CV14x range of infrared converter tubes
These devices, manufactured by EMI, are high-vacuum image converters. At one end of the device is a semi-transparent silver - oxygen - caesium film facing a cathodo-luminescent screen of zinc orthosilicate having a manganese activator which emits light in the green part of the spectrum.
The phosphor screen is held in a metal ring so that it is approximately 3mm from, and parallel to the photocathode.
An electrical connection is made to the photocathode by means of a thin metal ribbon which is sealed through the glass. One of the wires that holds the support ring of the phosphor screen is lead out as its connection.
The device works with approximately 5 to 7kV. It only has a resolution of a few line pairs/mm.
The finished tubes were sealed off with a vacuum of around 10-6mm/Hg.
It was important not to expose the tube to light above that in which a normally dark adapted eye can see in a dimly lit room.
There is currently no information on the differences between the CV14x range so it is likely there were slight modifications carried out and that each one required a new CV number.
The tube had to be almost 2" long so that the phosphor screen could be protected from reaction with caesium during the formation of the photocathode. When a tube was first assembled, the screen plate was held in its support ring close to the cathode window. Immediately before the tube was pumped down a hand-held magnet was used to pull out the spring-loaded retaining pin holding the screen plate in place. The screen then fell onto the end window and was flipped over by the operator (in an action rather like flipping a pancake) to get the screen side against the window. After this the tube was sealed to the vacuum pump.
After this a vacuum valve disconnected the tube from the pump, a small pressure of oxygen was introduced and a glow discharge started in the tube. This was stopped when the correct amount of oxide had formed, and the excess oxygen was pumped out. The caesium source was then slowly activated to allow the silver-oxide to acquire photosensitivity. A small lamp was used to excite the photosensitive cathode, the progress of activation being measured by a microammeter and small battery connected across the tube. The end point was determined by a falling off of the photo current, at which event the reaction was stopped and the tube then sealed off.
The grid structure that can be seen in some of the exhibits was a film of platinum created by vapourisation through a mask. This made connection with the phosphor and stabilised the potential over the whole screen area. There were other schemes employed too, such as continuous semi-transparent metal films, and aluminium coatings.
A paper published in 1947 detailing the CV14x range can be found here (418k PDF).
Information kindly provided by F. John Marshall (fjohn.marshall 'at' att.net) who used to work on these devices while at EMI.
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