Can CDs hung from rear-view mirrors int rfere with radar? How?

Was listening to the radio this morning, and the hosts professed ignorance as to a recent trend towards hanging CDs from rear-view mirrors (their complaint was that if sunlight reflects off of it, other drivers are momentarily blinded).

They requested listeners to call in and explain if they knew. The first caller said that some air fresheners look like CDs, but then another caller said that she had heard that they interfere with radar due to the metal in them (here in VA, by the way, radar detecters are illegal).

While this second person was on the line, the show’s producer broke in to say that a cop had called and, while not wanting to go on the air, confirmed the story.

Anyone have any confirmation that this is true? I mean, it’d be easy for someone to say “I’m a cop, this is true.” And just HOW would they interfere with radar detecters? Bounce it back? Thanks.

Um, bouncing the radar signal is supposed to be bounced back from the car, so I don’t see how a CD bouncing even more back would do the car driver any good.


I have a lot of trouble swallowing this mainly because radar detectors have no trouble clocking your speed from behind, where they would have to deal with three reflective things (two side view mirrors & a rear view). If a single piece of shiny plastic could cause so much of a disruption then surely three genuine mirrors would make radar detection (from behind) down right impossible.

And anyway, how could it possibly work- by reflecting the radio waves? That’s what makes radar work in the first place. Radio waves are reflected off your car & the round trip travel time is measured for a quick calculation of your speed.

My guess is that since most of a car is made up of metal and/or reflective surfaces, a CD hanging there would make no difference whatsoever. Then again, I believe computers are powered by gnomes.

Hey, bouncing it back was just a wild guess on my part, sorry if I didn’t make that clear. Perhaps it would bounce it in unintended directions, I don’t know.

But still, there was supposedly a cop that called in to confirm it, so I take the allegation a little more seriously than I normally would. Besides, with detectors illegal in VA, I’ll take any small advantage I can! :wink:

Far be it from me to disparage an officer of the law, but a cop confirming it doesn’t necessarily make it so.

If the CD bounces some of the radar beam in an unintended direction, that wouldn’t help. A car isn’t a flat plane; it will be scattering the beam all over the place, too.

If you’re thinking that maybe the CD reflects a killer ray back at the radar gun and overloads it or causes a false reading, I doubt it. I’m not going to bother with a calculation since I don’t have the specs for a radar gun on hand, but I’d guess that cross-section of the CD is small enough that even if it’s a perfect reflector with coherent backscatter its reflection wouldn’t be significant compared to a big ol’ block of metal like a car.

Now, laser guns might be a different issue entirely . . .

*Originally posted by Podkayne *

Sounds good enough, but what are the chances the laser hits the CD? And if it did, it would be for a fraction of a fractino of a second. (You are moving remember.)

I think that this is just one of those UL/rumors that people believe on blind faith without actually thinking about.

The mentality behind the belief? “Why would I spend money on a detector (or run the risk of getting caught having one when it may be illegal) when I can just use a CD?”

I dub this way of thinking the ‘stick it to the man in the simplest way possible’ mentality.

I agree with the above. I can’t see how one reflective metal surface of a CD would add any effect to the overall metal of the car and it’s reflective mirrors.

This reminds me of the theory that jamming aluminum foil under your wheel covers will also block, or distort, radar. I don’t see how that would work either.

Okay, I think I found out how this factoid got started.

First, here’s a website describing how microwave police radar works.

It has to do with the Doppler effect and the speed of the oncoming car. I don’t see how a CD hanging in the front windshield would affect this.

However, there’s another kind of police radar that works with laser beams, known as “lidar”.

Lidar works by measuring the reflectivity of your car. Here’s a website that tells you how to figure out how reflective your car is, and it mentions that frequently the lidar will target on your license plate. Therefore, I believe the train of thought runs, if you hang up something shinier than a license plate that is swaying back and forth, the lidar might target on that and become confused.

And BTW, just because someone calls a radio program and identifies himself as a cop doesn’t mean that he is. :wink:

I’m not sure I understand what difference it would make if the lidar targeted the license plate, the CD or the headlights if they’re all moving at the same speed.

It’s a total crock. There’s only four ways to defeat radar sensing:

(1) Early detection (via radar detectors)
(2) Absorption (don’t reflect the radar)
(3) Controlled reflection (reflect the radar away from the source)
(4) Jamming (return a false signal to the source)
#2 requires expensive materials and even then has limited effectiveness. #3 sounds a lot easier than it is because it’s very difficult to build an object that doesn’t have some edge or curve that can return a reflection.

