This is topic Polarized lens cap filter in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.


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Posted by Andrew Carr (Member # 9264) on 03-13-2018, 02:12 PM:
 
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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Posted by Marco Giustini (Member # 4544) on 03-13-2018, 05:15 PM:
 
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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Posted by Carsten Kurz (Member # 5396) on 03-13-2018, 08:06 PM:
 
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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Posted by Andrew Carr (Member # 9264) on 03-14-2018, 11:13 AM:
 
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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Posted by Randy Stankey (Member # 64) on 03-14-2018, 11:30 AM:
 
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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Posted by Andrew Carr (Member # 9264) on 03-14-2018, 12:03 PM:
 
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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Posted by Mark Gulbrandsen (Member # 72) on 03-14-2018, 03:40 PM:
 
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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Posted by Carsten Kurz (Member # 5396) on 03-14-2018, 06:23 PM:
 
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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Posted by Randy Stankey (Member # 64) on 03-14-2018, 06:43 PM:
 

quote: Carsten Kurz

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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.

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