From: Dave Stampe-Psy+Eng <dstamp@watserv1.uwaterloo.ca>
Subject: Re: head tracking
Date: Tue, 26 Nov 91 00:52:49 -0500



> From motcsd!lance@apple.com Mon Nov 25 15:42:08 1991
> From: lance@csd.mot.com (lance.norskog)
> 
> About Dave Stampe's LED tracking system:
> Another possible system uses microwaves.  
> 
> Semiconductive material, when excited by such a microwave
> beam, emits a second microwave at either half or double the
> excitation frequency (I can't remember which).  When security 
> people "sweep" for bugs, they literally sweep around a microwave 
> beam, checking for the secondary emission.  The semiconductor
> can be powered or unpowered, it doesn't matter.
> 
> You can build Myron Krueger's Exoskeleton (see "Artificial
> Realities") by wearing bracelets/anklets/necklets with big
> semiconductive crystals on them.  A base unit in the corner
> of the room would have to sweep in different directions
> very quickly; some kind of whirling base or phased array
> of senders would be needed.  
> 
> The advantage of this technique is that the body is transparent
> to the microwaves, and thus the the occlusion problems of
> LEDs are gone.  The disadvantage is that the health effects 
> of long-term low level ionising radiation are becoming known
> as bad, bad, bad.
> 
> I have no idea how well it would work, and I'm not volunteering.

Not too good.  Problem is that microwave wavelengths are too long to 
get a sharp beam: you'd get discriminations of an inch or larger.

You could use an infrared beacon, though: use a quartz-halogen lamp (100W)
with an infrared filter, a condensing lens, and strip mirror spun by a
shaded-pole motor ($5 at surplus stores, 1800 RPM +/- 5%).  Not very small,
but cheap.  IR detectors on the suit and 3 units (2 horizontal, 1 vertical)
would give you about 5mm resolution in XYZ at 20 samples/sec.  Not
good enough for head tracking, but OK for body position sensing.

- Dave Stampe