How Do Theater Projectors Work

[Peter Ellner]

I'm just curious as to how projector lenses (non-anamorphic ones) work in comparison to the lenses placed on a camera. Since I have only dealt with the latter, I actually realized that I don't understand how the former works.

 

For example, since the projector lens is usually very far away from the screen compared to its distance from the film it's projecting, the focal length of the lens must be measured in meters, right? So how is it able to focus so closely and yet project a focused image so far away?

 

And on the topic of film projectors, I'm interested in understanding how film projector shutters work. Is it just like how the film was exposed, where there are 24 frames projected ever second, but each frame is only shown for 1/48 of a second, followed by 1/48 a second of black, and then another frame, etc.? If so, then over the course of a 2 hour movie, are we technically watching 1 hour of dark screen, if for every 1/48 a second of visible frame there is 1/48 of a second of black screen?

 

And, I truly apologize about all my ignorance, but I can't help but notice that computer screens don't show flicker while the same movie in a theater does. Is it because each frame of the movie is shown for precisely 1/24th of a second on the computer with no time between frames while the theater has a period of darkness as the film advances, or something else?

 

Thank you so very much!

[Simon Wyss]

Which theater projects with flicker?

[Brian Drysdale]

A projector has at least 2 shutter blades, so you shouldn't perceive flicker. If your theatre has flicker I'd complain.

 

Here's how a projector works:

 

http://en.wikipedia.org/wiki/Movie_projector

[Antti Näyhä]

For example, since the projector lens is usually very far away from the screen compared to its distance from the film it's projecting, the focal length of the lens must be measured in meters, right? So how is it able to focus so closely and yet project a focused image so far away?

 

Projection lenses are usually within a range of, say, 35 mm and 150 mm. You have to remember that the projected image is also several meters wide.

 

And on the topic of film projectors, I'm interested in understanding how film projector shutters work. Is it just like how the film was exposed, where there are 24 frames projected ever second, but each frame is only shown for 1/48 of a second, followed by 1/48 a second of black, and then another frame, etc.? If so, then over the course of a 2 hour movie, are we technically watching 1 hour of dark screen, if for every 1/48 a second of visible frame there is 1/48 of a second of black screen?

Yes, exactly. The 48 Hz flicker is indeed visible to some people, and because of that, better projectors use three shutter blades to achieve a 72 Hz flicker.

[Phil Rhodes]

The general idea is that you make it flicker more, on order to make it, er, appear to flicker less. If you see what I mean. Ahem.

[John Sprung]

Cameras use cam driven pulldown claws, and typically spend about half the cycle time pulling down (180 degree shutter).

 

Projectors use a Geneva mechanism to drive an intermittent sprocket, which pulls down twice as fast. The shutter typically has two equal blades (90 degrees), one of which covers the pulldown, the other produces a blackout of identical duration, but opens to reveal the same frame again. So, you get 24 unique images per second, but 48 flashes of light.

 

Whether you see flicker depends on how many flashes per second you see, and how bright they are. At a given rate, brighter makes it flicker more. At a constant brightness, the faster they flash, the less you see it as flicker.

 

 

 

-- J.S.

[dan kessler]

I'm just curious as to how projector lenses (non-anamorphic ones) work in comparison to the lenses placed on a camera. Since I have only dealt with the latter, I actually realized that I don't understand how the former works.

 

For example, since the projector lens is usually very far away from the screen compared to its distance from the film it's projecting, the focal length of the lens must be measured in meters, right? So how is it able to focus so closely and yet project a focused image so far away?

 

 

 

There's really no difference in principle between camera lenses and projector lenses.

 

You know that the film is located a just short distance away from the lens in a camera,

and that is where the lens focuses an image. The subject is on the other side of the lens,

only much further away.

 

The same geometric arrangement exists for a projector lens. The only difference is the

direction in which the light rays are traveling.

 

The relationship between object, image, and focal length for a simple positive lens

is expressed as:

 

1/d1 + 1/d2 = 1/f

 

where d1 is the distance from the subject to the lens

 

d2 is the distance from the lens to the image

 

f is the lens focal length

 

 

Take a look at some simple ray diagrams and play with the formula.

Very interesting stuff for anyone who's serious about their photography.

[Peter Ellner]

Cameras use cam driven pulldown claws, and typically spend about half the cycle time pulling down (180 degree shutter).

