Ferrari Mailing List: Lidar, license plates, and ?

[Hans E. Hansen (FList]

Regarding chatter on another thread about IR light reflections needed
so you-know-who can determine your speed......

Consider what would happen if you tried to measure the speed of
a flat mirror.  The beam would hit and be deflected off at a perfect
angle reflecting the angle at which it arrived.  Odds are huge that
the narrow beam would not just happen to accidentally be reflected
back in the exact direction to arrive at the Lidar sensor.  The mirror
would have to maintain a very precise position.  So, in
effect, it would be nearly impossible to measure the velocity of
a flat mirror in this way.

What about a car?  Take a laser pointer and "paint" the front
of your car.  You'll find spots where you can see the beam
is bouncing back by direct reflection.  This will often be on
a curved surface.

However, some surfaces will scatter the beam - sending
it back at many angles.  The pearlescent paint many states
use for license plates does just this.  Point your pen laser at
such a plate.  You'll see a bright "starburst" that changes little
with the angle of the laser beam.  Thus if the Lidar hits your
plate you are probably screwed.

What to do?

Oregon's standard plate has a white pearlescent background
that is probably perfect for laser reflection.  However, they also
produce a "Crater Lake" plate which has a photograph of the
dark blue lake with your plate number in white numerals.
Fortunately, there is no pearlescent quality at all to this design.
Red laser (and presumably IR) does not reflect at all from the
blue, and - lacking the pearlescent - the white doesn't scatter
the beam.  It just reflects it pretty much as a flat mirror would.
Pretty cool.  This plate does well with the laser pen
test.  I have them on all my cars.

So.....

I've been thinking.  Yes, I know, I shouldn't do that.  UV is quite
easy to block.  Plain window glass does reasonably well.  Now
I know that UV is on the opposite end of the light spectrum, but
would it be a stretch to imagine that there are materials that do
not pass IR well?  Blue photographic filters are basically opaque
to IR.  What if you mixed transparent blue tint (available at
hobby shops, if not other places) with clear and coat surfaces
such as offending license plates?  I'm wondering if a reasonably
light tint would be sufficient to block IR.  If so it wouldn't be all
that noticeable.  It wouldn't be hard at all to test this in the visible
with a pen laser.  With only slight difficulty, one could devise
a IR LED to illuminate the surface and a IR phototransistor to
measure the reflectance.  Radio Shack (Shido Rack?) used to
sell IR LED/phototransistor pairs.  Maybe they still do.

If I find time, this would be an interesting experiment.  But until
then, maybe one of you might try???

Hans.