Projector lenses, lamps and polarising material

[P3D Peter Penhallow <100767.21@xxxxxxxxxxxxxx>]

A contribution from Colin Clay, dated 3rd January 1997.

I have very much enjoyed reading recent contributions on stereo projectors,
sent to me by George T. I agree with most of the replies and I have the
following comments.

Projection Lenses:
Provided that the rear element of the projection lens is large enough to
accept the image of the lamp filament, there will probably be no
significant improvement in brightness in changing from an f/3.5 to an f/2.8
lens. If the f/2.8 lens is better designed and made, or has superior
coatings on the surfaces then the change could be worthwhile.

Comparison between 500w and 750w projector lamps.
I wonder if the 750w lamps have a larger filament which, when imaged onto
the rear element of the projection lens is too big, so that some of the
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extra light is lost.

I certainly agree with Stephen Spicer that the use of 24v 250w or 36v 400w
quartz halogen lamps in TDC projectors is well worth considering. I have
found toroidal transformers particularly successful for supplying the
voltages, and a 500 watt transformer with two 25 volt outputs costs about
#30 ($50) in the UK. The much smaller filaments of these lamps decreases
the size of the light beam through the projection lens, effectively
stopping it down, and this probably accounts for most of the increased
sharpness of the screen image. I consider that some changes to the
projector optics are desirable to achieve optimum results and I will send
some suggestions to George T. for possible inclusion in his proposed book
on the TDC projectors.

Polarising Materials.
Some years ago I obtained samples of polarising material from Polaroid
Limited and carried out some tests. I considered that the best grade was
HN38 which transmits 38% of the light. A pair of filters with the
polarising planes parallel passes about 32% of the available light, and
crossed polarizers pass about 0.2% with more at the blue end of the
spectrum. Thus over 2/3 rds of the available light is lost in the
polorizers which explains why it is so hard to obtain adequate brightness
for stereo projection. It could well be that the Edmund Scientific, more
expensive scientific grade of polarising material, is of the HN38 type.

Peter Penhallow for Colin Clay
(ps apologies if this is the second copy, having e-mail problems)


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