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Projectors With Cold Light

Vlad Soare , May 15, 2003; 02:28 a.m.

Slides' colors begin to fade after a certain projection time (longer for E6, shorter for Kodachromes), for two reasons: heat and UV.
Is there a projector that uses cold light? If not, then why? As for UV, wouldn't it be enough just to put an UV filter between the bulb and the slide?


Bob Atkins , May 15, 2003; 11:58 a.m.

I think the answer is that it's not just heat and UV. If you hit a slide with enough visisble light for long enough, it will fade.

Projectors do use filters, both to cut down on heat and UV and all have forced air cooling to keep slide temperature down.

My guess is that the major problem with a cold light source (essentially a fluorescent bulb) is that you can't get small, high intensity, cold light sources with a continuous spectrum thoughout the visible. Fluorescents also tend to put out UV so you'd need the same UV filtering as for hot lamps, and of course high intensity visisble light will still cause problems.

Eric Domazlicky , May 15, 2003; 12:49 p.m.

Some people have been making their own LCD projectors that use semi-cold flourscent lights or a bunch of white leds as a light source. (More info). They are not near as bright as hot light sources of course. Not sure you really want to make your own slide projector though, and I doubt it would help with your fading much. Why not make copies of your slides if you are projecting them often and store the orginials in a nice dark place?

Christoph-Erdmann Pfeiler , May 15, 2003; 01:35 p.m.

From a practical point of view - use duplicates for presentations and keep Your original slides (or negatives) cool and moisture controlled in a dark area.

Or digitize them and print them for use.

Steven Clark , May 16, 2003; 08:02 p.m.

Does this also apply to scanning? Film often seems to come out of a scanner pretty warm at times and with scan times around 2 minutes it makes you start to worry.

Bob Atkins , May 16, 2003; 11:53 p.m.

As far as I know all scanners use cold lights (fluorescent tubes or LEDs).

Christoph-Erdmann Pfeiler , May 17, 2003; 11:20 a.m.

Light sources for scanners depend on the construction principle - flat bed scanners and equivalently built filmscanners need a homogenous, extended light source - cold light is ideal. Drum scanners use as small as possible light sources - Tungsten bulb or similar.

John Kim , May 17, 2003; 12:56 p.m.

From what I understand of physics, the problem isn't heat per se. Conducted and convective heat is pretty easy to eliminate with a fan. Radiated heat in the EM band (infrared) is pretty easy to filter out - a sheet of glass does a pretty good job (this is why greenhouses work). The real problem is the light itself; more precisely the natural process by which materials turn white light into different colors. Transmissive materials like slides generate colored light by absorbing all the "wrong" colors. This absorbed light mostly gets turned into heat. So in that respect, there is no such thing as "cold light." Within the visible spectrum, any balanced "white" light which produces a projection of a certain brightness will cause the same heat damage to a slide regardless of the light source. The best you can do is draw enough air over the slide to cool it and prevent heat buildup.

Jeffrey Rodgers , May 19, 2003; 07:27 p.m.

I've been trying to design what you descibe, but keep getting stuck. the problem is that a very bright light is needed, and a continuous color spectrum, so many light sources are either: not bright enough, too diffuse, or color is off. The best way to control the heat is special mirrors (they are better than heat absorbing glass), but the mirrors are very expensive, and diffuse sources like fluorescent are hard to concentrate all the light output into the aperture of the projection lens... the rays are all at different angles so concntrating some means scattering others. This is OK if you don't need so much brighness, like with an enlarger (I have designed a black and white enlarger using a compact fluorescent with parabolic relfector) My best advice is to just get a 150 watt projector and use it at closer distances to screen... less heat and still fairly bright image.

David H. Hartman , May 21, 2003; 03:44 a.m.

The dichroic reflectors used by many projector bulbs "filter" harmful heat (infrared) and probably UV light, the heat absorbing glass filters more. John Kim’s ‘all the "wrong" colors’ make good sense to me. Jeffrey Rodgers’ "150 watt projector and use it at closer distances" makes good sense also. I doubt that it’s worth the trouble to try and make a cold light slide projector.

I hate showing slides at camera clubs because sometimes they stop and discus the photo at length or worse something else while your slide cooks. I’m be glad when all projection is digital. No finger prints either.

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