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Author Topic: Polarized lens cap filter
Andrew Carr
Film Handler

Posts: 12
From: Toronto, ON Canada
Registered: Jan 2016


 - posted 03-13-2018 02:12 PM      Profile for Andrew Carr   Author's Homepage   Email Andrew Carr   Send New Private Message       Edit/Delete Post 
I do passive 3D installs and am currently working on an odd setup.
Where we typically setup 12"x12" circular polarized glass filters in front of table mounted projectors, we are working on a design which won't allow for this.
I was asked to look into the possibility of "lens caps" with polarized filters in them but have never seen anything like that.
Do they exist?

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Marco Giustini
Film God

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From: Reading, UK
Registered: Nov 2007


 - posted 03-13-2018 05:15 PM      Profile for Marco Giustini   Email Marco Giustini   Send New Private Message       Edit/Delete Post 
I guess you are talking about a z-screen. I guess the reason why a lens cap z-screen is not popular is because of heat. Z-screens will absorb 50% of light as a minimum and they do get warm. If the z-screen is lens-mounted, that makes heating a real issue IMHO.

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Carsten Kurz
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From: Cologne, NRW, Germany
Registered: Aug 2009


 - posted 03-13-2018 08:06 PM      Profile for Carsten Kurz   Email Carsten Kurz   Send New Private Message       Edit/Delete Post 
I guess Andrew talks about simple passive filters in dual projection setups?

Depending on the lens front, I see no reason why this should not work. Are you talking about DCI compliant projectors? If the lens has no suitable means to mount filters, I guess you could even modify the protection caps that come with the lenses. But of course, while a Z-Screen or similar active device is more heat sensitive than ordinary static filters, the latter still need to be able to withstand the local energy density - which, due to the beam spreading, will usually be higher close to the lens front than a few inches away. You also need high quality coatings to minimize back reflections I guess.

- Carsten

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Andrew Carr
Film Handler

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From: Toronto, ON Canada
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 - posted 03-14-2018 11:13 AM      Profile for Andrew Carr   Author's Homepage   Email Andrew Carr   Send New Private Message       Edit/Delete Post 
This setup will be a passive, dual projector system.
Also, we're looking at smaller Christie units, DHD851's.
The problem is that the projector mounting won't allow for a filter bracket to be mounted in front of the projector so we're trying to figure out the best way to get passive filters in front of the lenses without fabricating a cumbersome contraption.

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Randy Stankey
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From: Erie, Pennsylvania
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 - posted 03-14-2018 11:30 AM      Profile for Randy Stankey   Email Randy Stankey   Send New Private Message       Edit/Delete Post 
I don't know if this will work but here's a stab in the dark...

There is a company named Cokin Filters who makes lens filters for still photography.

https://cokinfilter.com/pages/filters?gclid=EAIaIQobChMIoI6Gzpzs2QIVw0sNCh3nagwOEAAYASAAEgLHpPD_BwE

The idea is that their filters slip into a holder that screws into the threads on the front of the lens. This allows you to rotate and slide the filter into any position you want in order to achieve the desired photographic effect.

The filter, when placed in the holder, stands off from the front of the lens by a fraction of an inch. Cokin's idea is to allow the photographer to set the filter at different distances, again, to achieve a desired effect. It also allows the photographer to stack filters to combine effects. (eg: A star filter and a graduated orange filter can create a faux sunset effect.)

However, that little gap might give you enough space for air flow to keep the lens and filter from getting too hot.

I'm not saying that you should run right out and get a set of Cokin filters but, maybe, if you studied how they do it, you might be able to reverse engineer something that will fit your needs.

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Andrew Carr
Film Handler

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From: Toronto, ON Canada
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 - posted 03-14-2018 12:03 PM      Profile for Andrew Carr   Author's Homepage   Email Andrew Carr   Send New Private Message       Edit/Delete Post 
That looks like an interesting option.
There are lots of filters for cameras available but that design letting us insert our own material may work.
I'll check them out to see is the size is appropriate.
Thanks for the info!

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Mark Gulbrandsen
Resident Trollmaster

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From: Bountiful, Utah
Registered: Jun 99


 - posted 03-14-2018 03:40 PM      Profile for Mark Gulbrandsen   Email Mark Gulbrandsen   Send New Private Message       Edit/Delete Post 
It may or may not be an option depending on how much heat there is at the front of the lens. I used this company when I worked at CLACO and did the Zion Canyon Giant Screen digital conversion. I had to supply both linear and circular polarizers as some giant screen movies are intended for linear polarization only. The filters in their frames could be switched in about a minute This company was nice to work with and supplied exactly what I needed in a short amount of time. I did place an air blower on the polarizers because we were running 6K lamps in the Christies and they did get warm without the blower after a 45 minute run.

API

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Carsten Kurz
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From: Cologne, NRW, Germany
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 - posted 03-14-2018 06:23 PM      Profile for Carsten Kurz   Email Carsten Kurz   Send New Private Message       Edit/Delete Post 
The Cokin system usually depends on holders being screwed on front filter threads on photographic lenses. They do offer systems for large diameter lenses as well, e.g. for Medium Format cameras. If the projection lens is not threaded at the front, you're out of luck.

I have seen correction filter caps for projectors being made by simple machine turning from aluminium, to be slid on the lens bezel.

https://www.heimkino-boutique.de/wp-content/uploads/2016/09/cine4home_light_power_filter_fuer_tw7300 _tw9300_tw9300w_inkl_einstellwerte_large.jpg

I could think of other ways, but they all would come out more or less d.i.y style.

3D printing would be another option.

- Carsten

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Randy Stankey
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From: Erie, Pennsylvania
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 - posted 03-14-2018 06:43 PM      Profile for Randy Stankey   Email Randy Stankey   Send New Private Message       Edit/Delete Post 
quote: Carsten Kurz
The Cokin system usually depends on holders being screwed on front filter threads on photographic lenses. They do offer systems for large diameter lenses as well, e.g. for Medium Format cameras. If the projection lens is not threaded at the front, you're out of luck.
Understood. As I said, my point was not necessarily to buy that particular product but, to use it as an example.

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Gordon McLeod
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From: Toronto Ontario Canada
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 - posted 03-15-2018 09:14 AM      Profile for Gordon McLeod   Email Gordon McLeod   Send New Private Message       Edit/Delete Post 
Lindsay Optics might be able to provide you with what you are looking for as they make lots of filters

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Andrew Carr
Film Handler

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From: Toronto, ON Canada
Registered: Jan 2016


 - posted 03-16-2018 03:09 PM      Profile for Andrew Carr   Author's Homepage   Email Andrew Carr   Send New Private Message       Edit/Delete Post 
Good to have some options! Thanks all!

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