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Topic: 2nd shutter blade on a SIMPLEX

Jeff Moore
Film Handler

Posts: 7
From: Lakewood, CA, USA
Registered: Sep 2000

posted 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?

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John Eickhof
Jedi Master Film Handler

Posts: 588
From: Wendell, ID USA
Registered: Jan 2000

posted 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'

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Tom Kroening
Expert Film Handler

Posts: 214
From: Janesville, WI USA
Registered: Oct 1999

posted 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.

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Wade Brashers
Film Handler

Posts: 18
From: Doylestown, PA
Registered: Aug 2000

posted 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.

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Scott Norwood
Film God

Posts: 8146
From: Boston, MA. USA (1774.21 miles northeast of Dallas)
Registered: Jun 99

posted 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).

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Phil Hill
I love my cootie bug

Posts: 7595
From: Hollywood, CA USA
Registered: Mar 2000

posted 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

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Pete Lawrence
Expert Film Handler

Posts: 192
From: Middleburg, PA
Registered: Aug 1999

posted 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.

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John Eickhof
Jedi Master Film Handler

Posts: 588
From: Wendell, ID USA
Registered: Jan 2000

posted 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

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Jeff Moore
Film Handler

Posts: 7
From: Lakewood, CA, USA
Registered: Sep 2000

posted 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.

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Brad Miller
Administrator

Posts: 17775
From: Plano, TX (36.2 miles NW of Rockwall)
Registered: May 99

posted 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.

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Randy Stankey
Film God

Posts: 6539
From: Erie, Pennsylvania
Registered: Jun 99

posted 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!

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Brad Miller
Administrator

Posts: 17775
From: Plano, TX (36.2 miles NW of Rockwall)
Registered: May 99

posted 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.

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Dave Cutler
Master Film Handler

Posts: 277
From: Centennial, CO
Registered: Jun 2000

posted 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.

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Pete Lawrence
Expert Film Handler

Posts: 192
From: Middleburg, PA
Registered: Aug 1999

posted 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?

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John T. Mellor
Film Handler

Posts: 52
From: Htafield, Pennsylvania, USA
Registered: May 2000

posted 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

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