(Shift-click on each above link to allow side-by-side viewing of both text and images.)
Electrical readout of the Selectron was a "challenging" task at best.
The electrical output, indicating the logical state of a selected bit, was tiny compared to the electrical noise
coupled from the other electrodes in the tube. Optical output of the Selectron had no such
limiting issues -- capacitive coupling of the address and control signals did not stimulate the phosphor dots,
making optical readout a good choice. The major problem is that in storage mode all written bits are illuminated.
Readout of a single bit disables all but the desired location. This means that all the other potentially
lit phosphor dots, perhaps 255 of them, must decay in light output: Several milliseconds would be wasted
waiting for the rest of the bits to "go dark." By gating the Faraday cage and phosphor dots the
storage section would be decoupled from the readout section. All the dots would remain dark
during the storage period except only the selected dot to be read -- if set.