Film-Tech Forum ARCHIVE: curved gates

This is topic curved gates - why? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Scott Norwood (Member # 30) on 10-24-2002, 05:57 AM:

With respect to the article on aperture plate distances: what is the supposed advantage of curved-gate projectors? Cameras have straight gates, why shouldn't projectors? I've run quite a bit of film through straight-gate Century C's and never had any sort of problem with them; I've only had one experience with straight-gate Simplex X-L's, but they seem fine, too. I'm not a Cinemeccanica fan, but the straight-gate V5 seems to be very gentle on film. So why are nearly all modern projectors using curved gates?

Posted by Pat Moore (Member # 380) on 10-24-2002, 08:07 AM:

Scott;

The curved trap and gate was implemented because film, being the plastic medium it is, loves to flex and bend. Theory is that if you curve it into a pre-defined shape along one axis, it minimizes other flexing the film can do as it passes through the trap. In general the theory works really well.

Pat

Posted by John Pytlak (Member # 331) on 10-24-2002, 08:40 AM:

Curved gates really help maintain focus stability as power levels increase. Several technical papers published in the SMPTE Journal years ago by Willy Borberg of General Precision Laboratory, and E. K. Carver, Fred Kolb and Paul Preo of Kodak, led to the development of curved projector gates.

By gently curving the film, the print is more resistant to flexing and changing focus position as it is heated by the intense focused energy of the lamphouse.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
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Posted by Jack Ondracek (Member # 1466) on 10-24-2002, 12:49 PM:

Is there any "standard", suggesting at what wattage a curved gate would be adviseable over straight... or do the variables make this less a science than art? Considering different lamphouse characteristics, glass/metal reflectors, and the varied approaches to heat baffling behind the aperture, this seems about as subjective a subject as when to use water cooling.

I use both straight and curved. The most notable result we saw was at the drive-in, where we had been fighting the focus problem with our old Brenkerts. Switching to Simplexes with curved gates made that problem a thing of the past.

Posted by Pat Moore (Member # 380) on 10-24-2002, 01:49 PM:

It is a variable thing, Jack, with all that mentioned in the mix and more. We built 870 projectors with air stabilization on that very large frame of film. The trap was compound -- curved above and below the aperture, but flat in the image area because the lens needed a flat plane to focus on.

In general, though, I think the curved gate and trap is the simplest and most effective method of controlling the tendency of film to flex in normal theatre operations. Modern lenses are design with that field curvature in mind. Heat is probably the second biggest influence affecting focus quality.

Pat

Posted by Randy Stankey (Member # 64) on 10-24-2002, 02:39 PM:

Correct me if I'm wrong, but I was under the impression that the reason gates are curved is to keep the film within the "plane of focus" of the lens.

Since the plane of focus isn't really a flat plane, but more of a parabola, curving the gate helps the entire frame of the film stay closer to the "focal parabola" ( ) of the lens, thus minimizing the phenomena where the center of the picture is in focus but not the edges, or vice-versa.

This is not to say that putting a curve into the film doesn't ALSO help keep it from buckling or waving as runs but I thought this was the main reason.

Posted by John Pytlak (Member # 331) on 10-24-2002, 03:28 PM:

Perhaps Dwight Lindsey of Schneider, or someone from Isco can jump in here.

AFAIK, a lens can be designed assuming EITHER a perfectly flat film plane, or some film curvature. I recall that one of the reasons modern lenses are so much better than those of a few decades ago is the recognition that the film is NOT perfectly flat at the moment of projection.

Posted by Tim Reed (Member # 137) on 10-24-2002, 04:11 PM:

Lens speed is a factor, too. Focus is more critical with faster lenses.

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Better Projection Pays!

Posted by Steve Guttag (Member # 268) on 10-24-2002, 05:20 PM:

I'll go round and round on the "flat-field" issue...

First off...as the film is heated in the aperture...it swells towards the light source so even with a straight gate, the film is not flat. Furthermore, as the film is pumped by the shutter (it's on and its off...) the film is in in constant motion fore and aft. The relaxation of the film between the shutter and flicker cycle is not enough for the film to completely relax. With 3-blade shutters this can be even worse since the film is hit three times and the film is actually moving to different extremes each time.

It is easy to see that with higher wattages, the effects become more noticable. Many screening room projectors are still straight gate to this day. Since they are in the 500-1000 watt range, the heat really isn't an issue. Order a Simplex PR-1020 and see what you get. Studio gates also have overlapping pad areas.

