Re: Cat's eyes.

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

Thanks John. On a webpage (reached from the second URL
you posted)

<http://www.reflexite.com/microprism_technology.htm>

they show a sphere and a flat mirror acting as a retroreflector.
The index of refraction of the sphere was presumably chosen
to make sure the parallel beam of the incoming light is focused
where the mirror and the lens touch. Such a system would
be highly sensitive to the angle between the beam and the
mirror surface. Any deviation from 90 would destroy the
retroreflectivity. It would change the
"refraction-REFLECTION- refractin" sequence into the
"refraction-refraction-REFLECTION-refraction-refraction"
sequence.

But a sphere whose back surface is coated with a reflecting
layer would be multidirectional. Is this an acceptable model
for the glowing eyes of a cat when pupils are widely open?
What is the evolutionary advantage of eyes with reflecting
retinas?

On the above-referred webpage they write "Candlepower-
Typically 1000 cpl". Also "Typical brightness of white is
800 CPL". (The second specification is for another tape.)
Does this make any sense? I would expect the brightness
o be proportional to the intercepted luminous flux. And
why is cpl the unit of brightness?

John D. wrote:
> > Where can one buy it [retroreflective tape used on
> > trailers, road signs and sport jakets)?

> A web search turns up many answers to this question
> <http://www.altavista.com/cgi-bin/query?q=tape+near+retroreflect*+and+pric>
>
> http://www.altavista.com/cgi-bin/query?q=tape+near+retroreflect*+and+pric*&pg=aq>
>
> Also, you can probably find some at the local sewing supplies store.
>
> > How does it work?
>
> Multiple versions exist:
>
> 1) In some the tapes I've seen, there was a transparent front layer that
> was flat on the front and had zillions of pyramids embossed on the
> back.  The back of this first layer was reflective.  Then there was a
> second, optically-inactive layer, and then some adhesive.
>
> 2) There are other ways of doing it.  For marking roadways, they do it by
> putting down relatively ordinary paint, and then (while the paint is still
> wet) dusting it with retroreflective glass beads.  There are also tapes
> that use glass beads.  There's even a federal procurement specification for
> such beads: TT-B-1325