This is topic 2nd shutter blade on a SIMPLEX in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Jeff Moore (Member # 614) on 10-17-2000, 10:26 PM:
It is my understanding that are two shutter blades on a Simplex projector. What is the function of the second blade?
Posted by John Eickhof (Member # 323) on 10-17-2000, 11:21 PM:
Most modern projectors employ a 2 bladed shutter, 1 blade for blocking the light during pull-down, and the other to provide a second light cutoff (frequency of frames per second X 2 or 24 FPS x2 = 48 Cycles per second) This part of the blade is called a 'flicker' blade. If it was not present, a
frequency of 24 time per second is very visible to the eye and the flicker would be quite annoying. If you are dealing with older 1930-40 vintage simplexes, they had an optional 'front' shutter blade to counteract the regular rear shutter and cause a more even light cut-off from both top & bottom of picture at the same time. The century projectors used a pair of counter-rotating shutters between the lamp and the gate.
Motiograph used a rotating 'barrel' type shutter that did the same thing. Front shutters were eliminated when cinemascope lenses were introduced due to the length of the lens itself interfering with the shutter blade. In short throws or in studio theatres, 3 bladed shutters are used to decrease flicker even more and prevent excessive light on the screen. So much for 'shutter-101'
Posted by Tom Kroening (Member # 201) on 10-17-2000, 11:23 PM:
The way I understand it, if the projector flickered once ever frame (while the frame changed) You would see the flicker on the screen. WIth the second blade it effectively doubles the flicker speed and tricks us into thinking that the picture is not flickering. They do make a three bladded shutter which helps even more, but it doesn't allow as much light on the screen.
Posted by Wade Brashers (Member # 581) on 10-17-2000, 11:34 PM:
Persistence of vision is about 17 frames per second, most 16mm projectors only flash the frame once at 24fps. With 35mm projectors the double blade is mostly to keep the light off the frame so it doesn't burn, its still technically 24 frames per second you're just not seeing each frame the whole 24th of a second, if you did it would probably look the same because the ammount each frame is off screen in between frames would be the same.
Posted by Scott Norwood (Member # 30) on 10-18-2000, 12:30 AM:
Actually, most 16mm projectors (at least the classroom-style portable ones) have triple-bladed shutters, since most of them can be switched between 16/18fps ("silent speed") and 24fps ("sound speed"); with a two-bladed shutter, flicker becomes obvious at 18fps but is less so with a triple-bladed shutter (at the expense of light output).
Posted by Phil Hill (Member # 371) on 10-18-2000, 01:47 AM:
Thank you John for the most accurate description/reason for the "flicker blade" in two and three-blade shutters used in common (8,16,35,70mm...including 1570) motion picture projectors.Some of the special venue 48 fps and 60 fps (Showscan) projectors do not use a flicker blade and use only one light interruption per frame...only during pull-down... the frame rate itself provides flickerless viewing as described by Scott.
However, most all 16mm projectors (and especially the "school" type ONLY have 2-bladed shutters. The selectable 2 & 3 blade types are usually found in the high-end 16mm projectors that run 16/18 and 24 fps.
And, of course, there is the 5-blade shutter for (NTSC) telecine, but I won't mention that...OOPS! 
>>> Phil
Posted by Pete Lawrence (Member # 130) on 10-18-2000, 08:37 AM:
Is there anything else involved with converting from a two to three blade shutter, other than changing the blades? Perhaps a gear or shaft change? I have Simplex E7 heads and the small amount of flicker I see is still annoying on my eight foot wide screen. I know that reducing the light level helps with the perception of flicker, but I don't want to do that. The difference between two and three blade shutters on 16mm machines it very noticeable. The three blade picture is far more pleasing, even if it's slightly dimmer.
Posted by John Eickhof (Member # 323) on 10-18-2000, 12:08 PM:
There are no changes required to the gear train of a standard projector to use a 3 blade shutter. Just install it and time it like any other. I have 3 blades in stock for any projector that uses a flat blade. ie:
Super, Standard, E-7, Century. Email for details. John
Posted by Jeff Moore (Member # 614) on 10-18-2000, 01:12 PM:
So, the extra blade is to cut down on flicker. But how much light output is sacraficed as compared to a single blade Christie, and at what two point of the frame cycle is the light being blocked? 
It all seems easy looking at the manual, but next to impossible when trying to picture it while watching the shutter blade.
Posted by Brad Miller (Member # 2) on 10-18-2000, 01:47 PM:
Just to clarify, the single bladed shutter Christie units actually spin twice as fast. There is still 48 flashes per second, just like all other 2 bladed shutters. The improvement is an ever so slightly faster open and close time that allows just a tad more light (like a double shutter) since the shutter is spinning double speed.
Posted by Randy Stankey (Member # 64) on 10-18-2000, 02:22 PM:
Soo... Why, then don't they just rev that puppy up to about 3,000 RPM? There'll be NO flikcer, then!.... And then, when the projector starts to LIFT OFF you can back 'er off just a hair...
HUMOR, OF COURSE!
Posted by Brad Miller (Member # 2) on 10-18-2000, 04:03 PM:
Just in case someone wants more clarification as to why Randy's humorous post cannot be a reality, it's because the shutter must completely block the light for the entire duration of the intermittent's pulldown. Otherwise, vertical streaking would be seen on screen.
Posted by Dave Cutler (Member # 515) on 10-18-2000, 04:11 PM:
How much improvement in image quality is gained with replacing a 2 bladed shutter with a 3 bladed one in a simplex projector? I have a PR1014 and would be interested in seeing what it looks like if it is worth it.
