Film-Tech Forum ARCHIVE: A stoopid film-heat question

This is topic A stoopid film-heat question in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Rick Long (Member # 213) on 01-30-2003, 12:18 AM:

Each time I change a xenon bulb i always check for heat on the film. I do this by "scissoring" my index and middle finger and closing them very lightly on the film. (On Simplex this is most easily done where the film drops down to the sound-drum). This is usually done on dark film (where I suppose, heat build-up would be most evident).

The results are almost always the same when I check during the performance.

Checking the heat of the film when the film has just started, and maybe ten minutes or so after the film has been running, results are quite cool.

About an hour later, however, when performing the same check, I often find the film temperature quite hot!

What has changed?

The xenon lamp is drawing the same current.

Water cooler and aperture fan are still running.

Heat filter is in place.

The temperature of the film leaving the platter and even travelling around the upper-feed sprocket remains the same.

The film is only being held by the edges in the gate.

So why does it now feel hot? What changed?

Posted by Paul G. Thompson (Member # 655) on 01-30-2003, 12:28 AM:

The bulb focus is probably out of adjustment. Very briefly, re-target the bull's eye, put the lens in, and flush the field so it is even light on the screen. Consult the instruction manual for the lamphouse, it should go into much greater detail.

Don't worry about this being a stupid question. The only stupid question is the one that is not asked... [Wink]

I add this on an edit....re-aligning the lamp's focus and bulb position is necessary 99% of the time when you install a new bulb.

[ 01-30-2003, 03:07 AM: Message edited by: Paul G. Thompson ]

Posted by John Pytlak (Member # 331) on 01-30-2003, 08:16 AM:

Some links:

http://www.kodak.com/US/en/motion/newsletters/pytlak/fall97.shtml

http://www.kodak.com/US/en/motion/newsletters/pytlak/winter97.shtml

Several possible explanations of why your print gets hotter after the projector "warms up":

1. The gate rails are getting hot, and this heat transfers to the edges of the film, and eventually across the width of the print. Be sure your lamp is properly focused, and not overfilling the aperture and spilling onto the metal parts of the gate. Water cooled gates are a very good idea when using larger lamps.

2. The dichroic heat filters are becoming less effective as they heat up. This may be due to a change in the infrared "cut" as the filters expand with heat which can change the spacing of the dichroic coatings. But more likely, if the only heat filter is the dichroic coating on a metal mirror, the effectiveness is decreased as the metal of the mirror gets really hot. Be sure the air flow of the lamphouse is sufficient and properly flowing around the mirror to keep the mirror itself cool. Use an additional dichroic heat filter in the light path, that reflects the infrared to a heat sink.

Remember, the heat energy that is being rejected by the dichroic heat filter(s) has to go somewhere (conservation of energy). That somewhere is either a metal mirror, or a heat sink. Proper airflow is needed to remove that heat and keep them cool, or they will lose effectiveness as they get too hot.

3. Darker scenes absorb more radiant energy than light scenes. Silver-image Black-and-white prints absorb more infrared energy than color prints. Maybe the latter part of your test print has darker scenes?

Posted by Gordon McLeod (Member # 33) on 01-30-2003, 10:17 AM:

I have placed a thermalcouple probe in the apperture and have noticed that in particular one brand of Xenon lamp seems to produce far more IR (heat) than others for the same amperage and screen light level and distribution
I even had a PE lamp shatter a shortfocus lens (with film running) 4K @135 amps with 16fl in the centre and 14 in the corners so lamp focus should not have been an issue. I also have had those lamps regularly blister the dichroic coatings off reflectors PE lamps [puke]

Posted by Christopher Seo (Member # 7) on 01-30-2003, 10:41 AM:

Rick,

I have no answer to your question, but one simple test you could try (if you haven't already) is to run the bulb for an hour with the hand douser closed. If the film still runs hotter as you describe, then that would eliminate everything after the douser as the culprit.

Question: Is it possible for thermal expansion of the xenon lamp to move the arc out of the point of focus?

Posted by John Pytlak (Member # 331) on 01-30-2003, 11:18 AM:

Christopher Seo asked: "Question: Is it possible for thermal expansion of the xenon lamp to move the arc out of the point of focus?"

Possible, but not by a significant amount. However, other components in the lamphouse may shift position slightly as they heat up. Final lamp alignment and focus would normally be done on a lamp that had been on for awhile.

I still suspect heat is building up on the dichroic mirror, making it less effective as it gets really hot.

Posted by Mike Olpin (Member # 1115) on 01-30-2003, 03:51 PM:

quote:
Don't worry about this being a stupid question. The only stupid question is the one that is not asked...

