As an AV technician, I worked on a lot of Kodak Carrousel
slide projectors.  The illumination of the slide is fairly similar
to what needs to be done to concentrate light in to the pipe.

        The older Carrosels had a lamp that used to remind me of
a radio tube.  The filament was actually several parallel coils
of wire in front of a reflector that directed the light in the
general direction of the optics and away from the stuff that
would melt in a heartbeat were it exposed to that little Sun in a
box.

        The optics consisted of three lenses.  The one closest to
the bulb is a heat lens and has no refractive properties, but
which absorbs infrared light.  Without it, slides melt to a
blistered mess in seconds, (no kidding), and with it, they can
stay in the gate for an indefinite time.

        The other two lenses are called "condenser" lenses and
are double convex like a magnifying glass lens.  They broadly
focus the lamp in to a wide beam that covers the rear surface of
the slide.  The only other lens is the one which you see aimed
at the screen.

        One could imagine the light pipe in about the same spot
as the slide.

        The newer Carrousels have a bulb that looks exactly like
a flashlight lamp except that the bulb and reflector are one
costly and short-lived module.  It shines at right angles to the
optics, but there is a front-surfaced mirror at a 45 degree angle
between the lamp reflector and the optics which creates the light
path.  The first lens, again, is the heat lens which is the
same heat lens as is used in all Carrousels pub there is only
one condenser lens to make the broad beam.

        You will probably need the heat lens to keep from
melting the pipe as someone already mentioned.  If you use
recycled parts, be very careful of the heat lens.  New heat
lenses are quite sturdy, about 8 millimeters or 1/4 inch thick.
After use, they still look just as sturdy, but they get heat
stressed.  They have a nasty habit of violently shattering at
some random time after being released from the clamp that holds
them in place.  The glass flies everywhere and usually stays in
big pieces, but big sharp ones.  If one goes off in your hand,
the sensation is like having a big spring released.

        If it goes off near your eye while you inspect it, you
may need to learn Braille to stay literate.

Martin McCormick WB5AGZ  Stillwater, OK
OSU Center for Computing and Information Services Data Communications Group