--- Bob Howard <mgbob@juno.com> wrote:
> David,
> Is the reflected laser beam reflected sufficiently away from the
> laser
> gun's receiver if the licence plate is tilted down a few degrees,
> or dispersed if the licence plate is bowed?
No, because the nature of a retro-reflector is that it
bounces the light back in the direction from which it
arrived. The easiest way to accomplish this is to imbed
thousands of small glass beads in the surface. This is
the method used on street signs and painted lines on
roads. In fact, when the lines are painted on the road,
the painters scatter handfulls of tiny glass beads over
the paint before it dries to achive the retro-reflectivity.
This is also the way a glass-beaded projection screen is
made, which it why such screens work well in rooms with
a lot of ambient light if you're sitting near the axis of
the projector, but don't work well if you're off the the side.
It's also the way front-projection photography works.
Remember the "stone age" scenes in "2001"? How the tigers'
eyes glowed? That's because the "background" was projected
via a semi-silvered mirror in front of the camera onto a
glass beaded screen behind the actors. Just like any
animal in the headlights, the tigers' eyes reflected some of
the light back to the camera.
If you look at a sphere, and remember that the angle
of reflection is equal to the angle of incidence, you
will see that no matter what angle the light enters the
bead, it will be bonced straight back to the source.
David Breneman david_breneman@yahoo.com
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