When Management Graphics adapted their film recording technology to support motion picture film formats, it was quickly adopted by movie studios to bring special effects from their computer memory images on to film. There were some problems however, and one of the most serious was the difficulty in obtaining the full brightness range found in typical scenes, especially when they included lights—candle light, desk lamps, car headlights, streetlights. Any light source, even a glimpse through a window to the bright outdoors, would cause a large flare in the final film frames, washing out detail in the scene. Our customers complained, and we started down a path to research and solve the problem.
We understood what the fundamental issue was: halation, an effect caused by the glass faceplate of the cathode ray tube used for creating the image. The bright spot on the phosphor screen was internally reflected at the glass surface which then illuminated the phosphor coating. If phosphor were black, this would not be a problem, but phosphor coatings are white, as are most materials made of fine powder, and it resulted in this internal reflected light overexposing the film. In the absence of a black phosphor, there were few other ways to mitigate the halation effect.
An example of halation on a photographic film plate. The circular haloes and flare are apparent around the street lights in this 1910 image.
One of our customers was incorporating our film recorder into a full workstation system. Quantel, a company in Newberry, England, had become successful in the early years of digital video and was looking for a way to expand its editing tool offerings into the motion picture market. Quantel’s engineers understood the halation problem as well, but they didn’t want to rely on our figuring out a solution: they had an aggressive development schedule.
The coils that provide the magnetic force to move the electron beam
Cathode ray tubes are a remarkable technology that incorporate many seemingly magic principles of physics. Thermionic emission causes electrons to “boil” off a cathode, high voltage electric fields accelerate and focus them, and magnetic fields steer them to the anode screen where they energize phosphor molecules, which then re-release that energy as visible light!
While developing the electronics to control the CRT and make
all this magic happen, we often had to “bring up the spot”, showing the
electron beam in one static location, where it could be examined visually and
measured with various instruments.
This was always a tricky maneuver, because if the electron
beam were allowed to become too intense, the energy transfer to the screen
would cause it to heat up to the point where the phosphor would suddenly
vaporize, leaving a dark spot that could never be lit up again. Usually, this burn-hole was in the middle of
the imaging area, and so the CRT, the single most expensive component in the
system, would become instantly unusable.
To avoid this event, there was a standard procedure for bringing up the spot: apply voltage to the CRT and gradually increase it, sneaking up on the level where the spot would become visible, and then avoid going too far. The safe operating zone was rather small.
But even the best procedures cannot anticipate every possible
experiment and test that might be needed while inventing new technology. Consequently, there were situations where the
beam accidentally and unexpectedly reached the critical phosphor burn level,
and whoever was conducting that particular test suddenly realized that they had
crossed the threshold and now the CRT had a permanent blemish. They had become a member of “the burn-hole
club”.
The burn-hole club comprised everyone who had suffered this
unexpected event. It was both an
embarrassment, and a badge of honor. It
was awful to realize that an expensive component was now worthless, but on the
other hand, the tests and experiments that we were conducting were on the
cutting edge of our knowledge, the term “cutting edge” implying that injuries
were part of the process. Only brave
researchers dared to push this edge forward.
I am not a member of this exclusive club, but that is
because I had skilled technicians who knew far better than I how to conduct the
tests I requested. They were on the
front lines of the technology. And as a
result, they were the ones first inducted into the burn-hole club.
There is one incident that deserves special mention. It occurred during the development of the
brightness calibration method, a critical part of making accurate exposures
onto film. We used a photocell at the
far edge of the screen. It needed to
“see” the spot, and measure how bright it was.
The task of figuring out how to do this was assigned to Rick Keeney, who
became a master of writing code to control the complexities of driving a
cathode ray tube.
To solve this particular problem, Rick came up with a clever
algorithm to position the beam directly under the photocell. The exact horizontal and vertical position of
the photocell is not known, at least not at first. It needs to be located. So Rick made a first best guess, and then
refined it. By moving the beam slightly
horizontally and vertically and seeing if the light seen by the photocell
increased or decreased while doing so, the beam position could be
estimated. Move the beam to where the
light measurement was strongest, and that would be the location directly under
the photocell.
But if the photocell light level was low, it was hard to
know which way to move, so increase the intensity of the electron beam and try
again. And if that didn’t help, then it
was likely that the beam was not quite where it was expected. So move it over a little bit and try again. This was the strategy for locating the beam
and calibrating its brightness. It worked
fairly well… until it didn’t.
On one occasion, the beam could not be detected at all. The algorithm increased the intensity trying to measure the light, but as it did so, burned the phosphor in its path. When it failed to detect the beam, it moved over a little bit and burned the phosphor there too. Since the beam was still not detected, it was moved a little more and tried again. The algorithm didn’t have a limit check on position, so it marched all the way across the full width of the screen, leaving behind a tire-track of vaporized phosphor.
This was a spectacular example of damage by electron bombardment, and Rick Keeney, in addition to being an Academy Award winner, also holds the prime honor in the burn-hole club. And I have the privilege of curating the resulting damaged tube.
The trail of damaged phosphor, leading right off the edge of the screen.
Rick with a more public acknowledgement of his skills in pushing the cutting edge of science and technology.
Those who know me would be stunned to learn that I have a
gun collection. I acquired them in the course
of my work trying to make computer images on film in the 1990s. They are electron guns, the mysterious
workings at the business end of a cathode ray tube.