Always remember. There are no stupid questions, just stupid people. (j/k) [evil]

Posted by Mark Gulbrandsen (Member # 72) on 01-30-2003, 08:52 PM:

I have placed a thermalcouple probe in the apperture and have noticed that in particular one brand of Xenon lamp seems to produce far more IR (heat) than others for the same amperage and screen light level and distribution
______________________________________________________________

Gord, it has to do with the quartz they are made from, and how much anti ozone(really just anti IR/UV) is in with the glass.......in other words the no ozone feature in the glass varies from lamp to lamp, and manufacturer to manufacturer....perhaps cause of getting around patents and such.
Mark @ CLACO
www.clacoequipment.com

Posted by Rick Long (Member # 213) on 01-31-2003, 12:08 AM:

First let me thank you all for your contributions. I found Kodak's notes (referenced by John Pytlak very helpful).

Still a couple of questions remain.

I failed to mention the equipment with which i am finding this situation of overheating.

Simplex Millenium (although noticed the same effect on 1060's 1050's an Cinemeccanica Vic.8's)

In all cases, an IR filter was in place at the front of the lamphouse.

Also, in all cases the bulb in use was Osram 7000w/hs.

John Pytlak has mentioned (in other topics) that in order to acheive decent screen light, the formula should be about 5 watts per square foot. What then should be the biggest picture I can project using this bulb?

My question still remains where is the heat coming from? (Yes, I know its the bulb). The IR filter should provide, to some degree, a division between any heat developed in the lamp and the aperture. (Talking new IR filters here). Other accessories such as water-cooling the gate and fan-forced aperture cooking should help this situation even more.

Proper lamp cooling was mentioned also. Does cooling the mirror help cool the light emitting from it?

If the film is getting hot due to improper focus, causing a larger than normal "spot" on the aperture and therefore causing the trap to get abnormally hot, should this not result in a diminished light? (The available light not all being focussed into the aperture? Should I put an ice-machine across the mains feeding the motor to cool the water to compensate for the increasing heat?)

What part of the film in this case is getting hot? If it is the edges and the heat transferring to the centre of the film, given the speed at which the film is travelling, would it have migrated to the centre of the film by the time I "finger-meassure" the heat?

Speaking of "finger-measuring, is there a reliable way to check temperature on the film (or at the aperture) and what are the maximum values one can safely achieve?

Again, thanks.

Posted by John Pytlak (Member # 331) on 01-31-2003, 08:01 AM:

Replying to Rick Long:

>In all cases, an IR filter was in place at the front of the lamphouse.

That's good. But where is it reflecting the infrared to? It should reflect the energy to a heat sink, and NOT back into the light path.

>Also, in all cases the bulb in use was Osram 7000w/hs.

That is BIG for 35mm.

>John Pytlak has mentioned (in other topics) that in order to acheive decent screen light, the formula should be about 5 watts per square foot. What then should be the biggest picture I can project using this bulb?

Depends upon the efficiency of your lamphouse and optics, and the gain of your screen. With a gain=1 matte white screen, should be able to light a 24 x 58 foot screen. A properly curved gain screen could be larger.

>My question still remains where is the heat coming from? (Yes, I know its the bulb). The IR filter should provide, to some degree, a division between any heat developed in the lamp and the aperture. (Talking new IR filters here). Other accessories such as water-cooling the gate and fan-forced aperture cooking should help this situation even more.

Over 1/2 the energy from a xenon lamp is NOT visible. Ideally, you want only VISIBLE energy going through the film, and the UV and IR filtered out.

>Proper lamp cooling was mentioned also. Does cooling the mirror help cool the light emitting from it?

As noted, in a metal mirror with a dichroic coating, the metal mirror itself is the heatsink. So it needs to be cooled to be efficient.

>If the film is getting hot due to improper focus, causing a larger than normal "spot" on the aperture and therefore causing the trap to get abnormally hot, should this not result in a diminished light? (The available light not all being focussed into the aperture? Should I put an ice-machine across the mains feeding the motor to cool the water to compensate for the increasing heat?)

Improper lamphouse-to-film distance and misfocusing of the lamp to overfill the aperture will reduce light output, and cause excessing heating of the area around the gate. Water cooled gates are needed for really large lamps like yours, and refrigeration of the water can be used (but don't keep the gate so cold that moisture condenses on it).

>What part of the film in this case is getting hot? If it is the edges and the heat transferring to the centre of the film, given the speed at which the film is travelling, would it have migrated to the centre of the film by the time I "finger-meassure" the heat?

The edges of the film get hot by contact with the metal components of the hot gate. The image area is heated by absorbing radiant energy.

>Speaking of "finger-measuring, is there a reliable way to check temperature on the film (or at the aperture) and what are the maximum values one can safely achieve?

A thermocouple can be used. The radiant energy level (watts per square millimetre) is the key determinant in film damage potential, but when the film heats much beyond 200 degrees Celsius, serious damage is likely.

Kodak published an excellent paper in the September 1983 SMPTE Journal that discusses all this. "Projection Performance of Theatrical Motion-Picture Films Using Xenon Short-Arc Lamps" by Paul Preo will answer all your questions.

Posted by Rick Long (Member # 213) on 02-01-2003, 12:06 AM:

Again, thank you John for some very helpful information.