The RCA Selectron -- US Patent 2,442,985 Selectron Optical Readout
Jan A. Rajchman, Radio Corporation of America
Applied: 08 January, 1947
Granted: 08 June, 1948
The '985 patent covers the an all optical readout system for Selectrons. Reading the miniscule currents of the electrical output of a Selectron was non-trivial. The high voltage (and current) switching signals would easily overwhelm the output. The optical readout of the Selectron gave a way to easily detect the presence or absence of a bit. A photomultiplier tube would detect the presence or absence of light when a single bit was interrogated. Optical shielding is easy: steel and black paint.