Curving the gate stretches the film and controls it a bit better. In fact, VistaVision really has it best since it curves the film on the long direction.

Steve

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"Old projectionists never die, they just changeover!"

Posted by Gordon McLeod (Member # 33) on 10-24-2002, 05:22 PM:

But I believe BTS SUpra's had terrible problems with there curved gate not holding focus well according to an article I read once in the BKSTS

Posted by Steve Kraus (Member # 476) on 10-25-2002, 11:33 AM:

If lenses are now designed with for a curved focal plane then one has to assume that focus with a straight gate is going to be compromised. As Steve points out, even with a straight gate there is going to be some curving from heat but surely that is not as much as with a deliberately curving film path. Let me anticipate those who are going to point out the wonderful depth of field of many lenses by reminding them that depth of field refers to how quickly you go out of focus as you move away from the plane of focus (points become circles of confusion) and does not itself change the fact that there is still a definite plane of perfect focus and if that plane is curved a lot more than the film does on a straight gate then it has to compromise focus across the frame.

Posted by Pat Moore (Member # 380) on 10-25-2002, 12:25 PM:

It's not that lenses are made to look at a curved plane as much as they will accept an image to be within a certain area, +/- the perfect image plane, and still be able to focus the image on screen. That's why the same lens works on a flat or curved film trap, as long as the film is within its "depth of field". The lenses of today are designed to accommodate that "flex" of film in and out of the desired focal plane. Typically, "slower" lenses (higher f-numbers) would have larger depths of field while passing a bit less light.

This is what makes motion picture film projection lenses unique, their ability to accept the fact that the film image is a rather fast-moving target and that is does flex in and out of a focal plane. All these lenses look great if they are looking at a glass slide. The difference is when they are projecting the moving picture image and how they handle that.

Pat

Posted by Richard Fowler (Member # 893) on 10-25-2002, 02:12 PM:

The Supra projector which was the first to have a curved gate in a modern sense did have to work against lenses of the day and hot reflectors. The trap was fairly large which may have contributed to the flutter problems....you have to start somewhere

Posted by Christopher Seo (Member # 7) on 10-25-2002, 03:16 PM:

I'm curious as to whether a higher gate tension is needed to maintain a steady image in a curved gate. It would seem that the film tension is relatively high at the aperture, where the gate pushes in the farthest on the trap bands, and less at the top and bottom where the gate rails curve away from the trap. On the other hand, a straight gate should probably impart uniform pressure to the film throughout the gate and would thus seem to require less overall tension.

Posted by Pete Naples (Member # 761) on 10-25-2002, 04:08 PM:

BTH SUPA did indeed have dreaful problems, well I think it was early versions that did. If I recall right the aperture was in front of the film, so more of the film was hit with heat from the lamp. There were mod kits for early ones, and later ones had the design altered somehow. I can't comment in more detail as I only ever saw one pair in the flesh, they were earlies, and to be quite honest, once was more than enough.
Likely as not, Bernard can tell us more of the SUPA, I'm really not old enough!

Posted by Ken McFall (Member # 845) on 10-25-2002, 04:47 PM:

Reading this post reminds me that there is indeed nothing new in the world, ideas come and go and return hailed as the next best thing since sliced bread!!

On the topic of BTH, I've had the pleasure of meeting Billy Bell who worked for BTH and was a mine of very fascinating stories of the delevopment of the Supas.... It seems the issue of heat is nothing new and was one of the problems that BTH faced.

I've used a number of different projectors, many of them now obsolete, BTH, Kalee 8-12-18-19-20-21, Simplex, Philips (Kinoton), Cinnemechanica, Westar etc etc. All of them were inovative in their own way.

I recall a problem with a vic 4, nice machine if you sealed the rear door. We used to scratch the middle of the frame every now and then. We checked the usual suspects, rollers and non-rewind, but found nothing wrong. It turned out to be the flat gate. Where the film entered the top of the gate the guide rollers had a tendancy to cup the film slightly, polyester prints especially as they were softer. As the apature plate was very close to the film surface it could touch and scratch. The solution was to force the film to enter the top guide rollers at a slight angle giving it more stability and thereby not allowing it to cup. A curved gate solves this problem perfectly. The curve does not have to be in the apature plate opening area, to be effective. Look at the Vic 8 as an example of this. The gate is not acutally curved as such but has a change of angle mid way enough to give the film extra stability as it passes the apature plate in a flat plane. How old is the Vic 8 design?

