Does an EPG windshield (as supplied on a 1997 Audi A8) interfere with
readings on police laser guns?  Is there technical support for your
answer?

Hello cyndi,

This is indeed a tough question! The quick answer is no. Explaining
this theory will take a little understanding of frequency, frequency
filters, and the doppler effect. I am relying on my own personal
knowledge, and where I can, I will post some links to relevant sources
to back this theory up.

Q. Does an EPG windshield interfere with readings on police laser
guns?

As you will see in the first site I list below, police radar guns work
on the premise of the doppler effect. The radar signal is 'shot' out
from the gun, and is bounced back off of your vehicle. The police
officer in question needs a reflective surface to have the signal
bounce back from, and the window does not play any role in the
equation, since it allows the radar signal to pass through it. I'll
explain this in a moment :-)

The typical places a police officer would aim his radar at include :

- license plate
- rear bumper
- headlights
- grill

Data to confirm the above - Radar Facts
http://www.k40.com/K40Radar/Radar%20Facts.htm


Q. Is there technical support for your answer?
-------------------------------------------------------------------------------

I want you to get a solid answer to your question, so we can assume
that the police officer did aim his gun through your back window, that
was of the EPG composition. Here is what I propose :


The EPG line of windows is manufactured with a PVB (polyvinyl butyral)
layer, sandwiched between two layers of ordinary glass. They
distribute this product under the name 'Saflex'. Here is the Saflex
automotive branch website:

Saflex
http://www.solutia-autoglass.com/

Within this site, they have a chart that graphs the products overall
performance of sound reduction for a given frequency range. You can
view that here :

EPG Noise Reduction Chart
http://www.solutia-autoglass.com/engNoiseReduction.jsp

This plastic layer that is inside the two layers of glass acts as a
filter for a specified frequency range. It is somewhat of a dual pass
filter, letting all all the frequenies above 8000 Hz and below 2000 Hz
to pass through as near normal.

Looking at the frequencies for police radar, we can see that we don't
even come remotely close :

10.525 GHz              X Band Police Radar Police Radar
24.150 GHz              K Band Police Radar Police Radar
34.200 GHz-35.200 GHz   Wideband Ka Police Radar Police Radar
34.300 GHz              Photo Police Radar Police Radar


In addition, the composition of the PVB contains no significant
metals, so the radar signal passes right through the PVB layer and the
glass. Here is the compositional make up of the PVB (Butvar). It is a
PDF document so you will Adobe Acrobat to view it :

Prositech Compostion of PVB
http://www.proscitech.com.au/msds/c060.pdf

In case you can't get a view of this, here is the compostion :

Polyvinyl Butyral
Diethyl Butyral
Water
Butyraldehyde
Ethanol
Ethyl Acetate

Here is a reference from Radio Shack/Tandy that confirms that radar
frequencies do pass through plastics.

Tandy Radar Report
"Radar signals will pass through glass or plastic, but not through
metal."
http://support.tandy.com/support_auto/doc32/32789.htm


So the conclusion is - this layer has no effect on the radar signal as
it passed right through it, much like glass. I would contend that the
propagation time (time to get through it) is changed ever so slighty,
but on a scale so small it has no effect.


To assist with this answer I searched Google for :

polyvinyl butyral
://www.google.com/search?q=polyvinyl+butyral

Radar signals will pass through plastic
://www.google.com/search?q=Radar+signals+will+pass+through+plastic

epg glass
://www.google.com/search?q=epg+glass


I hope this answers your question. Should you need further
clarification, or more resources, please ask before rating this answer
as I would love to be of further assistance.


Thanks for the very challenging question!
SgtCory

---

Clarification of Answer by sgtcory-ga on 24 Nov 2002 22:53 PST

Hello cyndi,

A little clarification. I used the term 'radar gun' and 'laser gun'
interchangeably. A laser guns works on almost the same principal. It
shoots out pulses of infrared light, and times how long the reflection
takes to come back. In this case, there would still be no effect,
because the beam is not aimed at your windshield, rather your plates.

