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From: dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC)
Newsgroups: sci.electronics,misc.misc,misc.wanted
Subject: Re: Distance measuring using lasers
Message-ID: <1191@unccvax.UUCP>
Date: 6 Nov 88 13:35:39 GMT
References: <113@ericom.ericsson.se>
Organization: Univ. of NC at Charlotte, Charlotte, NC
Lines: 29

In article <113@ericom.ericsson.se>, etxbrfa@kklm01.ericsson.se writes:
> How to measure distance with lasers?
> I've been thinking a lot about the problem, and my conclution is:

>   It's impossible  (at least with submillimeter accuracy)

> Still it's done. And it's done by cheap devices.

> The technology of today must be pushed far beyond it's capabilities. The
> speed of light is far to high.

	Naaaah.  Try amplitude modulation and a timer.

	Actually, it is pretty trivial to build a stable phase meter
in the 1-100 mHz range with an accuracy of 0.1 degree. Let's just say
(for nice numbers) that an AM modulated laser beam at 100 mHz has a
wavelength of 10 feet. With 0.1 degree resolution, this works out to
(3600 divisions/10 feet) or 30 divisions to the inch.  Naturally, you
can put a sychronising pulse in there on every so-many cycles of the
modulating frequency to eliminate the ambiguity of multiple wavelengths.

	For your run-of-the mill cheap laser distance meter, I would 
think that 1/30 inch would be quite acceptable for many applications.
No doubt there are techniques which use interferometry with, say, a
small frequency synthesizer and null detector (or, for that matter, 
plain old interferometry) to get yeee-haaa accuracy.

York David Anthony
DataSpan, Inc