A CD hanging from the mirror is not going to affect the absorption or reflectivity of the rest of the vehicle. This is because radar receivers expect some level of noise and bypass. They use an averaging technique and your little CD just doesn’t figure into the average in any significant way.

So this leaves only jamming as a possibility. To jam the radar sensing equipment, you need to send an amplified, red shifted radar signal back to the source. Therefore, jamming is an active thing and CDs are passive things. They have no facility to alter the reflected radar signal back to the source.

Now, I know what some of you are thinking… “Wait a second. The laser radar could hit my dangling CD and reflect back an altered signal. After all, we’ve all seen the prismatic effect of light reflecting off of CDs.” Unfortunately, it doesn’t work that way. The laser versions use an encoding technique called “spread spectrum”. It’s actually similar to the encoding used in cellular and cordless phones to insure others can’t easily listen in on your conversations. One of the reasons that lasers were added to police speed sensing equipment is that they can’t be jammed. To jam the laser system you would need to send a properly encoded signal to the receiver BEFORE you received the original. Einstein tells us that this can’t be done… I tend to believe him.

I am sure that a strong enough noise signal over the proper range could jam it without knowing how the original signal is encoded. How powerful would that have to be? I don’t know, but I would bet that it is possible. Spread-spectrum is more resistant to interference, but it isn’t immune to it if there is enough noise at a high enough level over the right spread (range) of frequencies.


You’re right. I oversimplified a bit. It is possible to jam by swamping the receiver with random noise, but that is instantly detectable by the equipment and since it is illegal, you’ll still get nailed.

What happens if I cover my entire car in AOL CD-ROMs with the shiny-side out? Do I become invisible? Will I be able to fly? Bulletproof?

I don’t know but I’ve heard you can make a pretty good solar oven out of those puppies!!

Do this, Vix – cover your car in CD-ROMs like you said and drive 120 mph past a radar cop. You might also want to moon/flash/flip him off at the same time. Tell us what happens.

When you get out.

p.s. Those would be AOL-Time-Warner CD-ROMs now.

I vaguely recall reading somewhere that those curved plastic whatzits that people put over their license plates are supposed to momentarily scatter the laser and/or radar beams; the laser/radar guns supposedly work best when reflected off a flat surface, and as your plates are generally the flattest surface at the front or the rear of your car, the cops try to aim the beam at them. The scattering effect will do no more than give you another half second in which to spot the cop parked up the road and slam on your brakes, however.

No cite, but I think in was in Forbes in 1994, in an article about super-duper radar detectors for the obnoxiously rich.

Apparently, most of the spped guns have a line of sight range of up to about 1/2 mile. Drivers usually only look a couple of hundred yards ahead of them. It seems the best defense would be to try and spot the cop before he spots you.

Without revealing any classified details, I can confidently say that hanging a CD from your rear view mirror will do absolutely nothing to save you from the mean, ol’ radar/laser-equipped speed cop. Slow down, look farther ahead, and/or drive smarter (provided that’s possible).

I doubt there are any cost-effective ways to beat the current generation of police radar/laser equipment. Get used to it (or become a millionaire and buy some REAL EW equipment – it’ll take that kind of money).


Wait a minute. I don’t know what I’m talking about here, but something is ringing a bell. Allow me to make a fool of myself, please.

(guess) CDs give off that pretty prismatic look because they’re breaking up and reflecting everything except the particular shade of red that it’s supposed to be absorbing from the laser.

But cop lasers work on reflection, not absorption. So if it were targeted by a laser, even one that shifted wavelengths, wouldn’t a CD break that beam up and therefore not be interpretable by the reciever?

Sofa King wrote:

No, the CDs give off the prismatic look because the closely-spaced grooves act as a diffraction grating, which bends the different wavelengths of light at different angles (like a prism). White light has all colors, so you see a rainbow.

The CD might break up the portion of the laser beam that actuall hits it, but this are other reflective things on your car to reflect it back. The CD can’t subtract from the overall reflection, and depending on its orientation, it might make the reflection slightly larger (and therefore easier for the detector to measure your speed).

My idea for a laser-absorbing car is to have it made up of a series of flat panels, each (2n+1)/4 wavelengths apart, so that the reflected signals interfere with each other. I’m looking for funding…