One of the problems with the BTH design was that the base side of the film was pressed aginst a fixed plane. Therefore focus could easily drift as the emulsion moved away from the fixed plane. Most, but not all, modern projectors press the film's emulsion against a fixed plane so the focus does not drift in the same way.

There's been the standard twin blade shutter, the contra rotating twin shutter, the drum shutter, the single blade double speed shutter of the Kalee 21 and of course the No shutter FP20 pulse lamp. All with their virtues and supporters. I wonder which of them will make a BIG come back in the next few years.

I remember the novel way the Kalee 8 framed the picture by moving the whole gate assembly up and down in relation to a fixed apature plate. The system was simple and I'm sure was very easy to manufacture. God how I miss some of these old machines and their character.

We don't know we're born now-a-days.....

Posted by Steve Guttag (Member # 268) on 10-25-2002, 05:31 PM:

Actually,

Most projectors use the base edge as the reference edge. That is, the curved gate that uses tension bands (Century, Simplex, Norelco DP-70) have the base be the reference edge.

One thing straight gates do have over curve gates is that they have the emulsion on the reference edge so the focus wont drift from changes in stock thickness.

Steve

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"Old projectionists never die, they just changeover!"

Posted by Brad Miller (Member # 2) on 10-25-2002, 05:38 PM:

Check out the pictures of the UA Berkeley, Steve. We always had horrible focus issues there on those Simplexes with straight-gates between acetate and polyester film stocks. (As an example...polyester feature with a mixed bag of polyester and acetate trailers.) The problems went away when polyester film completely took over. These machines had the emulsion side as the reference. These auditoriums were long throw and the lenses were not super fast or anything special. (Run of the mill high-end mid 1980's ISCOs at the time.) I agree with the theory, but having seen this in person, I don't buy it.

There were also side to side focusing issues, such that you could get the center of the film in focus, but not the sides, or vice versa. Not enough that customers noticed and complained, but definitely deserving of an F in my book. The only straight-gate projectors I have used that I was impressed with was the very early Christies. (Disclaimer, no I have not worked with a lot of straight gate machines.)

Posted by Steve Guttag (Member # 268) on 10-25-2002, 07:32 PM:

I have worked with quite a few straight gate machines....I have no dislike for them. As the heat goes up, their short comings show up too.

I have nevr had a problem with the traditional Simplex straight gate...now the circa '70s reversing straight gate was a nightmare! Never saw one that wouldnt' love to scratch your film! Just changed out another pair...located at the University of Penn....they now have modern Simplex curved gates...and oh what a difference! Even on their 50' screen the picture is NICE (complete with Strong Ultra-80 lamphouses).

Steve

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"Old projectionists never die, they just changeover!"

Posted by Brad Miller (Member # 2) on 10-25-2002, 07:56 PM:

quote:
"As the heat goes up, their short comings show up too."

That's probably why. The HVAC in this booth was under-spec'd from day one and UA wouldn't spend a dime fixing it properly. Couple that with the lack of real outside exhaust systems (they were vented above the ceiling tile with mini blower fans) and add to that the Strong lamphouses (which run hotter than an equivalent sized Christie or Cinemecannica lamphouse), and the machines were HOT!!! Most of those Strongs have been changed out with Christie machines and there is a noticeable improvement in not only light output, but also less heat.

Posted by Bernard Tonks (Member # 809) on 11-01-2002, 01:25 PM:

Pete wrote,

quote:
BTH SUPA did indeed have dreadful problems; well I think it was early versions that did. If I recall right the aperture was in front of the film, so more of the film was hit with heat from the lamp. There were mod kits for early ones, and later ones had the design altered somehow. I can't comment in more detail as I only ever saw one pair in the flesh, they were earlies, and to be quite honest, once was more than enough.
Likely as not, Bernard can tell us more of the SUPA, I'm really not old enough!

Ken wrote,

quote:
On the topic of BTH, I've had the pleasure of meeting Billy Bell who worked for BTH and was a mine of very fascinating stories of the development of the Supas.... It seems the issue of heat is nothing new and was one of the problems that BTH faced.

