Michael Foster
Thu, 10 Jul 2008 11:15:34 -0700
--- On Thu, 7/10/08, Stephen A. Lawrence <[EMAIL PROTECTED]> wrote:
> So why not just use an interference filter? By
> multicoating a clear
> substrate and carefully choosing the thicknesses and
> refractive indices
> of the coatings you can selectively reflect certain
> frequencies. This
> is very mature technology. Old fashioned color enlargers
> typically used
> filters which worked this way. Those fancy gold film
> ribbons which you
> can get at any party supply store use the same trick.
>
> I'm not sure how easily this would let you reflect IR
> while passing
> visible light, tho, but I dare say some google searching on
> interference
> filers would turn up a fair chunk of info in it.
>
> I don't see the need for either nano particles here,
> nor for any kind of
> weird flourescence (which is a way of transforming
> frequencies you don't
> want into frequencies you do want, rather than a way of
> *sorting* the
> frequencies, which is what you really want to be doing
> here).
Such coatings already exist. In many common projector lamps and even
high-intensity desk lamps, there is a quartz halogen bulb inside a reflector
which reflects the visible, but passes the IR. This is called
a cold mirror. You want the reverse, referred to as a hot mirror, which also
already exists. The commercial versions of these things are far too
expensive for Jones' purpose. 3M manufactures a multilayer dielectric
film by extruding and stress tentering interposed layers of high and
low refractive index polymers and taking advantage of a phenomenon they
refer to as "giant birefringence". This stuff is also too expensive for
the purpose. Further, given the materials used, it probably wouldn't
survive in an outdoor environment. Those fancy ribbons you see in party stores
are made from rejects from the 3M process or are made by a similar
but cheaper process not precise enough for use as a hot mirror.
I have a method of making dielectric reflectors of this type
holographically. I've never implemented this method on a manufacturing
scale, but I don't see why it couldn't be done, given the demand.
M.