Film-Tech Forum ARCHIVE: Couple of questions about Intermittents and Platters

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Posted by Shayne Martin (Member # 1876) on 09-08-2003, 05:42 PM:

I'm a new member to the upstairs world and since my arrival I've come across a few questions that I'd like answered. 1) I understand that the way the intermittent works is that the shutter spins in time well my question is, why does it flash twice before the intermittent spins once. 2)What causes the brain and platters to become... get to the point where they need calibration. We leave them on all the time so I don't understand what might cause some to go quicker than others or at all for that matter. I had a third but when I remember it then I'll write back later. Thank you.

Posted by Chris Hipp (Member # 1788) on 09-08-2003, 06:01 PM:

That's a good question. I don't know.

Posted by Aaron Sisemore (Member # 145) on 09-08-2003, 06:04 PM:

#1: if the shutter only blocked the light beam once for each pulldown, the flicker at 24fps would be horribly annoying, therefore the flicker rate is 48 times per second (projecting each frame twice), greatly reducing the visibility of the moving shutter blade.

#2: It depends on the platters used. Some use mechanical switches and save for the switch or motors outright failing, they really don't 'go out of calibration'. In 'electronically controlled' platters like Christie, later Strongs, SPECO, etc., all electronics do drift with age and environmental conditions (temperature, humidity, etc) and as such the setting may drift away from the optimum. Also with many platters (Christie and SPECO come immediately to mind), the rubber drive wheels that move the platters wear with time and the speed that the platter moves does decrease proportionally.

-Aaron

Posted by Chris Hipp (Member # 1788) on 09-08-2003, 06:10 PM:

For a while, back when I was stupid, I thought that only every other frame was projected because of the shutter.

What type of equipment are you using?

Posted by Aaron Sisemore (Member # 145) on 09-08-2003, 06:14 PM:

Chris, all 24fps theatrical equipment basically uses the same system, even with variations on the theme like barrel shutters as on Motiographs and some Tokiwas, and single-blade shutters that rotate at twice the speed as on the Norelco AAII.

-Aaron

Posted by Mike Blakesley (Member # 26) on 09-08-2003, 06:17 PM:

Shayne, you better make that thread title more specific before you get yer ass moderated! [Smile]

Welcome to the forum.

Posted by Joe Redifer (Member # 3) on 09-08-2003, 06:38 PM:

Don't worry Mike, I moderated his ass (changed the topic title). [Smile]

Don't forget about 3 bladed shutters, Aaron! Also, as far as platters are concerned, they are mechanical and will not keep exact settings as parts wear over extended periods of time. They need to be tweaked occasionally. But Aaron really answered the question best.

Posted by Brad Miller (Member # 2) on 09-08-2003, 06:51 PM:

Shayne, when asking questions please be very specific as to the equipment you have in your booth. Few questions are so general that you can get a truly accurate answer without us knowing the brand and model of your equipment.

Posted by Torsten Jasper (Member # 1860) on 09-08-2003, 07:01 PM:

quote:
Joe wrote:
Don't forget about 3 bladed shutters, Aaron!

Yeah, don´t forget about 18 fps for soundless films. [Smile]

Greetings
T. J.

Posted by Aaron Sisemore (Member # 145) on 09-08-2003, 08:53 PM:

quote:
Don't forget about 3 bladed shutters, Aaron!

How many 'standard multiplex' booths run machines with 3-blade shutters? I would say very few. [Smile]

quote:
Yeah, don´t forget about 18 fps for soundless films.

Once again, how many multis run silents anymore? [Smile]

What about 30fps Todd-AO? 60fps Showscan? [Smile]

c'mon guys... lighten up [beer]

-Aaron

Posted by Thomas Procyk (Member # 1170) on 09-08-2003, 11:29 PM:

What sort of difference does a 3-blade shutter make? Reduced flicker?

=TMP=

Posted by Jon Miller (Member # 163) on 09-09-2003, 12:11 AM:

Yes, reduced flicker, but at a price of some loss of light. A three-bladed shutter would be just the ticket for an overlamped very-short-throw home system like some of us have, if you could only find one easily... [Frown]

(More lamp power than is necessary for a given screen size can cause flicker.)

I think some commercial screening rooms use projectors equipped with three-bladed shutters to get a smoother image at a very short throw.

Posted by Manny Knowles (Member # 1171) on 09-09-2003, 12:27 AM:

It's been a while, but IIRC this is due to something known as Critical Flicker Fusion or CFF.

There was a study that demonstrated that 50 flashes per minute is generally perceived by people as steady illumination. The test subjects could not detect any flickering.

Mainstream cinemas play films at a speed of 24 frames per second, so if we doubled the flicker rate by using a two blade shutter then we would be approach the CFF.

A three blade shutter raises the flicker rate to 72 per second which is considerably higher than the nominal CFF for humans. As mentioned, the trade-off is reduced light output.