 

Projectors use a Geneva mechanism to drive an intermittent sprocket, which pulls down twice as fast. The shutter typically has two equal blades (90 degrees), one of which covers the pulldown, the other produces a blackout of identical duration, but opens to reveal the same frame again. So, you get 24 unique images per second, but 48 flashes of light.

 

Whether you see flicker depends on how many flashes per second you see, and how bright they are. At a given rate, brighter makes it flicker more. At a constant brightness, the faster they flash, the less you see it as flicker.

 

 

 

-- J.S.

 

This clears quite a bit up. Thank you. So just to see if I understand correctly: in film cameras, the shutter typically has an angle of 180 degrees, meaning each frame of film is expose only once and for 1/48 second. In theater projectors, the shutter has two blades, each with a shutter angle of 90 degrees, meaning each frame of film is projected twice for 1/48 second each time. And on an LCD computer monitor...?

 

Also, the flicker I notice only happens sometimes, but now it all makes sense. I notice it more in the corners of the screen or when I'm not looking directly at the screen (peripheral vision is more sensitive to motion), and when there is a white background (brighter light makes the rods in the eye able to perceive the flicker better). Thanks.

[K Borowski]

I think it's a gross oversimplification to say that ALL cine projectors have double-bladed shutters and project 48 times a second. I've been to several that had standard shutters.

[Marc Roessler]

In general film projectors either have (in effect) two bladed (48 Hz) or three bladed (72 Hz) shutters. This is true for 8mm, 16mm, 35mm and 70mm projectors.

 

A "three bladed "shutter usually has three blades. The Kinoton series with geneva cross film transport has this three bladed shutter as an option.

 

For the regular, "two bladed" (48 Hz) shutter there are serveral ways to achieve this mechanically:

 

- a two bladed shutter as a flat disc or as a cone shaped shutter (for getting closer to the gate)

 

- a single blade shutter rotating at double speed (flat: Kinoton FP20/30, Philips DP75; cone shaped: Philips DP70 afaik), thus making the transition phase between open and closed shutter shorter and reducing flicker

 

- a revolving drum with two opposing sides open (when seen radially), thus also forming an 48 Hz shutter

 

 

The three bladed shutter has a light loss of 50% over the two bladed shutter, this means you need to double your light output (and thus your lamp wattage.. not a linear correlation but you get the idea). As many big screens are already running the largest lamp (4 or 7kW) with water cooled lamphouse, dichroic heat filter and water cooled gate it's simply not possible for them.

 

 

Karl, can you elaborate a bit on the make and model of those "standard shutter" projectors? I have never encountered anything like that so far, neither with US nor European projector models?

 

Greetings,

Marc

Edited August 18, 2011 by Marc Roessler

[John Sprung]

I think it's a gross oversimplification to say that ALL cine projectors have double-bladed shutters and project 48 times a second. I've been to several that had standard shutters.

 

Three blade shutters are available, Century is the company that makes them. A long time ago, Universal had a pair of Century's in a screening room, one with a two blade, and the other with a three. It took a while to figure out why one of them needed so much more current to the lamp.

 

A balanced single blade geared at double speed works just like a two blade shutter, except that edge transit is twice as fast. We had that on the AA-2 Norelco 35/70 machines. They're a damn buzz saw, they go so fast.

 

 

 

-- J.S.

[John Sprung]

And on an LCD computer monitor...? I notice it more in the corners of the screen or when I'm not looking directly at the screen (peripheral vision is more sensitive to motion), and when there is a white background (brighter light makes the rods in the eye able to perceive the flicker better). Thanks.

 

LCD and plasma are actually quite nice for screens in the shot. They usually change from frame to frame very quickly, and have usable picture up most of the time. But always test them to be sure. Try a rolling shutter with a very small angle to catch them advancing frames.

 

Yes, peripheral vision is more sensitive to flicker. You can even see 60 Hz flicker in your peripheral vision, if you look away from an old time CRT TV set.

 

 

 

-- J.S.

[K Borowski]

John: It's possible that's what I worked with. I just know it wasn't three bladed from when we had to re-time them to eliminate ghosting.

[Peter Ellner]

In general film projectors either have (in effect) two bladed (48 Hz) or three bladed (72 Hz) shutters. This is true for 8mm, 16mm, 35mm and 70mm projectors.

