Film-Tech Forum ARCHIVE: where best to put filters to "stop down" the light

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Posted by Adrian Hauser (Member # 4723) on 04-06-2008, 01:16 PM:

Where is it best to put filters to stop down the light output. We have two lenses on our
projector both with markedly different light outputs. One lens is set to 16 FL
and on the same etup the other is about 30 FL. Currently I adjust the light output
but you can imagine this would be a bit tedious. They are both ISCO lenses, one scope and the other academy.

We also need to make filter packs
for each to make both lenses hit 5800k. Can these be perpendicular to the film plane or is it recommended they are off axis?

Many thanks,
Adrian

Posted by Mark Gulbrandsen (Member # 72) on 04-06-2008, 02:03 PM:

I used to se a stainless steel screen in the light path between lamphouse and projector. This doesn't affect color twemperature only light level. Any place other than in front of the lens. Some Schneider lenses have room for an adjustable iris inside them. This is also a good idea.

Mark

Posted by Randy Stankey (Member # 64) on 04-07-2008, 12:06 AM:

Would it be, per chance, that your scope lens gives a brighter picture than the flat lens?

Posted by Adrian Hauser (Member # 4723) on 04-07-2008, 03:27 AM:

Yes Randy,
The scope lens is the culprit in the brightness stakes. One option is to find a better matched pair of lenses but as always its going to be a trade off between cost of lenses or a cheaper solution. The lenses arent that bad. Could be better but workable none the less.

Mark, why do you suggest to not put a filter in front of the lens. As we are a DI grading theatre we need to hit 5800K and so need to adjust our white point ever so slightly. From 6300 to 5800 to be precise. We did have a filter pack sitting parallel to the film plane before I broke it. If this is a bad idea what are my options?

Thanks,

Adrian Hauser.

Posted by Frank Angel (Member # 248) on 04-07-2008, 06:33 AM:

A friend of mine had a similar problem with going from 35mm to 70mm in a relatively small screen auditorium. He told me he used a simple iris (basically an aluminum gobo material -- same as they use in theatre lighting instruments) that was placed over the lens opening. I can't remember if he said it was at the front or rear of the lens. Said he used a protractor to cut the circle true. I would imagine, if it doesn't cause any vignetting, it would be better than filters in that it would eliminate the question of color temp changes that density filters might introduce. And cheap too.

Posted by Tim Reed (Member # 137) on 04-07-2008, 08:21 AM:

quote: Adrian Hauser
why do you suggest to not put a filter in front of the lens

Adrian, Mark's referring to a scrim, not a filter. Since it's made out of wire, you can't put a scrim in front of the lens as it will obscure the picture. Color correction gels (and/or ND filters) can go in front, however.

Posted by Mark Gulbrandsen (Member # 72) on 04-07-2008, 08:45 AM:

This is why Christie came out with the motorized optical bench on the SLC. It adjusts the lamp focus between flat and scope in order to attempt to maintain a more even light level between the formats.

IN any grading situation you would never want to place any filter in front of the optical path no matter how high of quality that filter may be. Any filter in front will cause some diffraction of the light leaving the lens. No matter how insignificant it may be you still want the optical train as perfect(and color matched) as possible!! The iris in the lens internally als Schneider is good and I would also vote for the scrim because it won't degrade the optical quality of the image nor change the light color temperature. Filters behind the film plane are ok and won't optically degrade the image... I don't know of a scope lens that has a threaded front to allow screwing on or adapting filters. You always end up taping them on and it makes a mess out of everything.

Mark

Posted by Larry Myers (Member # 738) on 04-07-2008, 09:43 AM:

If in fact the scope lens is brighter then someting is wrong with your flat lens. It should be the other way around. That is, the scope lens should be about 16 FL and the flat should be close to 30 FL. The reason is, the flat lenses light is not being spread out 2x in one direction.

Posted by Richard P. May (Member # 3598) on 04-07-2008, 10:20 AM:

In our laboratory screening room, the "natural" light from 1k lamps is too bright for the screen size. We have placed neutral density gel filters, which are in a home-made frame, directly in front of the lenses. The light intensity can be easily corrected, and we see no effect at all on the projected image.
The filters are simply taped to the front of the Norelco FP20 cases, in a position that they do not get in the way of the door.
We previously had the filters on the rear of the porthole glass. This wasn't good, as the light bounced back from the glass onto the filter, then to the front again, putting a sort of ghost image on the screen whenever there was a bright part of a scene.
Previously, I had made small filters and mounted them in a 35mm still photo slide mount. These were placed in front of the projector gate, behind the lens. On a Simplex XL it worked fine, as the gate assembly almost invites such a unit. The Norelcos aren't built that way, and it was hard to keep it in place. There was never any melting or fading of the filters, regardless of the fact they were right in front of the film.
The same technique can be used for color correction filters if the lamps are not putting out evenly colored light.
The theory of the scope lens only transmitting half of the light of a flat lens to the screen doesn't work in practice. We only find a difference of about 3 fl drop, and to compensate for this, swing the ND filters out of the light path.
Don't forget, the light is being spread double only in the horizontal direction. To make a 50% reduction I think it would need to be both twice as high and twice as wide.

