Why don't film strips burn up?

Technically-oriented discussion of classic films on everything from 35mm to Blu-Ray
Robert Moulton
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Why don't film strips burn up?

Unread post by Robert Moulton » Sun Sep 19, 2010 6:12 pm

I can't believe I've never had this question enter my head before:

If a motion picture film will burn up if stalled during projection then why don't film strips?

If the answer is a simple matter of the intensity of the bulb then why is a less powerful bulb passable?

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Mike Gebert
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Unread post by Mike Gebert » Sun Sep 19, 2010 6:37 pm

My last experience with a film strip was probably 6th grade, but offhand, yes, I think you're looking at a much less powerful bulb intended for projection in a 20-foot-square classroom, not down 200 feet of auditorium onto a 25-foot-tall screen. It may also help that the image is up there continuously, unlike a movie frame which merely interrupts blackness for an instant.
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Marr&Colton
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Unread post by Marr&Colton » Thu Mar 24, 2011 5:58 pm

Actually the simpler answer is that filmstrips were printed on a very stable SAFETY film, and as was already mentioned, the bulbs were usually 300 watts or less and heat filters were used in the projector.

In the pre-1951 days of NITRATE film, there was a serious danger of fire if the film not only lingered in front of the hot light source but jammed in the projector or touched the hot lamphouse.

Since the universal adoption of SAFETY film in the early 1950s, film will just melt if exposed to excess heat or flame.

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Mike Gebert
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Unread post by Mike Gebert » Thu Mar 24, 2011 6:03 pm

Well, yeah, but a film strip doesn't melt, either.

Although referring to film strips in the present tense is probably anachronistic by now.
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Rick Lanham
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Unread post by Rick Lanham » Thu Mar 24, 2011 9:19 pm

One time our teacher informed us that she would be
showing us a "strip film." She never did seem to
notice her mistake, despite some laughs.

Rick

Eric Grayson
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Unread post by Eric Grayson » Mon May 09, 2011 7:37 pm

The bulb for a 35mm movie projector has to be much brighter because there's a shutter blocking off the twice per frame (1/24 sec.) That means you're losing 50% of your light just as a start. You're also farther away from the screen, and light transmission falls off by an inverse square: 2 as far away = 1/4 of the light transmission.

The optics also make a difference. On a modern projector, your aperture is smaller than a filmstrip's. That loses you light too...

Jay Schwartz
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Unread post by Jay Schwartz » Tue May 10, 2011 12:41 pm

The simple non-detailed answer is: The filmstrip projector is DESIGNED to show non-moving film, so of course it's not going to melt the film.

As already mentioned, this is accomplished by heat filter glass and lower wattage lamp.

If you used these on a motion picture projector, the image would be too dark. The shutter is a big reason. I'm not so sure about the aperture making a big difference (I think a filmstrip frame is identical to a full-frame, i.e., silent-era motion picture frame).

As for safety film versus nitrate: There is no 16mm nitrate film, and that is the gauge that was used most in schools. Even when schools used 35mm film (like before 16mm was introduced in 1923), it was mostly safety film.

All filmstrips would have been safety base, I have to imagine, but this is just my common-sense guess.
Last edited by Jay Schwartz on Tue Dec 27, 2011 1:06 am, edited 1 time in total.

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mndean
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Unread post by mndean » Tue May 10, 2011 4:34 pm

I have some industrial film strips, and they are full-frame silent. Of the ones I have, a few are nitrate (I tested them all) but most are safety. They date from just after WWII to the late '40s. My guess is whoever did the printing used what printing material they had available. The ones I have weren't meant for a school, but were for instruction on machinery use, and came with records and printed material also. The negatives they were printed from were certainly nitrate, the codes are readable.

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nitrocellulose
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Re: Why don't film strips burn up?

Unread post by nitrocellulose » Fri Aug 26, 2011 9:47 pm

A projectionist from the days of nitrate told me that of the two nitrate fires he had, one was because he threaded up before letting the film trap cool down (you were supposed to give it a couple minutes). Even after the light source was removed, the metal was too hot for the film to just sit there without moving.

On the 35mm Simplex projectors I use the gate is too hot to handle for a little while after you kill the lamp, so I consider this plausible.

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