Re: IR filters

[DeWitt Harrison <de@aztek-eng.com>]

On Thu, 17 Sep 1998 18:38:39 -0400, Robert Houk wrote:

>   I have a couple PIAA 959s in my garage, they seem ideal candidates for 
>   this modification.  All metal, they should take the heat of a big bulb.  
>   Small, so smaller IR filter needed, and tidy.  How big a bulb?
>
>How much power really is needed? The source laser *must* be down in the
>milliwatt range, and must be reading microwatt levels reflected back. It
>seems like a flashlight pointed at the source would completely swamp any
>reflected signal, let alone requiring megawatt QH bulb assemblies . . .
>Just kinda curious.

Check out the Radar Reporter at www.radar.co.nz/index.html. Read
his various thoughts and experiences with radar and lidar jamming.
It won't directly satisfy your technical reservations but seems to add
credibility to the "it takes power to jam" concept. For example,
one means of defeating an inadequately powered IR source is to
point the lidar at someplace else on the car, e.g. a headlight instead
of the license plate. Apparently easy to do.

A few points come to my mind. First, the laser system is very
narrow band and may be able to reject or ignore the majority
power from the interfering  _broad_ band IR source by precise
filtering. Second, the pattern from the interfering source will be
wide so very little of the total output power will even fall on the
lidar receiver. Thirdly, lidar uses light pulses; so as long as it can
resolve small pulses riding on top of a the continuous IR signal, the
round trip time can still be measured. You basically have to
completely saturate the receiver front end.  All these factors
multiply in a 1/100 X 1/100 X 1/100 kind of way so it's not too
surprising that your 50W light source only results in microwatts
of useful interfering power.

A light source which puts most of its power into the correct
wavelength and in a well directed pattern would require
the least power to be effective.  Sounds like an interesting
challenge, eh?

DeWitt Harrison
Boulder, CO
88 5kcstq