Martin McCormick wrote: > 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. Many (older) projectors are provided with a "pinhole" slide (cardboard) which permits adjustment of a mirror. My previous post mentioned the correct alignment of the mirror for a single-coil filament; the corresponding procedure for a multi-coil filament is to adjust so that the reflected image of the coils is aligned exactly between the direct image; again so that together the light source appears as a single uniform square. > The other two lenses are called "condenser" lenses and are double > convex like a magnifying glass lens. Or plano-convex x 2. > They broadly focus the lamp in to a wide beam that covers the rear > surface of the slide. The condenser lenses are adjusted to project an image of the filament well beyond the slide itself. In general, they project the filament image inside the objective lens so that as much light as possible is used effectively. An excellent example of this is the overhead projector whose efficiency critically depends on focussing the beam onto the objective lens - that is what the huge Fresnel condenser does. Of course, this is made difficult by the fact that the objective lens actually moves to focus the image of the slide. > One could imagine the light pipe in about the same spot as the > slide. No. For a light pipe, you have only one condenser lens (may however be compound) and it focusses the light on the entry aperture of the light pipe. The objective (of the design, not lens!) is to focus on a quite *small* aperture compared to a slide projector where uniform illumination of a *large* slide is required. Even 8mm projectors! > The newer Carousels have a bulb that looks exactly like a > flashlight lamp except that the bulb and reflector are one costly and > short-lived module. With the reflector pre-focussed. At one lecture I recently attended, I arrived late (*not* unusual, really). In my absence, it appears they had successively consumed *five* bulbs trying to get one projector to operate before they ran out of bulbs. I presume the late model machine has a SMPS and this one had a regulator malfunction. > You will probably need the heat lens to keep from melting the pipe > as someone already mentioned. Presumably - *although* - glass pipes should not melt as they should transmit heat instead of absorbing it. But in general, the intention to these units is usually to provide a cold light source. Inservice training on these sources is often a bit slack. I got *very* stroppy with a nursing sister this week who wanted to switch the light source off during a part of a procedure where the "scope" was - temporarily - out of use. She was concerned about it setting fire to the white "sponge" in which I had wrapped it. She later said it had happened once with a green one; I pointed out it was unlikely with a clean white one and has certainly never happened to me in the past ten years. My concern was with the ever present risk on turning off the light source that it may not start the next time. In the middle of a half-finished procedure, that *really* puts a spanner in the works, even more so than the odd charred sponge. -- Cheers, Paul B.