RE: IR filters

["Buchholz, Steven" <Steven.Buchholz@kla-tencor.com>]

> Some have asked why you would need soch a powerfull IR source to 'blind'
> a LIDAR gun, given the fact that these units should operate in the mili
> or micro watt range.  It has to do with beam widths.  The LIDAR gun has
> a very narrow beam width.  That means that as the signal gets to the
> car, bounces, and returns to the receiver it is maybe 6 ft. wide.  The
> intensity of the signal is still very strong, since its power is only
> spread over a small area.
> 
> The laser jammer OTOH doesn't know where the REO is, so it must shine IR
> energy in all forward directions (and backwards if you want to cover
> your tail) much like a broad high beam pattern (not ultra-narrow pencil
> beams) to make sure it illuminates the laser gun.  So the IR energy of
> the jammer is spear over a big area which could be 1000 times larger
> than the laser gun's beam, and in comparison, is 1000 times weaker.  For
> those of you with a radio background, this is equivalent to the "antenna
> gain".  For the rest of us, this is equivalent to your adjustable
> spot-flood flashlight.  In spotlight mode, the flashlight is very
> bright, but only illuminates a small area.  In floodlight mode, the
> light is weaker, but illuminates a much larger area.  Conservation of
> energy and all that.  HTH
> 
... sorry, but while the beam that illuminates the evildoer is very tightly
controlled, the return beam is quite uncontrolled.  Depending on what it is
reflecting from there will be a different amount of dispersion of the
return.  Actually, the likelihood that any random vehicle will have a
specular reflection that just happens to be returned to the detector is
almost 0 ... even though the cops like to aim at the large vertical
surfaces, they are depending upon the surface they hit to scatter the light
just a bit.  No matter what the illuminated cone created by the LED, the
REO's receiver is going to see a direct transmission by the LED, which is
quite likely to be more intense than the reflection of the pulse he sent
out.  

As I stated in my earlier post, the idea is to create noise that makes it
impossible for the receiver to pick the return beam up.  It is usually much
easier to jam a signal that is being reflected than it is to actually
receive the original signal.  Of course, the way things are going, the state
will "improve" the design of the retro-reflective license plates we now use
to include IR wavelengths.  Perhaps I shouldn't have said that ... they
might not have thought about that idea yet ...

Steve Buchholz
Somewhere USA