The BTH Supa’s known as a couple of Elephants looking out of portholes. Pete’s summary is correct. Billy Bell used to service my Western Electric equipment at Cranleigh, so I also heard many fascinating stories like Ken. Large 2” hub spools, the take-up ran anti-clockwise! Billy told me that the best Supa he ever installed was over 50 years ago, trouble free to this day. It is in a glass case at the London Science Museum!

I did a days relief at the Classic, Brixton (now Ritzy) which had the Supa Mark 2s, they looked much better with round spoolboxes and seperate xenon lamphouses.

The last Elephant Supa’s I saw running over ten years ago, was in a twin cinema in Wadebridge, Cornwall. They were well maintained and quiet running. The Projected Picture Trust Museum at Bletchley Park has a BTH Supa on display.

Posted by Stephen Furley (Member # 1277) on 11-02-2002, 12:57 PM:

Bernard,

I haven't seen SUPAs for very many years, except for the one at Bletchley Park. I remember the one on display at the Science Museum, but I don't think it is there now, I haven't seen it since the new Photography and Cinematography gallery was opened, years ago now.

In the box at the Phoenix there is a photograph of a young Peter next to one of these machines, at the Odeon Finchley, I think. Of course, the Odeon Leicester Square had three of them at one time, they were there at the same time as the Kalee Vista machines. The Telekinema at the Festival of Britain also had a pair, together with a television projector, which was replaced by a Bell and Howell 609 16mm machine after the Festival, when the Telekinema became the original National Film Theatre for a few years. The SUPAs would not have been ideal for the 3-D prrojection, with their 2000 foot capacity. Interestingly, the festival prints were all made on safety base, so I don't know iif they would have been allowed to use larger spools, or if the regulations at that time would have prohibited it, even though no nitrate was being used.

I assume that all Cinerama prints, and tracks were always safety, as this process always used large spools.

Posted by Ken McFall (Member # 845) on 11-02-2002, 02:15 PM:

While on the subject of old equipment, slightly off topic I know, I used to work in Peterhead Scotland for some 9 years as projectionist and manager. We had BTH projectors, with the driven sound head, can't recall the model no. I loved them everyone else hated them... bastards to set up until you get to know them then they were beautiful. Scotland, especially further north, was a living cinema museum. There were three types of BTH including the supas, Kalee 8's, 12's, 18' 21's. Ross GC3's,Westars that had a strong hold in Scotland for so many years and the original Western Electric sound on disc (not in use of course). It was a bit hard to find the disc's, I've said this before there's nothing new today!!

There were also two cinemas with BTH amplifiers giving very good sound considering.

All of these projectors were still earning their keep some 13 yaers ago and some possibly still are. It was always fun to visit a cinema and find these machines still in use. They were serviced by Westrex right up until they shut up shop.... god knows what they do for spares.

Posted by Gordon McLeod (Member # 33) on 11-02-2002, 11:00 PM:

I once sent some drives back to DTS labled "att ERPI division" but the sense of humour was lost on them

Posted by Pete Naples (Member # 761) on 11-03-2002, 03:11 PM:

Bernard, as ever a mine of information! Thank you. I once ran a show on a pair of SUPA's, it was only when I started the machine that I discovered that the take-up ran backwards! Much swearing to be heard comeing from that booth.

Ken, was that the Playhouse, Peterhead? I was up there a couple of times in the last few years, it's now closed. Last time I was in there, one Westar and one FP20 were the kitin use. Another Westar lay in the foyer and various Kalees were slumbering in other areas of the building.

If we disregard the modern cinemas here in Scotland, basically we run Westars. AFAIK there are no Kalee, BTH, or Ross running now. A few Phillips FP20 and the odd DP70 and DP75 are to be found, but basically it's Westrex. I tripped overa couple of Ross heads backstage at one of my services the other day, they were replaced with the omni present Vic 5 some years back.

Posted by Ken McFall (Member # 845) on 11-03-2002, 03:46 PM:

Hi Pete,

Thats the stuff I put in a long long time ago. The FP20 came from Grampian Television, no c/o shutter or lamphouse table or heat sheild. The westar(s) came from one of the companies Bingo halls, Regal Forfar if I recall correctly. The AEI lamphouse came out of the Victoria Dundee, now long gone, with a Westinghouse rectifier that I converted to diode. The Peerless conversion came from Morganite and was installed by Ernie Chinnery.... happy days

It was quite amazing that so many small town cinemas had survived so long, The Kings Brechin was all BTH, amplifier as well, and I was happy to be able to let him have a couple of mech's when the FP20 was installed.

