck out of an IR LED(s),
>possibly an Optek OP290 or Motorola MLED81.  Wazing would mean at least 1 Amp
>current pulses.  At this current the duty cycle of the LED drops to 10% and I
>would need to cycle five of them in turn to get the 50% required.
>
>Has anyone done something like this?
>
>Stan Burton (DND/CRAD/DRES/DTD/MSS/AGCG)                 sburton@dres.dnd.ca
>(403) 544-4737     DRE Suffield, Box 4000, Medicine Hat, AB, Canada, T1A 8K6

A method which will directly (almost) provide you with the information that
you require is that which is commonly used for coordinate measurements of 
human body landmarks (eg. the SELSPOT system by SELSPOT AB, Sweden, and the
WATSMART System ??).  These use lateral photoeffect detectors [Lindholm and
Oberg, 1974; Woltring and Marsolais, 1980] to determine the position of a
spot of light projected (using a camera lens) over its surface.  In escence,
the detector is a large photodiode with four cathodes (one on each of the
four sides of the square detector crystal) and a single anode (the back of
the crystal).  A spot of light will produce currents which are proportional
to the position of the spot on the detector's face.  Let's try some ASCII
graphics in 2-D (so the detector has two cathodes to detect linear position)



              -------------------------> 1
             |                       |
             |                       |       |\
           -----   cathode 1         |       |  \
           XXXXX                     ________| +  \
           XXXXX|                            |      \        -------
    light >XXXXX|                            |        \_____ divider __ output
           XXXXX|     |                      |        /      -------
           XXXXX|-----| |--                  |      /           |
           XXXXX|   + |    |         ________| -  /           1 + 2
           XXXXX|         --- gnd    |       |  /
           XXXXX|         \ /        |       |/
           XXXXX| anode              |
           XXXXX                     |
           ----- cathode 2           |
             |                       |
             |                       |
             --------------------------> 2

If the dot of light from the LED is the ONLY source of light, then the position
of the projection is given by this circuit regardless of the level of illumina-
tion (the divider is used to normalize relative to total received power).  When
this circuit is used in the presence of other sources of light, then the way of
"tuning" to it is to sample for a few msec the background signal, light the LED
for an equal time and sample that signal separately.  The difference between
the two is the position of the LED.

Hamamatsu photonics sells linear and 2-D lateral photoeffect detectors, and
they also sell the required signal processing electronics.  The ready-made
systems by SELSPOT and others are NOT cheap ... 

Lindholm, L.E., and K.E. Oberg, "An Optoelectronic Instrument for Remote
On-Line Movement Monitoring", Biotelemetry, 1, 94-95, 1974.

Woltring, H.J., and E.B. Marsolais, "Opto-Electronic (SELSPOT) Gait Measure-
ments in Two and Three Dimensional Space - A Preliminary Report", Bull. Pros-
thetics Research, 46-52, 1980.

Most probably this technique can provide you with a much more accurate
measurement than you need, but I think that its simplicity beats scanning
a quadrant detector with a pan/tilt system until you reach zero difference.

            - David



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|  David Prutchi                 HC1DT                       |
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