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From: hollombe@ttidca.TTI.COM (The Polymath)
Newsgroups: rec.autos,sci.electronics,misc.consumers
Subject: Re: Correct Terminology (was Re: Radar Detectors (Ka band) ...)
Keywords: radar, laser, lidar?
Message-ID: <3930@ttidca.TTI.COM>
Date: 22 Feb 89 19:12:02 GMT
References: <603@icus.islp.ny.us> <7944@netnews.upenn.edu> <1895@tank.uchicago.edu> <5632@homxc.ATT.COM> <4057@alvin.mcnc.org>
Reply-To: hollombe@ttidcb.tti.com (The Polymath)
Organization: The Cat Factory
Lines: 36

In article <4057@alvin.mcnc.org> bishel@mcnc.org.UUCP (Geoffrey R. Bishel) writes:
}In article <5632@homxc.ATT.COM> h16@homxc.ATT.COM (D.JACOBOWITZ) writes:
}>A laser will spread 3.5 feet in 1/5 mile?
}>Are you sure?
}>That doesn't sound like coherent light.
}
}You've got to remember that the beam is passing thru the air, and dust, dirt,
}water molecules, etc. will all diffract the beam to some extent.  Thus you get
}a spreading of the beam.  

About 15 years ago, when lasers were relatively new toys, I was living in
London, England.  One night I went to a film festival on Oxford Street.
Evidently, some other event was going on as well and, for decoration, they
had what looked like two "neon" tubes, one red, one blue, hanging in the
air from Regent Street to Charing Cross Road, a distance of about a mile.

I soon realized that an unbroken, unsupported length of neon that long was
impossible, and the effect had to be created by two laser beams aimed down
the length of the street.

There was no obvious spreading of the beams, in spite of a light mist in
the air.

A commonly quoted statistic, at the time, was that a one inch laser beam
aimed from the earth to the moon would spread to a circle 50,000 ft in
diameter. 50K ft / 250K mi = .2 ft/ mi spread, or about 2.4 inches per
mile.

Any way you look at it, 17.5 ft of spread per mile isn't even in the ball
park.

-- 
The Polymath (aka: Jerry Hollombe, hollombe@ttidca.tti.com)  Illegitimati Nil
Citicorp(+)TTI                                                 Carborundum
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