Posted by Pete Lawrence (Member # 130) on 10-18-2000, 08:17 PM:
There was another thread discussing three blade shutters a while back. Search on "Question regarding light" for some interesting comments. The E7s I have require the blade that blocks the light (while the intermittent moves) to be about 90 degrees of shutter rotation. What does a three blade shutter look like, one 90 degree blade and the other two smaller? Three 90 degree blades?
Posted by John T. Mellor (Member # 468) on 10-18-2000, 10:02 PM:
If you look at some of the Older projectors you might find that the shutter started out in the front of the projector instad of the back. If you can find one check out Richardsons Blue Book Of Projection The only copy I can find in the Philadelphia area Is in the main library branch but they wont let it out of the building. I have one of the tools to line up the front and rear shutters on a Super Simplex Projector . On some of the earlier Simplex projectors the shutter size was adjustable . Or try the high speed intermittents that the drive ins used to get more light on the screen
Regards
John
Posted by John Eickhof (Member # 323) on 10-18-2000, 11:00 PM:
The three blade shutter for an E7 or simular projector is actually cut at about 100 degrees on all three blades, the three openings are approx. 20 degrees each, Thus, the light is scanned across the open aperture
making the picture even more flickerless.
The light loss is substantial though, and generally twice the recommended wattage
for academy brilliance is required. Conical shutters (like the XL) are cut at about 75 degrees closed with the openings being 45 degrees each, this is possible due to the shutter blade being so close to the film,
thus the efficiency is much greater and only
about 55 degrees of closed shutter are required to block the pull down. That is why a Simplex XL will put more light through the film than any other american made machine including the Christie. I stock 3 blade XL shutters too, but they are quite pricy!
Posted by Sean M. Grimes (Member # 445) on 10-19-2000, 03:13 AM:
but how does the light on screen of an xl compare to a 1060? I mean is the design really at all comparable?
Posted by Dennis Atkinson (Member # 345) on 10-19-2000, 08:20 AM:
Having worked with 3 bladed XL's in the past, the light output is cut by about 1/3.
With some other changes you can show silent films at 16fps with no flicker, which is why the 3 blade was needed in my case.
Posted by Pete Lawrence (Member # 130) on 10-19-2000, 07:26 PM:
Using John's numbers, that says about 16.7% of the light reaches the film, 83.3% is completely wasted. This sounds like something that was designed by a government bureaucrat. 
Posted by Jon Bartow (Member # 241) on 10-20-2000, 10:10 AM:
Perhaps the illustrius John Pytlak can correct me on this, but I seem to remember that three bladed shutters are also used in Film to video transfer
Posted by Scott Norwood (Member # 30) on 10-20-2000, 10:40 AM:
Actually, film-chain projectors use five-bladed shutters. This results in 120 flashes per second, which is easily divided by 30fps, which is the frame rate for NTSC television (actually, it's 59.94 "fields" per second, but it's close enough that there's no flicker when showing a film on television). PAL (European) television runs at 25fps; usually, films are just run slightly fast when shown on PAL television and the sound is usually processed sufficiently to make it sound fairly normal. Films intended for broadcast on PAL TV systems (documentaries, etc.; stuff that wouldn't be released theatrically) are often shot at 25fps. Some films work intended for NTSC television (commercials, etc.; again, stuff that won't be shown theatircally) is shot at 29.97 or 30fps in order to look "smooth" when transferred and to avoid the motion artifacts that one normally sees when watching 24fps film on NTSC television.
Now, most TV transfers are done on a "flying-spot scanner" rather than with an actual projector with an intermittent movement. I've never seen a 35mm film-chain projector and have sometimes wondered about what kind of intermittent movement they use--surely, it would have to be something along the lines of the "drive-in" fast pulldown type...
Posted by Jeff Moore (Member # 614) on 10-20-2000, 12:16 PM:
Scott, What exactly is a "flying spot scanner", and how does it differ from a regular telecine? By the way, thank to everybody for the answer!
Posted by John Pytlak (Member # 331) on 10-30-2000, 12:04 PM:
Information on "flying spot scanners" may still be found on the Cintel website:
http://www.cintel.co.uk/ In general, these telecines scan the film with light generated by a cathode ray tube (CRT), and convert it to an electrical signal using color filters and photomultiplier tubes.
Most books and faq's about film on television contain a description of telecine technology, both flying-spot and CCD.
As far as shutters, the shutter blade(s) need to block the light while the film is moving during pulldown, otherwise, you will see a streaking known as "travel ghost". For the most commonly used 4:1 "Geneva" or "Maltese Cross" intermittent, the shutter blade needs to be near 90-degrees, depending on how efficient the shutter is in "cutting" the light. At 24fps, only one interruption per frame would give very unacceptable flicker, so a second "flicker" blade (or two rotations of a single blade) are usually used to give two interruptions per frame, or 48 per second. For two 90-degree blades, the shutter efficiency is about 50 percent. To completely eliminate flicker, an even higher frame rate is needed, or a third blade can be used on the shutter to give 72 interruptions per second at 24fps. Three 90-degree blades are only 25 percent efficient --- that is, you only get half the light compared to a two-blade shutter.
As discussed in another thread, three-blade shutters almost completely eliminate flicker, and would certainly be an advantage on small or medium sized screens, where the lower efficiency can be compensated for by a larger lamp.
For silent films (16fps), a three-blade shutter is needed to achieve acceptable flicker (3 x 16fps = 48 interruptions per second).
------------------
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 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com