Mellon optical memory

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Mellon optical memory was an early form of computer memory invented at the Mellon Institute (today part of Carnegie Mellon University) in 1951.^[1]^[2] 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 magnetic-core memory in the early 1950s. It appears that the system was never used in production.

Computer memory and data storage types
General Memory cell Memory coherence Cache coherence Memory hierarchy Memory access pattern Memory map Secondary storage MOS memory floating-gate Continuous availability Areal density (computer storage) Block (data storage) Object storage Direct-attached storage Network-attached storage Storage area network Block-level storage Single-instance storage Data Structured data Unstructured data Big data Metadata Data compression Data corruption Data cleansing Data degradation Data integrity Data security Data validation Data validation and reconciliation Data recovery Storage Data cluster Directory Shared resource File sharing File system Clustered file system Distributed file system Distributed file system for cloud Distributed data store Distributed database Database Data bank Data storage Data store Data deduplication Data structure Data redundancy Replication (computing) Memory refresh Storage record Information repository Knowledge base Computer file Object file File deletion File copying Backup Core dump Hex dump Data communication Information transfer Temporary file Copy protection Digital rights management Volume (computing) Boot sector Master boot record Volume boot record Disk array Disk image Disk mirroring Disk aggregation Disk partitioning Memory segmentation Locality of reference Logical disk Storage virtualization Virtual memory Memory-mapped file Software entropy Software rot In-memory database In-memory processing Persistence (computer science) Persistent data structure RAID Non-RAID drive architectures Memory paging Bank switching Grid computing Cloud computing Cloud storage Fog computing Edge computing Dew computing Amdahl's law Moore's law Kryder's law
Volatile
RAM Hardware cache CPU cache Scratchpad memory DRAM eDRAM SDRAM SGRAM DDR GDDR LPDDR QDRSRAM EDO DRAM XDR DRAM RDRAM HBM SRAM 1T-SRAM ReRAM QRAM Content-addressable memory (CAM) Computational RAM VRAM Dual-ported RAM Video RAM (dual-ported DRAM)
Historical Williams–Kilburn tube (1946–1947) Delay-line memory (1947) Mellon optical memory (1951) Selectron tube (1952) Dekatron T-RAM (2009) Z-RAM (2002–2010)
Non-volatile
ROM Diode matrix MROM PROM EPROM EEPROM ROM cartridge Solid-state storage (SSS) Flash memory is used in: Solid-state drive (SSD) Solid-state hybrid drive (SSHD) USB flash drive IBM FlashSystem Flash Core Module Memory card Memory Stick CompactFlash PC Card MultiMediaCard SD card SIM card SmartMedia Universal Flash Storage SxS MicroP2 XQD card Programmable metallization cell
NVRAM Memistor Memristor PCM (3D XPoint) MRAM Electrochemical RAM (ECRAM) Nano-RAM CBRAM
Early-stage NVRAM FeRAM ReRAM FeFET memory
Analog recording Phonograph cylinder Phonograph record Quadruplex videotape Vision Electronic Recording Apparatus Magnetic recording Magnetic storage Magnetic tape Magnetic-tape data storage Tape drive Tape library Digital Data Storage (DDS) Videotape Videocassette Cassette tape Linear Tape-Open Betamax 8 mm video format DV MiniDV MicroMV U-matic VHS S-VHS VHS-C D-VHS Hard disk drive
Optical 3D optical data storage Optical disc LaserDisc Compact Disc Digital Audio (CDDA) CD CD Video CD-R CD-RW Video CD Super Video CD Mini CD Nintendo optical discs CD-ROM Hyper CD-ROM DVD DVD+R DVD-Video DVD card DVD-RAM MiniDVD HD DVD Blu-ray Ultra HD Blu-ray Holographic Versatile Disc WORM
In development CBRAM Racetrack memory NRAM Millipede memory ECRAM Patterned media Holographic data storage Electronic quantum holography 5D optical data storage DNA digital data storage Universal memory Time crystal Quantum memory UltraRAM
Historical Paper data storage (1725) Punched card (1725) Punched tape (1725) Plugboard Drum memory (1932) Magnetic-core memory (1949) Plated-wire memory (1957) Core rope memory (1960s) Thin-film memory (1962) Disk pack (1962) Twistor memory (~1968) Bubble memory (~1970) Floppy disk (1971)
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Writing

The main memory element of the Mellon device consisted of a very large (television sized) square 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.

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 to the cell was accomplished by an external cathode ray tube (CRT) arranged in front of the photoemissive side of the grid. Cells were activated 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.

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.

Mellon optical memory

Writing

Reading

Refreshing

References

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