Mellon optical memory
Encyclopedia
Mellon optical memory was an early form of computer memory
invented at the Mellon Institute in the 1950s. The device used a combination of photoemissive and phosphorescent materials to produce a "light loop" between two surfaces. The presence or lack of light, detected by a photocell, represented a one or zero. Although promising, the system was rendered obsolete with the introduction of core memory in the early 1950s. It appears that the system was never used in production, but it represents one of the typically odd earlier attempts to produce a useful high-speed memory system.
To set up operation, the tube was charged to a very high DC potential. If an external source of light struck the photoemissive layer, it would release a shower of electrons, amplified by the DC potential. The electrons would be pulled toward the positive charge on the phosphorescent layer, traveling though the vacuum. When they struck the phosphorescent layer, it would release a shower of photons (light). 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 "cells".
The process of electron emission causing photoemission 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. The Mellon system replaced the static charges with light, which was much more resistant to external influence.
(CRT) arranged in front of the photoemissive side of the grid. Cells were selected by using the deflection coils in the CRT to pull the beam into position in front of the cell, lighting up the front of the tube in that location. This initial pulse of light, focussed through a lens, would set the cell to the "on" state. Due to the way the photoemissive layer worked, focusing light on it again when it was already "lit up" would overload the material, stopping electrons from flowing out the other side into the interior of the cell. When the external light was then removed, the cell was dark, turning it "off".
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...
invented at the Mellon Institute in the 1950s. The device used a combination of photoemissive and phosphorescent materials to produce a "light loop" between two surfaces. The presence or lack of light, detected by a photocell, represented a one or zero. Although promising, the system was rendered obsolete with the introduction of core memory in the early 1950s. It appears that the system was never used in production, but it represents one of the typically odd earlier attempts to produce a useful high-speed memory system.
Description
The main memory element of the Mellon device consisted of a vacuum tube consisting of two slightly-separated flat glass plates. The inner side of one of the plates was coated with a photoemissive material that released electrons when struck by light. The inside of the other plate was coated with a phosphorescent material, that would release light when struck by electrons.To set up operation, the tube was charged to a very high DC potential. If an external source of light struck the photoemissive layer, it would release a shower of electrons, amplified by the DC potential. The electrons would be pulled toward the positive charge on the phosphorescent layer, traveling though the vacuum. When they struck the phosphorescent layer, it would release a shower of photons (light). 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 "cells".
The process of electron emission causing photoemission 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
Williams tube
The Williams tube or the Williams-Kilburn tube , developed in about 1946 or 1947, was a cathode ray tube used to electronically store binary data....
. 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. 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 cathode ray tubeCathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
(CRT) arranged in front of the photoemissive side of the grid. Cells were selected by using the deflection coils in the CRT to pull the beam into position in front of the cell, lighting up the front of the tube in that location. This initial pulse of light, focussed through a lens, would set the cell to the "on" state. Due to the way the photoemissive layer worked, focusing light on it again when it was already "lit up" would overload the material, stopping electrons from flowing out the other side into the interior of the cell. When the external light was then removed, the cell was dark, turning it "off".