116 vs. 716 and more (part III - conclusions)

[P3D Steve Spicer <s.spicer@xxxxxxxxx>]

George Themelis wrote:

> I like the simplicity and reliability of the TDC and would
>like to push its performance to the limit.  When I reach this limit and if
>I have the money, I might "upgrade" to a Brackett Dissolver or an RBT.



To push either TDC to the limit, I would recommend discarding the existing
lamps (either 500W or 750W), and replacing them with _low_volatge_ quartz
halogen lamps (either 24V/250W, or the newer 36V/400W). George, this may
sound mad, but believe me it is true. Then again, you would realise from
your experience with viewers that this is the best path to follow.

In Australia we have used the common 24Volt/250 Watt lamp for the 716, and
the result is a much better image. The light is much whiter, looks much
brighter, measures a little brighter, and the image appears much more sharp
and contrasty. (At Stereo Conventions we use this TDC with the 24V/250W
lamps as well as twin-RBT projectors, and there really is no obvious
difference visual in performance between them.)

For your interest, I have on file a copy of an article that appeared in our
Stereo Club's Newsletter a few years ago. I think I may have posted it to
the list before - anyway, it seems relavent to the current discussion. So I
have attached it below.

Steve Spicer
Australia



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Updating the TDC 716 De-Luxe Stereo Projector
(copied from "Victorian 3D Society" Newsletter)

This short note gives some technical advice on experiments with  the TDC
stereo
projector.

It may also apply to other projectors of similar vintage and design.
Our enthusiastic Club Member Max Hem recently obtained a TDC 716 Stereo
projector. This is the De-Luxe model and features two 750 Watt 240 Volt
lamps, with some nice 5 inch f3.5 main lenses. (TDC also made another model
- the 116 which used a pair of 500 Watt lamps.) Max will be using his
projector at some of our future meetings, so do try do have a good look at
it as
it is in top condition. It really does look as if it is brand new!

In Max's quest for the best in stereo projection, he set about up-grading the
lamp house by substituting some 650 Watt Quartz-Halogen 240 Volt lamps.
These are quite a common lamp and are used often in modern overhead
projectors. They are fairly expensive though at around $45 each. Needless to
say Max got quite a surprise when his Gossen light meter showed that the
light output was identical with either the old 750W or new 650W Halogen
lamp! The only improvement appeared to be a slightly better (higher) colour
temperature. Needless to say Max felt quite short-change having swapped
lamps with virtually no effect.

The next step was to compare the light output with Stephen Spicer's projector
which uses a pair of 250 Watt 24 Volt Quartz Halogen lamps. These tests
revealed that the light output as measured off the screen was identical to
the original 750 Watt or new 650 Watt Halogen lamp! This was quite a
surprise - a lamp 1/3 the power equalling the other lamps for brightness.
What was even more painful, was the fact that the polarisers in the TDC
were lighter than those on Stephen's projector, so a test with the polarisers
removed would have shown the 24V 250 Watt lamp giving slightly more
brightness than the 750W or 650W lamps.

After this Max decided to jerry-rig one 250W 24V lamp in one side of his
TDC, with a 650 Watt Halogen lamp in the other side just to see what was
really going on. The story was the same. The 250W 24V Halogen lamp was
1/3 of a stop brighter than the 650 Watt Halogen lamp. And one side of the
projector was running a lot cooler. But the other big surprise was the much
improve image quality with the 24V 250W lamp - not only was the light a
lot whiter, but the image was a lot sharper. If anything it is these last two
things which make the use of the low voltage lamp desirable.

These experiments have set straight a few things that have been said about
projector optics in our Club but have never been confirmed:

* The low voltage lamps can be made with a more physically rugged
filament (a high voltage lamp needs much finer wire) so the filament can be
run much hotter giving better whiter light and more of it Watt-for-Watt,
without the filament breaking (particularly during starting).

* The low voltage lamp has a smaller filament area, and therefore is closer
to a point source of light which makes better use of the condenser system in
the projector.

* A lamp with a larger filament provides a more diffuse light which
degrades contrast in the image and can subjectively affect the perceived
sharpness (similar to using a diffuser in an enlarger for printing).


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