Stonehenge and Solitaire

My visit to Stonehenge in 1994

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. 

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The Burn-Hole Club

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. 

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Behind Gamma’s Disguise

Electron guns from assorted cathode ray tubes.

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.

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