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Eerie and haunting;
the effects of the Image Orthicon tube



by Paul Huber
with Richard Diehl

It affected me immediately. A black & white episode of the original Twilight Zone series, beginning with the strange iconic music and Rod Serling’s narration of the circumstances, but it was different. The characters were more alive in some way – more fluid, almost living in the present. There was a stronger contrast too that seemed to amplify everything and give more impact.

Most of “The Twilight Zone” episodes were produced using film cameras, where it’s a sequence of still photographs taken one after another on the filmstrip. When played back, we perceive the series of pictures as continuous motion because of the ‘persistence of vision’ of our visual nervous system. Some episodes though, were produced with electronic cameras incorporating an Image Orthicon tube. The electrical signals from the cameras are recorded on magnetic tape, rather than film strips, and this was done to save money when production budgets were tight. There’s still a sequence of images, similar to film, but the scanning operation of the electronic tube involves some interesting physics that give it its special qualities.

After I watched that episode I did some research to find what made it look different. I suspected it was electronic versus film and that’s what led me to the Image Orthicon tube. That was years ago and the whole thing just stayed in the back of my mind, but recently the interest grew again and some Internet searching brought me to someone who has a passion for this stuff. His name is Richard Diehl, a.k.a. “Lab Guy”, and we discussed his interest and the underlying physics of this early but ingenious technology.

(Q) Where did your interest in video tubes come from?

(A) As a very young child, around age four, I was absolutely fascinated by our "big" black and white television. My memory says 23 inch, reality was probably 15 inch. I would put my nose to the screen and see the scan lines. "What the heck is that?" I'd wonder. It actually made me angry that the image wasn't completely flat and homogeneous! It became imperative that I figure out how that worked. Once in school, I discovered books on the subject. But, the age appropriate material was too short on detail. When in the second grade, Dad was stationed in Munich Germany, being in the Army, and we had to have our small Emerson 11 inch set readjusted to receive sound in Europe. The fellow came to the house. Using my mother to translate, I grilled that poor guy on what he was doing. He patiently explained audio sub-carrier to me, through mom, and the best part was I understood what he was talking about. I had just learned how radio stations worked on the dial! It made complete sense!

Also, at that same time, I already knew what the image orthicon was! The Ernie Kovacs show would come on and his character, Percy Dovetonsils, would say "Greetings over your orthicon tube."

I pestered my folks as to what that meant until they researched the question and told me it "was" the TV camera. Everyone who met that little brat back then also learned about the magical orthicon tube! We had no word for OCD or evil medications then. Thank goodness!

In my teen years, people started giving me old dead televisions and radios. I was fixing the radios at age 11 and my first TV at that time. One day Dad came home from a yard sale. He had a rather large black book in his hand for which he paid no more than a quarter. He said, "I found this book and I think it's going to give you a religious experience!" I thought it was going to be a Bible. It was, sort of. The ARRL (Amateur Radio Relay League) ham radio handbook! It started me down the road to video.

Ham radio slow scan TV was the big thing in the late 60s. One of the ham radio magazines had a construction article for an SSTV monitor. I built that at around age 13. Next came a copy of a vacuum tube camera by Dage. The source of video junk in those times was a surplus store called Denson Electronics, in Rockville Connecticut. Al Denson and I exchanged letters on the topic and he provided a schematic and some of the critical parts to construct the camera. It worked! Sort of.

I rebuilt this camera into a full transistor job when an article appeared in Radio Electronics magazine. I was able to get pictures from it after a fashion. But, there was a typo in the schematic that I was not made aware of at the time. But, it counted!

Then, I joined the Air Force in 1976 and things took off. I bought a reel to reel video tape recorder and camera outfit and was in like Flynn!

 

Top: Video clip from episode title "Twenty-Two".
Lower: Image from the airport scene showing an explosion. The dark halo effect is very pronounced due to the brightness of the flash. That brightness produced an excess of photo-electrons emitting from the front plate of the tube.

(Q) Do you have a favorite tube among the evolutionary stages (Image Dissector, Iconoscope, Orthicon, Vidicon)?

(A) If it is electronic and makes pictures, I’m fascinated by it. TV cameras, fax machines, printers, scanners, computer graphics, etc.

My favorite imager is the Image Dissector tube by Farnsworth. Poor guy. His tube is perfect for every type of imaging... except fast scan television! The faster you scan a dissector, the less sensitive it is. How ironic. The tube lacked an important property called charge storage.

Zworykin at RCA had a tube with this property, but loaded with other problems of its own, particularly related to scan geometry. Its photosensitive surface, the mosaic, was too thick to be scanned from the back side, and had to be scanned from the front. The electron gun was typically oriented at around 45 degrees aiming "up" at the mosaic. This resulted in a scan resembling the stroke of a windshield wiper. It was key stoned and required scan correction on the fly. In vacuum tube circuits, the last thing you want is more tubes! Cost, heat and power consumption shoot through the roof!

(Q) Where did the name "Orthicon" come from?

(A) The "orth" in the name orthicon refers to the linear sensitivity of the photosensitive materials used. From the Greek "orthos" that means just that: straight or erect. The "icon" part is taken from the Greek word eikōn, for "image" and/or is a contraction of the engineering terms "image converter." I believe that Vladimir Zworykin considered "icon," as in iconoscope, clever term construction.... and it is. Albert Rose and Richard Webb carried the terminology forward in the image orthicon. "Image" referring to the section derived from the work of Philo T. Farnsworth. No single person "invented" television.

