P3D Re: Broken Stereomatic 500 Condenser Lens

[Tom Hubin <thubin@xxxxxxxxx>]

Pixschack@xxxxxxx wrote:
> 
> One of the small greenish condenser lenses in one of  the 2 mirror boxes of
> my VM Stereomatic projector has broken.  I've removed the remains and the
> projector seems to work just fine without it.  What am I missing if I use it
> this way?  Anyone know where I might obtain a replacement?
> 
> Gary Schacker

That is probably Infrared Absorbing glass. More commonly known as heat
absorbing glass. Projection lamps radiate about 5% light and 95%
infrared energy. Slides absorb both and convert them to heat. The 95%
energy that is infrared is not visible so it is not necessary that it
pass through the slide. The 5% energy that is visible must illuminate
the slide to produce the picture at the screen.

So the infrared absorbing glass absorbs the infrared and helps keep the
slide from getting too hot and damaged. Of course, the pale green plate
gets incredibly hot. 300F and up. And sometimes they break.

If your projection lamp is 300 to 500 watts you should not risk your
valuable slides. Keep the illumination time to only a few seconds. Not
minutes.

To replace it, you will need to know the thickness of the plate and the
diameter. The glass type may be difficult to determine but Schott KG1
and KG3 are good choices. Having a pair custom made will cost $200 or
so. Usually you hunt for some common projector with heat absorbing glass
that will fit in your machine.

The TDC116 and TDC716 use 60mm diameter, 3.2mm (1/8 inch) thick plates.
Glass type unknown but I have seen at least 3 different glass types in
these 50 year old machines.

Kodak has 63mm diameter, 6mm (1/4 inch) thick plates for their common
projector. They sell both KG1 and KG3 glass for $32/plate. They
recommend KG1 for 300 watt lamps and KG3 for 500 watt lamps. You can use
either but I would suggest the lighter colored KG1. If you have a 500
watt lamp and there is a polarizer between the lamp and the slide only
38% of the light is getting to the slide anyway. Mono projectors don't
have polarizers so more of the lamp energy gets to the slide.

Anyway, I never did find a good stock supplier for TDC heat absorbing
glass. I know how to get them made but the cost and time are too high. 

Cutting down an existing plate to fit your machine does not work because
these plates are usually tempered so that they will survive the high
heat. Once that is done you cannot alter them easily. 

Best bet if you cannot find an exact replacement is to modify your
machine to take some glass plate that is easy to find.

Read http://www.clark.net/pub/thubin/aosd/photo3d/ir_study.txt if you
want a little more technical stuff on the subject.  

Just for fun, if you still have a good green plate on the other side,
try this test. With no slide, block the light coming out of each
projection lens with your hand. Feel how hot your hand gets when the
green filter IS in the condenser system. Feel how hot your hand gets
when the green filter IS NOT in the condenser system. There should be an
obvious difference.
 
Tom Hubin
thubin@xxxxxxxxx


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