Re: More laser stuff

[steveb@falcon.kla.com (Steven Buchholz)]

> And about the angle.. lazer gun works also by doppler effect. Only here, 
> pulses of light are used instead of the micro waves. At an angle, there 
> is a cosine of that angle is envolved between the guns direction and the 
> directionof the vehicle. And if you know, cos(90) is 0. Thus if you're 
> traveling at right angle to the gun's direction, there will be no change 
> in frequency of light impulses sent and light impulses recieved and the 
> gun will read 0 MPH.
> 
It is my understanding that the "LASER" speed detectors do _not_ use the 
Doppler effect.  Doppler effect assumes that you can sense the frequency 
change that occurs due to the relative velocity difference between the 
source/receiver and the target.  The "LASER" speed detectors measure the 
amount of time it takes for light pulse to reach the target and return, 
and they use the change in this flight time to estimate speed.  The thing 
I find most interesting with this method is that if the primary return 
point changes then there will be an associated change in the flight time, 
and thus the measured speed.  I guess if you had the right kind of Mylar 
streamers attached to your car you might be able to screw up the reading 
as well (OK, I'm assuming that Mylar is a good IR reflector).

All of the statements that were made WRT the fact that the measurement 
is affected by the cosine of the angle between the sensing direction and 
the actual velocity vector do apply for this method of measurement as well.
To get a reading of "0" you would need to be traveling in a circle centered 
on the REO ... :-)

Steve Buchholz
s_buchho@kla.com
San Jose, CA (USA)