In my line of work/research/slavelabor we use dichroic
mirrors/lenses/filters, etc everywhere.  So to quote an optics catalog:

"Dichroic filters transmit light of a certain color and reflect its
complementary color.  They have little or no absorption. They can be used
to separate a color from its complement by transmission and
reflection.  For color matching, they are superior to glass filters which
subtract light by absorption.  Dichroic filters are manufactured by vacuum
deposition of thin films onto glass substrate."

Remember that physics class when you talked about destructive and
constructive interference? Well, that is what these jobbies work on. The
dichroic film's thickness is on the order of a wavelength of light- they
actually will tune the thickness to match a desired color or
wavelength.  All of this is for a given light incident angle, of course.

So to get to Phil's question, i suppose if you wanted blue (or
whatever) light in your bus, you could purchase a lightbulb that has a
dichroic coating that will only transmit blue (or whatever it is tuned
for).  I think this is how those cheap blue head-lights work.  The
expensive (like those found in high end cars these days) blue-ish
headlights are a whole different thing.

-Matthew
84 wolfsy weekender

<For more info on my reasearch, see the top link below.>

Matthew Pollard          http://dynamics.chem.uidaho.edu/~dynamics
Dept. of Chemistry                      http://www.chem.uidaho.edu
University of Idaho                          http://www.uidaho.edu