Posted by John Pytlak (Member # 331) on 09-09-2003, 09:05 AM:

CFF:

http://webeye.ophth.uiowa.edu/dept/RESFELO/ResDay2000/puwat/sld012.htm

http://www.therubins.com/alzheim/alztest13.htm

http://www.physiol.ox.ac.uk/~pch/abstracts/arvo1998.html

http://www.nlm.nih.gov/cgi/mesh/2002/MB_cgi?term=Flicker+Fusion&field=entry

For most people, a film projected at 24fps with a two-blade shutter has acceptable shutter flicker at light levels below about 25 footlamberts. That is why standard SMPTE 196M sets an upper limit of 22 footlamberts for theatres. At higher framer rates, or by using a three-blade shutter, much higher levels of screen luminance can be used without significant flicker perception.

I've projected 35mm prints at 80 footlamberts with a three blade shutter (72 interruptions per second), and the images are nearly flicker-free and stunningly realistic. But prints timed for the SMPTE standard 16 footlamberts may sometimes show too much detail in the shadow areas at this very high level of brightness.

Assuming a 90-degree shutter blade, a two blade shutter is 50% efficient, and a three-blade shutter is only 25% efficient in transmitting light. So you typically need twice the lamp power if you move from a two blade shutter to a three blade shutter. One way to overcome this is to use a 5:1 "drive in" intermittent, or adopt the new Kodak "Quickermittent", that allow a more efficient wider shutter opening without travel ghost.

Posted by Frank Angel (Member # 248) on 09-09-2003, 11:48 AM:

John, are the Kodak Quickermittents available as practical replacements for the more common 35mm projectors or is it just a design concept? I once tried to put in a drive-in intermittent in a Simplex XL. It sounded like a 120v coffee grider plugged into a 240v outlet. It was scary. Plus the picture had more jitter than a the normal intermittent. And, it wasn't a simple exchange of one for the other -- drive shaft gears had to be changed too. I gave up. Sure would be nice if the Quickermittents were just swappable with standards.

I assume that with a Drive-in or Quikermittent, you then can cut back the shutter blades? I would imagine that in situations with large screens where they need more light (hence the desire for a quicker pulldown), they would already be pushing lots of light -- wouldn't the longer rest period bring the film closer to the threshold of heat damage due to the longer time light is passing through each frame?

Posted by John Pytlak (Member # 331) on 09-09-2003, 12:36 PM:

Prototypes were made for several different projectors, as a direct retrofit to the existing stars and cams. Kodak had a "show and tell" with several major projector manufacturers, both in the US and overseas. Unfortunately, still no takers on licensing the technology. (Serious inquiries still welcome to contact project leader Chris DuMont).

IMHO, the "Quickermittent" was somewhat noisier than a conventional 4:1 intermittent, but not excessive. The image on the screen was very steady (it's really the "dwell" of the star/cam fit that locks the sprocket in position that determine vertical steadiness). With polyester prints, the added acceleration did not produce unacceptable wear. More testing (e.g., long term run-in and wear characteristics) needs to be run, but this would be part of any commercialization effort by an interested manufacturer, since it would be specific to the tooling.

Of course you can use the extra efficiency in several ways: to increase screen luminance, to go to a 3-blade shutter, to use a stopped down lens for improved depth of focus, etc. The added light might allow the use of a more efficient heat filter, so you might be able to light a huge screen without risking heat damage or focus flutter.

The "Quickermittent" was one of the "Enhanced Theatrical Experience" projects, which also included a new lamphouse design, partnering with labs and printer manufacturers to improve steadiness, and the "Scope 1.5X" project revisiting Glenn Berggren's work with IscoVision.

Posted by Scott Norwood (Member # 30) on 09-09-2003, 05:35 PM:

Is the "quickermittent" any more likely to damage film than a "normal" intermittent? I've talked to a few people who have said that the drive-in intermittents (different technology?) are excessively hard on film.

Unfortunately, if something like this were to become popular, it would probably be used as a way to reduce the size of the lamphouse rather than as a way to improve image quality.

Posted by John Pytlak (Member # 331) on 09-09-2003, 07:48 PM:

As I said, there is no problem with polyester prints. The higher accelerations will put more stress on the perfs of old triacetate prints, which may be more brittle, so I wouldn't recommend it for archives or rep houses. But the increased efficiency would normally be needed by the biggest screens, which are usually first run and using polyester prints.

Posted by Torsten Jasper (Member # 1860) on 09-09-2003, 10:15 PM:

Hello,
even when I check the quality in our house, I aware some flicker when I stand nearby a projector.
But, when I check it in the audience room, the flicker is gone...

Posted by Manny Knowles (Member # 1171) on 09-10-2003, 12:03 AM:

One of my projectors flickers inside the booth but not on-screen. This is because it has a silver shutter blade and the light is being reflected back into the booth every time the shutter interrupts the beam. One of these days I'm gonna paint that shutter blade black.

Posted by John Pytlak (Member # 331) on 09-10-2003, 08:06 AM:

Flicker is usually more apparent when viewed with peripheral vision, i.e., "out of the corner of one's eye". When I travel in Europe, I notice the flicker of the fluorescent lighting and PAL television mostly when I'm not looking directly at the source.

Posted by Daryl C. W. O'Shea (Member # 1303) on 09-11-2003, 02:56 PM:

quote:
One of these days I'm gonna paint that shutter blade black.

Depending on the size of the lamp in use, you may not want to do that. You could end up with a warped shutter.