(Q) The Orthicon tube is known for the dark halo effect but I noticed two other effects as well: lifelike smoothness of motion and geometric distortion. Let's start with smoothness. Is that due to faster rate of frame or the electronic scanning versus the advancing of still images with film?

(A) Eventually, Albert Rose and a team at RCA took all the best attributes of the dissector and iconoscope and combined them. The orthicon tube was born. It had plenty of problems and each solution made it just that much more complex. It had a backside scanned mosaic and weird hybrid scanning. The vertical was electrostatic, the horizontal magnetic. This tube never passed the experimental phase. Finally, they added the "imager" portion of the dissector tube to the front of the orthicon and made the hybrid image orthicon tube.

The imager section has the same photo surface and electron lens arrangement as the dissector tube. The difference is at the back. The pin hole aperture is replaced with a charge storage surface. The electron image is cast onto the thinnest piece of soda glass known up to that time. It was made by blowing a bubble of glass, thickness measured optically with structured light (sodium vapor lamps as lasers were a few years off). When the thickness was correct, a beryllium ring was pressed into the bubble and the excess glass was cut away.

The electron image lands on this glass called the target. It is so thin, the electron charge can be read from the back side of the glass. High velocity light induced electrons deplete the charge on the glass by knocking out electrons. The scanning beam puts them back on each scan. The reflection of the scanning beam, what I think of as a "spray" - think water hose blasting a wall, is picked up at the back of the tube and amplified. The missing charge in this return beam IS the video signal!

As for the image smoothness artifact, it is caused by the lack of charge storage between scans, known as lag. Also, there is some spreading of the charge that evens out noise, giving the smooth appearance to large flat areas of constant brightness. The target is biased right to the hairy edge of balance. Just enough beam to erase the entire last image and no more. More beam ends up knocking out secondary electrons and surrounding bright spots with incorrect charge - dark halos in the output signal. Unlike the vidicon tube, which can scan out one image for several frames, the IO has no image lag at all. This is like the CCD. Each scanned frame is crisp and unique with no history of previous information. The effect is very pleasing to the eye.

The frame rates of film and television are different. Film operates at 24 frames per second and monochrome television operates at 30 frames per second. The subject of film scanning is too complex to delve deeply into here, just understand that film seen on television has a strange motion artifact called judder. Think of it as a form of blur, a loss of information. The direct video from the camera is free of this artifact and contains all of its original information. Some people describe the look as hyper-real. Film looks softer on TV most of the time. Film was so commonly used on early TV, that viewers became accustomed to this look over live video. So, when Twilight Zone switched from film, the harsh clarity of the image plus the perfect fluidity of motion seemed out of place and helped to enhance the feeling of tension and suspense. If you are interested in learning more about film to TV conversion, here is a resource.

(Q) What causes the geometric distortion? Does that have to do with the magnetic fields used by the tube?

(A) Geometric distortion is caused by the quality of the magnetic coils and to some degree by the optics. In my focus coil rewinding video you can see me going mad getting the thing perfect. Any imperfections show up as geometric error. Another source of geometric distortion is external magnetic fields, like that of the Earth. It is not uncommon to see the picture get all watery looking when the camera is trucked around the dolly. This is external distortion. The electron beam in the imaging tube is a low velocity type (+300 to +1,000 volts). Any electron charge or magnetism affects it adversely. In CRTs, for instance, the beam is very high velocity and is barely affected by these same forces (+16KV or more). The electrons are moving faster and so spend less time in the affecting field, which result in lower interaction.

(Q) The most notable association with these tubes is probably the dark halo effect. What about the tube causes that?

(A) The dark halo effect is when the bias is not quite right in the IO tube or the light is too bright. Secondary electrons are knocked out of the glass target and rain down around the bright spot and contaminate the image charge on the glass. If the "shader," a person who sat there tweaking the camera continuously in real time like automatic circuits still do to this day, was really good at their job, you would not see the halo effect. Some directors liked to have just the tiniest amount of shading in the image as it gives the perception of a sharper picture. It isn't really, but tell that to your brain!

(Q) For video capture, the CCD chip is a superior piece of technology, but I think it lacks something for enthusiasts. For me, it's almost a bit boring compared with the ingenious methods of manipulating electron beams inside complex glass tubes. What are your feelings on the CCD?

(A) The CCD is the pinnacle image capture devices. It has the lag free characteristics of the dissector and IO photo-target and PERFECT scan geometry. Charge transfer efficiency is so high as to be considered perfect. So, utilizing CCDs is great for producers. But, students of imaging will learn little from it as we learn from problems, not success! If TV had been perfect in my youth I might have followed a totally different path through life. □

 

Richard Diehl is a self taught engineer who became fascinated with television at a very early age. "I began building working radios and repairing televisions at the age of 11 in 1968. By the time I would have graduated from high school (I quit high school in disgust at the failing of my local school - lowest performance of any school in the state at that time) I had constructed two working vidicon television cameras. Joined the US Air Force in 1976 where I was a controller of communication satellite missions. I then moved to Silicon Valley in 1982 after getting an AA at a community college. One of my employers, years later, presented me with a certificate granting me the equivalent of a BS degree for the innovations I had brought to their product line (and so they could promote me inside their system, which required degreed engineers). I have worked at numerous video product companies over my career covering everything from normal television, video tape recorders, cameras, videoconferencing, machine vision systems, laser scanning systems, digital image processing, etc.. Most recently worked at a Silicon Valley IC manufacturer as a lab test technician. I am currently semi-retired."
Visit his website, LabGuy's World, for more information.

 

 

 


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