Re: polarizing filters

[P3D <s.spicer@xxxxxxxxx>]

>Date: Fri, 17 May 1996 07:53:20 +0800
>From: P3D henry chung  <magicmtp@xxxxxxxxxx>

>
>I would like to ask if any one knows where I could buy a pair of high
>optical quality heat resistant polarizing filters. I am building a pair
>of 35-mm film projectors for 3-d projection. They have lamp houses of
>5,000w  xenon arc.
>I had tried Hoya polarizing filters, they lasted 5 seconds and the
>centre part turns yellow and becomes transparent.


The first thing that came to mind was where are you putting the polarising
filters? In a conventional projector there are generally three choices:

1. Between the last condensor lens and the slide gate:
The TDC has them here. The advantage is that it cuts down on heat to the
slide. The disadvantage is that the heat may shorten the life of the
polariser, but you can normally arrange for some cooling from the system's
main fan.  Also, the slide could cause depolarisation of the light (depends
on film tyoe I believe)

2. Between the slide and the projection lens:
When I built my projector (2x250W/24V/QH lamps only) I first tried the
polariser here. I quickly destroyed the polarisers - the centers faded out
much as you describe, but may be not as quickly as with your xenon lamp. In
the particular place where I could fit them in my projector, the light beam
from the lamphouse was starting to concentrate down into a narrow beam so
it could pass through the aperture in the main projection lens. Hence the
heat was concentrated much more than that for Number 1 above - into a very
small area on the polariser. This is undesirable.

3. On the outside of the projector in front of the projection lens:
This is where I ended up putting my polarisers. They have worked fine here
for over 10 years. I don't like it as the polarisers are exposed to knocks.
I used standard camera polarisers (that rotate for adjustment), and clamped
them down with a bracket once set. The heat here is quite minimal. It could
be that if you haven't tried putting your polarisers here, it may be worth
trying. I like knowing that the light leaving the projector is as well
polarised as it can be, and this position is best for this.


Also, if you have already tried your polarisers at either positions 1 or 3
above, and they are burning out, how do you expect that the stereo-slide in
the gate will be able to tolerate the conditions? I hope you have
appropriate heat filters in your lamp house light-path.

Steve Spicer
Melbourne, Australia




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