 

A "three bladed "shutter usually has three blades. The Kinoton series with geneva cross film transport has this three bladed shutter as an option.

 

For the regular, "two bladed" (48 Hz) shutter there are serveral ways to achieve this mechanically:

 

- a two bladed shutter as a flat disc or as a cone shaped shutter (for getting closer to the gate)

 

- a single blade shutter rotating at double speed (flat: Kinoton FP20/30, Philips DP75; cone shaped: Philips DP70 afaik), thus making the transition phase between open and closed shutter shorter and reducing flicker

 

- a revolving drum with two opposing sides open (when seen radially), thus also forming an 48 Hz shutter

 

 

The three bladed shutter has a light loss of 50% over the two bladed shutter, this means you need to double your light output (and thus your lamp wattage.. not a linear correlation but you get the idea). As many big screens are already running the largest lamp (4 or 7kW) with water cooled lamphouse, dichroic heat filter and water cooled gate it's simply not possible for them.

 

 

Karl, can you elaborate a bit on the make and model of those "standard shutter" projectors? I have never encountered anything like that so far, neither with US nor European projector models?

 

Greetings,

Marc

 

 

On those projectors with 72 Hz shutters, does that mean that the film has to move faster through the gate, since there's less time between exposures?

 

And I'm not sure if anyone knows this, but why do projectors use a two or three bladed or really fast moving shutter that projects each frame multiple times but for short durations each time instead of simply projecting each frame once for slightly less than 1/24th of a second and then quickly advancing the film in the remaining time?

 

Seems like this would be better than having a high Hz flicker rate, no?

[Phil Rhodes]

I used to work at a projection facility that installed a good-but-not-great 3-chip DLP projector. Said projector ran two 300W lamps and produced an image much, much brighter than the 1.5KW Xenon 35mm projector on the same screen. I interpret this as mainly being down to two factors: first the fact that the DLP chips are larger than a 35mm frame and therefore there is a larger diameter throughout the optical system, producing a lower effective F stop; and secondly, probably mainly, because of the lack of shutter.

 

P

[John Sprung]

Actually, the high flicker rate doesn't look like flicker to humans. Our eye/brain combination can keep up quite well with 24 flashes per second, but not with 48 unless the image is quite bright. 72 lets you go even brighter before you see flicker, but requires a lot more light to get there.

 

 

 

 

-- J.S.

[John Sprung]

DLP gets its dynamic range by pulse width modulation. The bright areas of a DLP image are lit up pretty much the full 1/24 of a second that the frame is displayed, with brief off periods. The dark areas are mostly off, with brief flashes of light on them several times during the frame period. So, yes, you could think of DLP as somewhat like being able to project with a 360 degree shutter.

 

 

 

 

-- J.S.

[Hal Smith]

And I'm not sure if anyone knows this, but why do projectors use a two or three bladed or really fast moving shutter that projects each frame multiple times but for short durations each time instead of simply projecting each frame once for slightly less than 1/24th of a second and then quickly advancing the film in the remaining time?

 

If a projector were to be able to pull the film down between frames faster, there's a good chance it would rip out the sprocket holes. Film is a well developed technology where there are many, many factors involved in how things are currently done.

 

A quick example: The print film used in a commercial movie house uses a stronger polyester plastic base than camera film's cellulose triacetate base . It is so strong that if it is used in a camera, and there is a film jam, there's a good chance it will damage the camera's mechanism. A modern projector has to be built to handle that risk, cameras would weigh a ton if their mechanism was built strong enough to take a print film jam.

 

Kodak description of film structure

[John Sprung]

You could probably build a projector with a six point Geneva instead of the standard four point. That would be a natural way to do three blade shutters.

 

As for polyester camera stock, you could drive a camera with a rare earth magnet clutch to limit the torque and protect it from jams.

 

 

 

-- J.S.

[K Borowski]

At the same time, Kodak uses Panavision New York, supposedly, for their ECP testing. I'd imagine the real damage would be done at high speeds. I've seen a mishap at a certain film society where the projectionist was too busy twittering or something on his laptop to notice the problem, but if the camera is manned, it can be stopped before the mechanism loses the strength battle with the polyester base.

 

Nevertheless, there are high speed cameras that specify a polyester base; perhaps these are the ones that are moving so fast they don't require intermittent movements for exposure. . .