Retrotechtacular: Eidophor, an Unknown Widely Used Projector

If you own a video projector, be it a module small enough to fit in a mobile phone or one designed for a cinema screen, the chances are it will have a DLP at its heart. An array of microscopic mirrors on an integrated circuit, the current state of the art in video projection technology.

Perhaps you own an older video projector, or maybe a cheaper new one. If so the chances are it’ll have a small LCD screen doing its work, taking the place of the Kodachrome in something very similar to your grandparents’ slide projector or their grandparents’ magic lantern.

eidophore-patent-image-600pxLCD technology was invented in the 1970s, while DLP was invented at the end of the 1980s. So how did the video projectors that were such a staple of televised spectaculars in the preceding decades work? For that matter, how did NASA project their status displays on the huge screen at Mission Control? Certainly not with CRT technology, even the brightest CRT projectors weren’t up to filling a cinema-sized screen.

The answer came from the Eidophor (Greek: ‘eido’ and ‘phor’, ‘image’ and ‘bearer’), a device invented in the years before World War II by the Swiss physicist Dr. Fritz Fischer and granted a US patent in 1945. It featured a complex vacuum device in which an electron gun painted the video frames as a raster on an oil-covered mirror in the light path of a fairly conventional projector. High-voltage electric charges have the effect of deforming the surface of mineral oils, and it was this effect that was exploited to vary the effectiveness of the mirror as the raster was drawn. An unfortunate side-effect of tracing an oil surface with an electron beam is that a charge will build up on the oil surface, so the entire oil-covered mirror assembly had to rotate within its vacuum enclosure and pass under an electrode which removed any charge build-up.

Eidophor [by Topquark2 CC-BY-SA 3.0]
The resulting machine as seen in this 1952 issue of Popular Science was very large, complex, and expensive to run, but delivered by far the brightest and sharpest projected video available. In a literal sense they painted the backdrop to our culture, as they found a home not only in NASA’s control room but in television studios and at large televised events. This Shirley Bassey performance from the 1960s for example, or the spectacular video light show on this rather poor quality VHS YouTube clip from Seville Expo 1992.

You will probably be unaware of the exact date you last saw an eidophor performance. Quince Imaging tell us their last one was used at the TWA Dome in St Louis in July 2000. Eidophores may have become more compact over the decades but they remained costly to run, and through the 1990s they were suplanted by DLP devices that did substantially the same job with a lot less fuss.

It is not often that a search in the Hackaday archives for a technology returns no results, but the eidophor is one of those cases. Perhaps that is a fitting epitaph for a device that created its own show but never starred in it, that it is only its spectacular performances that live on.

40 thoughts on “Retrotechtacular: Eidophor, an Unknown Widely Used Projector

    1. There absolutely no need for a snipe. If you found a link, cool.

      Remember, google curates everything you consume. You cant guarantee that a search would pull it up for Jenny. Search history, location, time, OS, and cookies all can change googles returned search result.

  1. I can’t wait to see a teardown of this
    Dave? Clive? Mike? JW? Shahriar?
    Mehdi, perhaps? (That would be interesting, since I suspect some voltages invovled in there being right over the top of the scale…. ;-) )..
    Who would be the lucky one?

    1. They’re probably not as scary HV circuits as you’d imagine, after all the electron gun isn’t doing a huge amount more work than the one in a large CRT TV. So lots of volts, not so much current. But yes, still something to be careful around.

      1. I used to fix projectors and reverse projection screens and the basic principle applies – the longer the tube the higher the voltage.

        The compact projectors were a pain as the HV was carefully sealed into a space so small that it would arc over with the slightest electrical fault or mechanical disturbance.

        Small units were about 50kV and it’s all up from there.

  2. It goes to show how much we take for granted.

    That thing is huge and a peace of art.

    That is another perfect example of technology. I realy wonder what the next 20 years will bring.

    Great work..

      1. Kind of. We only mapped one city. The shots were taken on film with a rig that mounted 4 cameras on a Suburban. One frame was shot every few feet (I forget how many) and the resulting film was stored on an optical video disc. We then put three copies of the disc in three Pioneer players of the sort later used in Dragon’s Lair and computer controlled the playback as you ‘drove’ around the city. The video on the monitor was simply switched between the players using a computer-controlled video switcher.

        I wanted the UI to be the front half of some random car where we frosted the windshield and then projected on it using the Light Valve. I got voted down. :)

        We did however use the rear projection setup to play Star Raiders on the Atari 800. When you put the video on a big screen, pump the audio through 8 really big speakers and use a speech recognition system to act as joystick buttons it’s a much more immersive experience.

  3. I saw a modern example of this in use at a Thomas Dolby show in 1987. They used a GE Talaria projector to fill a medium-size movie screen with video. It was a really decent, bright picture – way brighter than even a 9″ tube CRT projector could do. The Talaria was about the size of a large 16mm projector.

    They must have nearly all been junked. There’s just one sitting on eBay at the moment with a BIN price of nearly $3K. It would be an interesting teardown if you could get it for free. Otherwise, eh, it’s like being sentimental about CRT projectors.

    1. As I understand it the Talaria was similar but slightly different to the eidophor, it shone the light through an oil-covered piece of glass rather than bounced it off an oil-covered mirror. Perhaps there will be a Hackaday reader who can elucidate.

