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Path: utzoo!utgpu!attcan!uunet!mcvax!enea!ericom!kklm01.ericsson.se!etxbrfa
From: etxbrfa@kklm01.ericsson.se
Newsgroups: sci.electronics,misc.misc,misc.wanted
Subject: Distance measuring using lasers
Message-ID: <113@ericom.ericsson.se>
Date: 31 Oct 88 16:03:14 GMT
Sender: news@ericom.ericsson.se
Organization: Ericsson Telecom, Stockholm, Sweden
Lines: 41

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.
To measure the time, would be simple, if the accuracy of the electronics
were good enough, but it isn't.
My thought is that you pulse the laser with an accurate frequency (say
1.0000000Mhz). If you start another clock at 0.9999990Mhz at the very same
time, the difference it time between the clocks would increase by 1ps every
cycle (an accuracy of 0.15mm). The problem is that you cannot get such accurate
clocks, and to start them excactly on the same time (difference less than
0.1ps). Even if you could, there would be other problems.
Suppose the problem with time measurement is solved. How can the laser-beam
be pulsed at 1Mhz with edge accuracy less then 0.1ps. Impossible again.
To light the laser takes at least a few micro seconds, and time varies.
Then you could ofcource have a constant beam, that is pulsed afterwards.
How? A mechanical solution is out of question, so is Liquid chrystals. I
don't have any idea at all.
Now suppose that problem is solved to. Then we have the problem with detecting
the reflex. A photo diode or transistor, is out of question, far too slow.
And even if they weren't, the logic to take care of the signal from them
would have to long (and unreliable) setup time.

I don't think there are more problems (at least not as tough as these basic
ones). But unfortunately, the logics of problems is an OR-function.

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.


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