RPM

Posted by Tim Reed (Member # 137) on 04-07-2008, 11:45 AM:

quote: Larry Myers
If in fact the scope lens is brighter then someting is wrong with your flat lens. It should be the other way around.

Bzzzt... (with apologies to Steve Guttag.) Scope is naturally always brighter because the frame area is larger. If flat and scope utitlized the same area on the film, then your theory would be correct. If anything, they should be close to the same, but in practice scope is almost always brighter.

Posted by Sean McKinnon (Member # 612) on 04-07-2008, 01:27 PM:

Larry,

Look at both your scope and flat aperature plates. The scope plate will have a larger aperature than the flat... Larger aperature means more light allowed through. That is one of the benefits of scope in my opinion there is more picture "information" and more light. IMO The reverse is actually true if your FLAT lens puts out a brighter picture than your scope lens then something is wrong.

Posted by David Graham Rose (Member # 1450) on 04-07-2008, 01:49 PM:

Greetings All

Schneider make a filter kit with holders at 10 degrees to the lens for exactly the purpose you describe. Follow the link to see what they have on offer.

From a snowy Cambridge

David Scneider Optics

Posted by Richard P. May (Member # 3598) on 04-07-2008, 05:40 PM:

Sean,
Your theory about the scope lens passing more light is logical when comparing with 1.85 format. My comparison was with 1.37 format, which concentrates more light and thus comes out a bit brighter than scope.

The Schneider filter kits do look great where precise modifications are needed, but run around $1000 per set. In a two-projector setup, this would buy lotsa gelatin filters.

RPM

Posted by Frank Angel (Member # 248) on 04-07-2008, 11:04 PM:

And LOTS of aluminum donut stop-down rings. Elegante can be quite expensive.

[ 04-08-2008, 07:00 AM: Message edited by: Frank Angel ]

Posted by Steve Guttag (Member # 268) on 04-08-2008, 06:47 AM:

First off...if 1KW is way too bright....then 3-wing shutters are the answer...they reduce the light (by 50%) AND eliminate the flicker...it is a win-win and they wont fade or degrade the image in any way...in fact, most say they improve it.

As to the Schneider filters...they are primarily to color match machines. While the filter KIT may be expensive...you only need one of them (and a proper meter). Once you know the corrective factor for your projectors...you only need apply the appropriate filters rather than the whole set.

Then again, depending on the lamphouse, you can get the mirrors to be matched, which will again, have a stable color temp over a long time, as opposed to filtration.

At Colorlab, we did successfully use filters in front of the projection lens to have precise 5400K and 3300K for so both xenon and tungstun could be properly timed.

Steve

Posted by Alan Gouger (Member # 543) on 04-08-2008, 09:02 AM:

I am enjoying this thread. I thought Xenon always threw the proper color temp never thinking reflectors etc could shift the color.
I recently installed a small 500watt Kneisley lamphouse and compared to my other xenon lamp house it has a very greenish blue tint to it. The reflector is glass. Im guessing its the reflector responsible for the tint? Is there differences in color temperature between lamps as well?

Posted by Frank Angel (Member # 248) on 04-09-2008, 08:06 PM:

I have seen a number of discussion on here about color matching reflectors and that it is fairly critical (and difficult) to get them perfect. But that has always been in reference to xenon lamphouses. Is that peculiar to xenon only? I have not heard that in reference to carbon arc. Is carbon arc less prone to variations in the reflectors?

Posted by Paul Mayer (Member # 355) on 04-09-2008, 10:17 PM:

The one time I had to match up a pair of dissimilar lamphouses I taped the color correction filters (1/4 CTO on that particular job) to the backside of the shutter housing (Simplex XL). That way I didn't have to project an image through them. That booth also did not have any glass in the projector ports, which kept the DP from claiming I was "loosing it in the glass." [Smile]

As has been noted elsewhere on these forums probably the easiest (if not cheapest) way to get matched color performance is to order reflectors that come from the same manufacturing batch.

As for intensity I thought most modern lenses have a provision for installing stop down rings as needed for each format to be shown.

Posted by Richard P. May (Member # 3598) on 04-10-2008, 10:17 AM:

Frank,
It's been a long time since I've used carbon arcs, but remember the color could change based on whether they were properly trimmed or not. Often changeovers showed a difference, especially on B&W films.
Wouldn't this make it particularly hard to balance the lamps?

RPM

Posted by Frank Angel (Member # 248) on 04-10-2008, 12:45 PM:

No doubt the color can change due to trim, but more than that, it changes with the temperature of the rods themselves. The trick is to fire up the incoming lamphouse a few minutes before the change-over. And no question, B&W films is where you see the color temp variation the easiest. With color film, especially when change-over is between scenes where there is little reference information for the eye, it is very difficult for the average person not looking for variation to tell color temp change.