If it were indeed aimed at your windshield, here is the scenario :

- EPG blocks UV light across a specified spectrum
- Laser guns use the infrared range (-> microwave)

Here is a diagram of the difference in wavelengths of the two
spectrums

Light spectrum
http://www.intl-light.com/handbook/ch01.html

As you can see the two are very far apart. The laser signal would pass
right through the minute plastic 'film', as evident in some samples at
this site :

Infrared pass through examples
http://sirtf.jpl.nasa.gov/EPO/Kidszone/infrared.html


I found one more good resource that I wanted to add. It explains why
tint (which often contains metal) would have an effect, and this
plastic filler/coating does not. It really helps put the answer in
perspective, and further backup our initial findings :

Tint and the Effects on Radar
http://www.laseradar.com/radar_faq.htm#4

SgtCory

For the researcher who takes this question, EPG stands for enhanced
protective glass. I was unable to come up with anything concrete in a
preliminary search.

tisme-ga

Hello
there seems to be some facts surrounding the windscreen mentioned on
the auto world site (along with stealth car polish!)
http://www.autoworld.com.my/EMZine/Review/viewarticle.asp?awReviewID=366&awCatID=RT.ATC.CAR.FS

There was an article in one of the UK motoring publications which
called the early A8's a stealth vehicle. From vague memory it was the
aluminium body work, angle of it's sides, double glazed windows and
tint coating that gave the A8 some stealth properties.

There is some speculative talk here of stealth cars on Google groups
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&newwindow=1&c2coff=1&threadm=19990617210651.25665.00001348%40ng-cg1.aol.com&rnum=4&prev=/groups%3Fq%3Dwindscreen%2BOR%2Bwindshield%2Bpolice%2Bradar%26hl%3Den%26lr%3D%26ie%3DUTF-8%26newwindow%3D1%26c2coff%3D1%26selm%3D19990617210651.25665.00001348%2540ng-cg1.aol.com%26rnum%3D4

"yuss" points out on Google Groups in the August 1988 issue of "Car
and Driver", the A8's windshield reduces the effectiveness of radar by
95% and laser by 75%
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&newwindow=1&c2coff=1&threadm=35ACA5C1.CC9EB601%40go-away.globalnet.co.uk&rnum=1&prev=/groups%3Fq%3D%2522audi%2Ba8%2522%2B%2Bpolice%2Bradar%26hl%3Den%26lr%3D%26ie%3DUTF-8%26newwindow%3D1%26c2coff%3D1%26selm%3D35ACA5C1.CC9EB601%2540go-away.globalnet.co.uk%26rnum%3D1

I did not manage to source any physics and technical explanation to
the types of radar or laser guns affected, or how they are rendered
less effective.

Kind regards
lot-ga

The data about frequency performance from 2 kHz to 8 kHz is for sound.
 Radar and lasers are electromagnetic radiation.  I don't know how
transparent the windshield glass is in the microwave region (radar),
but if it wasn't fairly transparent in the visible region (laser), you
couldn't see out.

At one time, somebody (Ford?) made a car with a defrosting front
windshield that didn't have the lines running across it for heating
elements like the back windshields do.  I seem to remember something
about a thin (*extremely thin*) layer of gold in the windshield.  It
might have been lines rather than a solid sheet, but to a radar gun
with a frequency of 10-30 GHz, any metal with gaps smaller than about
a centimeter would look solid anyway.

Frequency examples were given for the sake of example. Sound is
frequency. Without it there is no sound.

We could *assume* for an example - that the radar signals were
operating at a 8Khz signal. This signal would lose substantial power
level regardless. (If indeed this were even practical) The wavelength
of the frequency is what is targetted with the noise reduction effort.

I think the widnshield you are speaking of is actually a defroster
installed in the film, which would contain metal is the silicone film.
This would indeed cause problems. This is not the case with the
plastic resin it seems.

Sound has a measurable frequency, and so does electromagnetic
radiation.  They behave quite differently in various media (light
slows down in glass, sound speeds up, light travels fastest in a
vacuum, sound can't exist there, etc.).  The size of the scatterer
(car) is important in either case, but it's size relative to
wavelength. 8 kHz electromagnetic radiation (below AM) has a
wavelength of about 20 miles.  I doubt it could detect the presence of
the car.    8 kHz sound, on the other hand, has a wavelength of less
than 2 inches.  One of those sonar measuring tapes would have no
problem detecting the car.

You'll definitely get 100% agreement from me on your comments. My
point was that the two are distinctly different, and my answer in this
portion did leave alot to be desired. All of my facts and comments
equal one run-on sentence.

Thanks for taking the time to point that out. (Don't we all dislike
being wrong:-)

The point should have simply stated that the practical limits of the
glass layer deal with sound. I hope I didn't cause too much confusion,
and the reader will look at our comment banter before giving any type
of credit to the new type of science I created. lol

Thanks for your very appreciated help and guidance thenextguy -

SgtCory