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. One of enthusiastic Club Members (Max) 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. Fortunately the filament size was virtually identical (to match the existing condenser optics), so Max was confident of getting a boost in light output. 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 another modern 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 the 250W 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:
Max says that his projector is "just short of magnificent" now!