      Either way I too would love to peer inside one. Perhaps it’s as well I don’t have a spare 3000 notes :)

      1. The Talaria used a glass disc that rotated through an oil bath in a vacuum assembly, and which sat between two sets of transparent/opaque stripes forming a Schlieren optical system. The light from an arc lamp passed through the first set of stripes, then the glass plate, then the second set of stripes, then the projection lens. The two sets of stripes were offset such that in the normal state the light that made it through the first set of stripes was blocked by the second set so the picture was black. The electron beam created a diffraction pattern on the oil corresponding to the desired image which had the effect of directing some of the light through the second set of stripes and then to the screen. The basic single light valve model managed RGB by separating the light from the lamp into a green strip and a magenta strip, Then they could independently modulate the red and blue from the magenta by changing the diffraction pattern.

        There was also a dual light valve model which stacked one light valve responsible only for green on top of a second that only handled magenta. Then, a third model which stacked three light valves on top of each other, one for each of red, green, and blue. I worked with the first two, but not the triple model. They were used in quite a number of military flight simulators, with the peak probably being in the late eighties/early nineties. LCD, LCoS, and then DLP put an end to them.

  4. I worked on these in the early 90s. At that time they were under GE/Barco and generically call “Light Valves”.
    General Dynamics made big use of them in simulation. In that application GD used two “light valves” to paint the background onto the surface of 40′ domes. There were separate target projectors that were mechanically driven to represent the targets. All of these images were rendered in real time by dedicated Evans and Sutherland image generators. There were separate computers for the airframe simulation.

  5. When I worked at the BBC West London studios in the 70’s we regularly used Eidophors for back projection to simulate moving cars. For one episode of Z-Cars (a famous 60’s & 70’s cop show) we had two! One for the back window and one for the side. The second unit had shot two rolls of 16mm film with synchronized cameras so the moving views were in synch. It took a whole day to shoot the sequence, because of the time it took to set up the Eidophors, and we had two Rank Cintel Mk2 telecine machines running in synch to provide the inputs. One of the actors was the great Brian Blessed. He had a wicked sense of humour, and I still remember him corpsing and forcing retakes.

    Apart from 2 Eidophors and 2 Cintel MK2’s we were also using 3 EMI 2001 colour cameras, and recording onto Ampex VR2000 vtr’s. I reckon that lot would have cost around £500K (about US$700K then, and US$3M in 2016 money). I seem to remember that the Eidophors each had a team of 3 or 4 techs, but I can’t remember who owned them. I was the junior video engineer on the studio crew and had the job of looking after the studio end of the camera chains. Got a good look inside the projectors while they were being set-up. It took all morning to get them rigged and warmed up. Being colour they were in effect 3 projectors in one, and it took a lot of work to get the 3 colour images superimposed.

        1. Does the term ‘synchronized’ in camera speak mean each frame is being advanced identically at the same time? Because I would think, just from the little I’ve read about the Eidophors that they would have a much softer frame transfer than a normal film projector and therefore would be easier to prevent flicker from frame rate float. But really I’m just thinking out loud with little knowledge on the subject…

          1. The scene was shot with 3 cameras and switched “as live” to one VTR, so the scenes being back projected needed to be in sync. The two Eidophors were running of the same sync pulses so their frame rate was locked. Although it sounds like a very expensive way of doing it, at the time the technology for running multiple VTR’s in sync was very rudimentary, and most video editing was done on 2″ tape with physical splices! And that is a whole other kettle of fish!

            This particular show was normally shot with 3 or 4 sets in the studio, and 4 cameras. Scenes were shot in one take, and sometimes run together with cameras and actors moving between sets in real time. A 60 minute show could be recorded in half a day, and may only have 3 or 4 physical tape edits to complete it. I remember in the early 70’s working on sit-coms which were done in one take in front of a live audience.

    1. More neat information. :) Pity that the BBC erased or trashed most of the Z Cars tapes. :( Should there ever be a time machine or some way to retrieve information from the past without altering the past – a priority should be to record all the lost TV and radio broadcasts.

    2. I used to operate Eidophor’s at the BBC in the early 80’s. I worked for Link Electronics who also built most of the Cameras and Production trucks used by the Beeb in those days. We had the only colour Eidophor in the country, he BBC owned a Monochrome unit that was regularly used on Z Cars but that was before my time. You might remember George Gilbert from Zoom who was the master of the Black and White Eidophor and no doubt all the boys from scenic projection at TV Centre.

    1. What and where is Nerd Lite on the 20th April? As one of the few people in the UK who can claim to be able to operate an Eidophor I might just come along. Where it lighter I would bring one along as I know where is a working original. I di have an original GE5050 predecessor to the Talaria.

  6. I remember setting one up in ’88 to project images outdoors during a promotion for the NBC Olympic ‘Triplethon’ or something like that. It was a scheme that NBC devised to use all their cable channels to present simultaneous Olympic events.

    Anyway, the image was projected on a roughly 7.5’x10′ rear projection screen, and the Eidaphor–about the size of a large washing machine–was lifted with a forklift onto a scaffold and enclosed in a ‘tunnel’ of black plastic to keep ambient light from reaching the rear of the screen. As I recall (I spent most of the broadcast running tapes from the video truck) it was quite bright, even in daylight…almost like a billboard.

    During the same time-period, I also worked a lot of shows with the GE Talaria PJs. I always wondered if they used that technology to project in-flight movies on large aircraft like 747s…?

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