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From: kludge@grissom.larc.nasa.gov ( Scott Dorsey)
Newsgroups: sci.electronics
Subject: Re: Police Laser Speedtraps?
Message-ID: <1991May31.131219.16429@news.larc.nasa.gov>
Date: 31 May 91 13:12:19 GMT
References: <42808@cup.portal.com>
Sender: news@news.larc.nasa.gov (USENET Network News)
Reply-To: kludge@grissom.larc.nasa.gov ( Scott Dorsey)
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Organization: NASA Langley Research Center
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In article <42808@cup.portal.com> CPS@cup.portal.com (CHRIS PATRIC SMOLINSKI) writes:
>Some of us at work were speculating on just how those new police speed
>checking systems work - the ones that use lasers.  There was a segment
>oon the local news about them (and typical of local news, it was generally
>useless and non-revealing).

   This is a wild guess intended to begin speculation.  It should not be
considered any more than that:

   How about sending your laser signal through a beamsplitter.  Most of the
signal goes off to the car and is reflected back.  Probably a set of lenses
on the incoming path will be required to prevent a lot of extraneous stuff
from being reflected back too.  Then the incoming signal is heterodyned with
the reference signal that the beamsplitter forked off.  An interference 
pattern is generated, which is scanned into a computer and voodoo occurs and
the speed is read out on a display.
--scott