Your "Total load of crap" statement is perfectly correct.
However, there is a point that needs correcting elsewhere: unlike radar,
laser speed detectors DO NOT use Doppler shift. Their system works
entirely differently. They simply send out a pulse and time the return; in
round numbers it's about 1 foot per nanosecond (freespace speed of light is
300mm/nanosecond, or something like 11 13/16ths inches; in air it will be
slower). Then they do it again, and calculate how much closer you have
gotten. It is purely a matter of range at two times, and they calculate
"speed" based on the difference in distance and the time between pulses.
This leads to a potential fraud: if the cop "sweeps" the beam downward, he
may be able to greatly increase your apparent speed. Suppose the first
pulse hits your windshield or the chrome strip around it (many cars), and
the second is down on the hood ornament or grill. His unit will add the
difference in distance to the calculation, making you appear to be going
much faster. Grill and license plate will only make you be going another
half of a foot per interval, but if the interval is 1 millisecond the
addition is staggering! A parked car can give speeds that no street-legal
car can achieve. In court, it will be your word against the cop's, and he
will be backed up by the "High-Tech" tool. There was a very good article
on this a couple of years ago, I believe in Road & Track; maybe someone can
locate it. I believe Machine Design magazine also hit this in editorials.
Ron
----------
> From: W. Lee Hendrick <[email protected]>
> To: Francesco Fontanot G. <[email protected]>
> Cc: Volkswagen Mailing List <[email protected]>
> Subject: Re: OT: Laser Jammer???
> Date: Sunday, January 16, 2000 12:42 AM
>
> At 11:45 PM -0500 1/15/00, Francesco Fontanot G. wrote:
> >the other day i was told that the best laser gun jammer and inexpensive
> >trick is to put a CD facing outside of the car between the mirror and
the
> >windshield. The guy told me that the CD absorbs the laser rays from the
guy
> >and don´t reflect thems back to the gun so it would read something
wierd.
> >
> >Any ideas?
>
> Excuse my French, but it's a total load of crap. If CDs
> absorbed infrared laser radiation then CD players wouldn't work, as
> they all use an infrared diode laser to read the tracks of pits on
> the CD! The only arguement that could possibly be made is that since
> the CD has a periodic structure, it acts as a diffraction grating and
> would disperse the laser light. This is why you see the spectral
> pattern in a CD. Since the laser gun relies on the Doppler shift of
> the reflected light to calculate speed differential, it could
> potentially 'confuse' the gun. However, the laser is highly
> monochromatic, and any diffraction simply diverts the light into
> higher order which then propagates off to the side, and since the
> vast majory of light incident on a CD is specularly reflected anyway,
> this could just make your car _more_ visible to a laser gun! Laser
> guns have a relatively small angular divergence, and the beam width
> 100 yards is still quite narrow, on the order of a foot, I believe;
> wider than a laser pointer, but much narrower than radar. Therefore,
> the gun must be aimed accurately, but the benefits are better target
> discrimination and less stray radiation (tougher to detect), as
> compared to radar. Cops usually aim at the front liscense plate,
> which is retro-reflecting (at least in CA) and therefore returns the
> most energy. Removing the front plate, having pop-up (or covered)
> headlights and a dark color car are the best ways to make your car
> 'stealthy' to laser guns. It doesn't mean that you car is
> 'invisible', just that the detection range is reduced.
>
> Lee
>
> P.S. Sorry for the long-winded and somewhat technical explanation,
> but I felt obliged to respond because of my eduational background:
> B.S. Optics
> M.S. Electrical Engineering
> coming soon: Ph.D. Electrical Engineering - Applied Optics and Photonics
>
> W. Lee Hendrick
>
> [email protected]
> http://soliton.ucsd.edu/~hendrick/
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