Mellon optical memory
Mellon optical memory was an early form of
Computer memory and Computer data storage types |
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Volatile |
Non-volatile |
Description
The main memory element of the Mellon device consisted of a very large (television sized) square
The tube was charged with a high electrical voltage. When an external source of light struck the photoemissive layer, it would release a shower of electrons. The electrons would be pulled toward the positive charge on the phosphorescent layer, traveling through the vacuum. When they struck the phosphorescent layer, they would release a shower of photons (light) travelling in all directions. Some of these photons would travel back to the photoemissive layer, where they would cause a second shower of electrons to be released. To ensure that the light did not activate nearby areas of the photoemissive material, a baffle was used inside the tube, dividing the device up into a grid of cells.
The process of electron emission causing photoemission in turn causing electron emission is what provided the memory action. This process would continue for a short time; the light emitted by the phosphorescent layer was much smaller than the amount of energy absorbed by it from the electrons, so the total amount of light in the cell faded away at a rate determined by the characteristics of the phosphorescent material.
Overall the system was similar to the better-known Williams tube. The Williams tube used the phosphorescent front of a single CRT to create small spots of static electricity on a plate arranged in front of the tube. However, the stability of these dots proved difficult to maintain in the presence of external electrical signals, which were common in computer settings. The Mellon system replaced the static charges with light, which was much more resistant to external influence.
Writing
Writing to the cell was accomplished by an external
Reading
Reading the cells was accomplished by a grid of photocells arranged behind the phosphorescent layer, which emitted photons omnidirectionally. This allowed the cells to be read from the back of the device, as long as the phosphorescent layer was thin enough.
Refreshing
To form a complete memory the system was arranged to be regenerative, with the output of the photocells being amplified and sent back into the CRT to refresh the cells periodically.
References
- Echert Jr., J.P., "A Survey of Digital Computer Memory Systems", Proceedings of the IRE, October 1953. Reprinted in IEEE Annals of the History of Computing, Vol.20, No.4, 1998