If you start your arc 4 or 5 minutes prior to change-over, the plazma gets the rest of the rods behind the arc hot enough so as to reach the same temp as the outgoing machine. Color between the two then are very close. We also keep the rods in the lamphouse itself so the constant heat in the lamphouse helps pre-heat them enough so as to drive out any moisture in the carbon itself.

We still run carbon arcs....probably will until we can no longer get the carbon rods. Never saw any reason to change as it is not an automated booth. And that nice big fat arc that carbons generate makes it really easy to get very even illumination across the screen, edge to edge.

Posted by Tim Reed (Member # 137) on 04-12-2008, 10:26 AM:

quote: Frank Angel
it changes with the temperature of the rods themselves.... the plazma gets the rest of the rods behind the arc hot enough

They're not rods, they're carbons. [Wink]

Posted by Scott Norwood (Member # 30) on 04-12-2008, 10:38 AM:

So, Frank, where do you get your reflectors and do you have a problem with cracking?

I know of one theatre that may have to convert to xenon due to the lack of a reliable source for reflectors.

Posted by Bernie Anderson Jr (Member # 410) on 04-12-2008, 09:30 PM:

Wouldn't a 3 bladed shutter do the same thing? This is the first I'm hearing of the filter idea. I get alot of shutter flicker on my scope too being its a small screening room and a filter sounds like a great idea.

Posted by Richard P. May (Member # 3598) on 04-13-2008, 01:00 PM:

A 3-bladed shutter could very well cut down the light. The Norelcos have a 1-bladed shutter. Probably a replacement shutter could be custom made with a larger blade, but changing it would eliminate the flexibility of modifying the light if needed, such as when running scope films.

Posted by Bernie Anderson Jr (Member # 410) on 04-13-2008, 06:22 PM:

After reading this thread, I decided to take out my three bladed shutter and try it again. I remember now why I removed it. For some reason, I can't get shutter ghosts out of it, even with the fine adjustment (Super Simplex). If I clear up the top, the bottom starts to ghost and so on. Its as if the blades are just a hair to small to cover the movement of the film. I still get flicker and its only on the right side of the screen. Its not as bad as the 2 blade, but even with the 2 blade, I get more flicker on the right side of the screen than the left. Before anyone says anything, the power supply was rebuilt with new diodes and caps and they work and its focused and aligned properly. So its definitely something to do with the shutter.

Now my question is with the filters, what kind of filters are they? The Schneider Filters will not work, but the gel filters sound like they're a stage lamp filter used for stage work. If so, does anyone have the color number? And if this something that should be before the aperture? Or can it go between the aperture and the lens?

Bernie

Posted by Adrian Hauser (Member # 4723) on 04-13-2008, 10:38 PM:

OUr engineer is making up some glass "filter packs" to put in front of the Lenses.
They are Color Correction filters to be matched to each lens.
Does anyone know of how to measure the rating/intensity of the light out put
and convert that into a heat value, so that we can make sure we get the right specced glass. Ie that wont crack! OUr lamp is a 1k xenon.
If we can put it before the film plane I guess from comments here it would be better, but I guess then the light is even hotter.

The corrections are quite minimal ie:6300k to 5800k.

ADrian

Posted by Frank Angel (Member # 248) on 04-14-2008, 11:02 PM:

Scott, luckily, when I first got to this theatre, I was a young snot nosed kid and I overdid everything. I told them to get literally a closet full of carbons and 5 sets of reflectors. Only recently I had to get more carbons, and only changed reflectors twice because of some kind of caustic goop that dripped down from the exhaust pipe when rain water blew into the intake vent.

quote: Tim Reed
They're not rods, they're carbons.

Well, perhaps, but both work for me and both are technically correct. "Carbons" : the name assigned the specific functioning of a part in the arc lighting system; it derives from the main functioning component -- carbon, similar to the way "intermittent" is assigned to a specific functioning gear system to advance the film. If you said, the "film advancing gear system," you would still be correct.

Similarly, you can also refer to carbons in their more generic term which describes the part itself, i.e., carbon compressed and surrounded by a copper sheath (in some) in the form of a ROD -- hence, carbon rods. Perfectly understandable....it communicates what is intended and it's technically correct.

(How's that....did I wiggle my way out of that one?)
[Razz]

Posted by Tim Reed (Member # 137) on 04-16-2008, 03:04 PM:

They are not called rods, they're called carbons. Ask any old timer. [Razz]

Posted by Richard Fowler (Member # 893) on 04-16-2008, 07:42 PM:

A welder would call them "rods" [Big Grin]

Posted by Frank Angel (Member # 248) on 04-16-2008, 11:48 PM:

Are you saying I am not an old timer? Tim, you're too too kind!

Posted by Tim Reed (Member # 137) on 04-17-2008, 08:38 AM:

No, just a little misinformed.