From: saekung <saekung@gmail.com>

On 03/03/2016 21:50, jim bell wrote:

> >From: "dan@geer.org" <dan@geer.org>>Here's another one, http://www.nauto.com, which I mention as the
>> recording of license plates and everything else in the surround,
> Nearly 20 years ago, I first heard of "3M Louvered film", thin plastic sheets with opaque black plastic molded perpendicularly to the plane of the sheet. It would be very useful to help block the viewing of license plates from the side of the road, or above on poles or overpasses. Google-search '3M louvered film". http://www.apioptics.com/pdf/3M-Display-Film-Summary.pdf http://www.freepatentsonline.com/3919559.html http://www.freepatentsonline.com/EP0466460A2.html Two sheets installed perpendicularly to each other would render a license plate readable only from a limited viewing cone behind, or in front of, the car. Jim Bell

>I would think that is illegal in most countries.

A policeman might, and presumably has, argued that. But go to your typical car-parts store (in America), and you will see plenty of license-plate covers which shield the plate from dirt and dust. That doesn't mean that it is definitely 'legal', merely that most of the time, cops tend to look the other way if they can easily see the plate itself.

One actually claims that it prevents photographing the plate: http://www.ontrackcorp.com/original-protector.cfm?id=03

25 years ago, I did some research into making a car 'infrared stealthy'. The technology of (cheap) laser radar was just coming into play. I learned that such laser speedgun units used 905 nanometers IR, pulsed at about 25 nanoseconds. I concluded that covering a car's corner-cube plastic retroreflectors with an IR-absorptive glass (PPG's Azurlite absorbed all but 6% of 905 nm IR). Azurlite could also be used to cover a license plate, although it had a distinctive bluish color. Etching the inside surface of the glass with ammonium bifluoride etchant (art stores) greatly decreased the retroreflectance from the plate itself.

I found an LED, Stanley DN305, (since discontinued)which had a rise/fall time of about 10 nanoseconds. It would have made an excellent jammer. Presumably, better ones exist today.

Jim Bell