But yes, the mighty Westar did rule for so many yaers due to the fact that you could lease the equipment rather than buy it outright. Mind you with the cold winters that far north you needed the valve amps to keep the box warm.

Do you rememeber Bob Brown, he was our Westrex engineer for many years in Peterhead.

Posted by Thomas Hauerslev (Member # 566) on 11-04-2002, 02:51 AM:

You can read the story on how the gate in the DP70 became curved here

Cheers, Thomas

Posted by Leo Enticknap (Member # 534) on 11-04-2002, 03:29 AM:

Stephen writes:

quote:
I assume that all Cinerama prints, and tracks were always safety, as this process always used large spools.

And also because of the timings. Nitrate ceased to be manufactured in the US in February 1950, and the first Cinerama shows weren't until autumn 1952. Two and a half years would have been more than enough for residual stock supplies of nitrate to be used up, I would have thought.

Posted by Stephen Furley (Member # 1277) on 11-04-2002, 05:04 AM:

quote:
And also because of the timings. Nitrate ceased to be manufactured in the US in February 1950, and the first Cinerama shows weren't until autumn 1952. Two and a half years would have been more than enough for residual stock supplies of nitrate to be used up, I would have thought.

I couldn't remember the dates offhand, I thought it was later than that; if it was 1950 hen no prints would have been nitrate, at least not ones that ever went to theatres.

Do you know when triacetate stock first became generally available? Was there much of an overlap with Nitrate?

I know that Pathe was using diacetate safety stock for 28mm prints in 1912, and that similar stock was available in 35mm from about the same time, but was never very widely used.

Posted by Bernard Tonks (Member # 809) on 11-04-2002, 06:06 AM:

I was a trainee projectionist in April 1952, and safety film was definitely established by then. All releases were on safety stock; nitrate stock was being used up on newsreels only! In fact I well remember safety stock being around in 1951 when I was allowed to visit the box of my local Odeon once a week before leaving school to do some rewinding, as I was so interested to learn projection.

Posted by Leo Enticknap (Member # 534) on 11-04-2002, 06:16 AM:

Eastman Kodak introduced a new formulation of cellulose triacetate for release prints in the autumn of 1948. An article which appears in the SMPE Journal around then explains how it improved on the durability and mechanical tolerances of previous generations of acetate stock (I can dig out the full reference if you'd like it), to the extent to which Kodak felt able to market it as a serious alternative to nitrate. Eastman Kodak ceased manufacturing nitrate base for release prints in February 1950. I guess the European manufacturers may have gone on a bit longer, and there are all sorts of urban legends about nitrate continuing in the Eastern Bloc well into the 1960s.

Posted by Bernard Tonks (Member # 809) on 11-04-2002, 06:47 AM:

quote:
We had BTH projectors, with the driven sound head, can't recall the model no. I loved them everyone else hated them... bastards to set up until you get to know them then they were beautiful.

Ken,

That could have been the BTH D block soundhead and drive. The later model BTH was with the rotary magnetic drum soundhead (although called magnetic it was optical only, before stereo days.) I personally liked the combination of the BTH type KB picture head with the rotary magnetic soundhead. Never was a great lover of BTH sound, but they made some nice reliable racks, and could sound great with good acoustics!

Posted by Ken McFall (Member # 845) on 11-04-2002, 01:13 PM:

Thanks for that Bernard... our's were the rotary magnetic sound head. Gave very good performance if you knew how to set them up.

Posted by Pete Naples (Member # 761) on 11-04-2002, 02:33 PM:

Ken, I don't recall ever meeting Bob Brown, the Westrex man round these parts was more often than not, Ken Bush with the occasional George Coates thrown in for good measure.

What company did you work for in Peterhead? I've got some really old books someplace that talk about that cinema being part of the now defunct Caledonian Associated Cinemas empire. Last time I was in there the colour scheme would support that, flamingo pink and jobby brown! The CAC I started in was once those colours, and the few remaining ex-CAC's also bear those delightful shades in non public areas.

All this nostalgia, brings a tear to my eye! Ach it's ok, over the next two days I'm working in the Cameo and the Dominion, Edinburgh, you can't get any older or nostalgic than those two!