# Re: More laser stuff

```There is mystery here. The wave property of light is on our side:

the lazer works in infrared region, wavelength about 1k nm which is 1 micron.
The diameter of the focusing lenz of the lazer is one the order of 1 cm.
This the angle of
diffraction will be, with good acuracy assuming everything else is perfect:
1 micron / 1 cm = 1 * 10**-4
But the gun of the size used by police does not produce totally coherent
light. In fact a chamber of that size will produce about 10%-20% or .1 to
.2 of spectral dispersion.
This our angle is not 1 * 10**-3 rad. Factor in the fact that the lenz is
not perfect and hasn;t been cleaned since the gun left the factory and
the atmopheric dispersion during measurement over a distance of 100s of
meters and there you're.
The angle now is 1 * 10**-2 rad. This would give the lazer beam a spread
of about 10 m over a 1 km distance.

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.

Alex

On Mon, 16 Sep 1996, Mike Miller wrote:

> This is taken from the web article on Laser Guns:
>  <For example, with the LTI 20.20, even vehicles that are 5 feet to the side
> of the target vehicle could be the ones measured! The deviation grows
> increasingly with measurements taken on curves or with measurements that
> aren't directly in the line of travel (cosine effect), although to the
> advantage of the target driver, the speed displayed is less than the actual
> speed. So it's possible to measure vehicles that aren't in the direct vicinity
> of the target.>
> The whole article is available at http://www.motorists.com/laser.htm ( as
> previously posted)
>
> One thing I notice here is that as the angle of measurement increases, the
> difference in the speed indicated by the laser gun and actual vehicle speed
> increases in favor of the driver. This would indicate that traveling in the
> lane (on multi lane roads) farthest from where the laser traps are normally
> setup would result in the lowest speed reading. Here in WA state, I ususally
> see laser or radar traps set up using the left side of the road. So this says
> I should drive fast in the slow (right) lane.
>

```