Film-Tech Forum ARCHIVE: Toasting lenses?

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Posted by Chris Quiroga (Member # 872) on 06-01-2001, 04:32 AM:

Hello everybody,

My name is Chris Quiroga, and I am Equipment Manager for Doc Films, student film society of the University of Chicago (the oldest existing film society in the US! Check out our website at docfilms.uchicago.edu). I have been trying to learn as much as possible about projectors and other booth equipment in my short time as Equipment Manager, and I came across a post a few days ago that surprised me with its implications:

Someone said that it was potentially hazardous to leave lenses in a projector that is turned on if you don't have any film loaded. The figure they gave was like 30 seconds-- you shouldn't let the light go through a "naked" lens for more than 30 seconds. Well, I was wondering a few things:

- What damage can it cause if you leave your projector on without any film loaded for a long time?

- How can one tell if damage has already been done to existing lenses?

- How is it that a film print can offer much "protection" for the lens? In other words, is there any damage you do to a lens by playing regular film reels? What about long reels, like 6000-foot reels?

- Does it matter what kind of film print you are using-- when running a loop of RP-40, for example, should you also avoid running it for more than 30 seconds at a time, because it is mostly transparent?

I suppose the problem is likely one of too much heat hitting the lenses, but does a thin strip of celluloid really block all that much heat from hitting the lens?? In case it matters, we run Simplex XL projectors with 3000-watt bulbs. Thank you very much, in advance, for your responses.

Chris Quiroga

Posted by John Anastasio (Member # 607) on 06-01-2001, 05:23 AM:

Chris;
The problem with running a high-powered light source through a lens with no film is associated with the absorption of radiant energy by the lens and its casing. There's a lot of heat that's generated by your lamphouse. Remember that the film does absorb quite a bit of it. It's moving through the aperture, so each frame takes a little bit of the energy and then dissipates it through the machinery with which it comes in contact, effectively warming up everything around it but keeping cool itself. The heat is that way spread throughout the length of the film. Since the lens isn't moving, although it passes most of the visible light, it still takes on quite a bit of the infra-red. The longer wavelengths of light are refracted differently by the lens glass itself and lots of infra-red energy ends up striking the lense body, which is black, instead of passing straight through. Since it's just sitting there with nothing to filter it out, it can become quite hot. Stick your hand in front of the lens sometime even while a picture is being projected and you'll feel how much heat is still going through. The damage that will occur is cracking of the lens from uneven heating. You'd know one that's been heat damaged, because it'll be cracked. 30 seconds is really a round guestimate based on years of experience. It varies, of course from lens to lens and light source to light source, but over the years people have found that 30 seconds of exposure probably won't harm your lens...and it's long enough to check what you're going to check.

Posted by John Pytlak (Member # 331) on 06-01-2001, 06:57 AM:

Chris Quiroga:

John Anastasio is correct in his summary.

The film absorbs anywhere from about 25 percent (a scene with lots of white areas) to 99.99 percent (a night scene) of the visible energy before it gets to the lens. A 35mm print can be used with up to a 7000 watt xenon lamp as long as the lamp is properly aligned (no "hot spot" on the film) and uses an efficient heat filter or dichroic coating on the reflector to remove the unwanted infrared energy.

But without film, a really large lamp can heat a lens to the point of damage very quickly. A small lamp will have much less energy. One guide to how long you can put "white light" through the lens is to hold your hand just in front of the lens mount with the projector running WITHOUT the lens. With a small lamp, you will feel the heat, but not be uncomfortable holding your hand there for up to a minute. With a very large lamp, the heat is painful within a few seconds. Don't let the lens be exposed to the "bare" radiant energy any longer than you can comfortably hold your hand there. For your 3000 watt lamp, I would estimate that 30 seconds would be about the limit.

The SMPTE 35-PA film has about equal areas of light and dark. It absorbs about 90% of the radiant energy, so using a loop to check image size can be done for minutes at a time. But don't use a loop for critical evaluation of sharpness or steadiness, as the loop will heat up and dry out considerably from the energy, changing the way it focuses. Always use a ROLL of 35-PA for evaluating sharpness, focus uniformity, and steadiness.

P.S. I hope you are using motion-picture film and not "celluloid" --- Celluloid is a tradename for a thermoplastic made of cellulose nitrate and camphor.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Chris Quiroga (Member # 872) on 06-01-2001, 12:30 PM:

John (Anastasio)-- Thank you very much for your quick, concise explanation of the problems associated with running a film-less projector. It helps a lot to describe the problem to some of the other projectionists here.

John (Pytlak)-- Thank you for filling in some of the extra explanation. I'd like to try your rough test of running the projector with no lens in just to see with my hand how hot it gets, how fast. Also, your comment about SMPTE test film is very interesting. I noticed that the focus seemed to flutter a lot more on the loop than on any actual rolls we run, but I didn't know what might be causing that. Now, I suppose, it's possible (probable? definite?) that our loop is damaged. By the way, I know I misused "celluloid"; I caught myself right after I sent that post, and I couldn't go back to change it. We run actual film prints, (they're mostly polyester now, right?) not celluloid or cotton or whatever other random materials. I'm a Cinema Studies student, and I'm used to reading a lot about the film strip referred to as "celluloid" or the person on the strip as the "celluloid self." Cinema romanticism?

Chris

Posted by John Pytlak (Member # 331) on 06-01-2001, 01:26 PM:

Using a short loop of SMPTE 35-PA (RP 40) will DEFINITELY not be useful for evaluating steadiness, resolution or focus stability. The film perfs are stressed from repeated runs, and the heat deforms the film. A loop can ONLY be used for checking image area.

Posted by John Eickhof (Member # 323) on 06-01-2001, 03:11 PM:

I have seen burned lenses from improperly adjusted 1000 watt lamps! It is very important to have the lamp properly adjusted for an even field of light as not only lens damage can occur, but focus problems and even in severe cases, print burn damage may result! The newer 'air gapped'Lenses (lens elements spaced by open gaps) Are better at handling the heat from the lamp, but the older lenses with elements cemented together are quickly burned! You can see them very obviously as there will be a brownish burn area, usually between the first (closest to film) and second elements, as well as white light on the screen will be 'tan'! Take care of your lenses!

------------------
John Eickhof President, Chief Slave
Northwest Theatre Equipment Co., Inc.
P.O.Box 258
Wendell, ID. 83355-0258
208-536-5489
email: jeickhof@nteequip.com