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From: cjc@praxis.co.uk (Chris Clee)
Newsgroups: sci.electronics,misc.misc,misc.wanted
Subject: Re: Distance measuring using lasers
Message-ID: <3300@newton.praxis.co.uk>
Date: 8 Nov 88 17:21:30 GMT
References: <113@ericom.ericsson.se>
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Reply-To: cjc@praxis.co.uk (Chris Clee)
Organization: Praxis Systems plc, Bath, UK
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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.
>
>Am I wrong? Apparently I am, because these devices does excist. But what
>is wrong. I can't se any errors in the statements above. All of you who
>can see errors, please E-mail. I'd be most grateful to know.

Laser light is coherent; that is, it is all of one frequency.
If the laser tape measures operate in the way that I would think
that they do, they would split a beam into two parts. One will be 
routed internally to an opto sensor. The other passes out of the 
aperture of the instument, strikes the object to be measured and 
reflects back into the instument and onto the same optosensor.
Where these two beams combine, there will be a path difference
between them. This path difference will vary as the distance to
the object varies. The path difference between the beams causes
interference. So as the instrument is moved towards or away from
an object, the path difference will vary and produce alternately
constructive and destructive interference at the opto sensor.
Hence the instrument has a resolution of one half of a wavelength
maximum.

This is just a bit of Physicist's lateral thinking : I'm not absolutely
sure that that's how it's done, so please flame me if I'm wrong - I'd
be interested to know how they work myself - but the "timing the beam"
idea seems a bit unlikely although scientists DO measure variations in the 
orbit of the Moon by bouncing short laser pulses off the roof of some lander
on the surface !

Chris.