The Hodges biography of Turing has lots of in-passing mention of fascinating technology like this. I think my favourite was the use of a CRT as a memory array (by picking up the charge on the fluorescent screen and feeding it back to the electron gun to refresh it!) which suggested to Turing the idea of using light to stimulate the feedback cycle and thus writing directly to memory with a very real "light pen"!

I'm probably borking up the details there but my point that the biog is great stands.

I guess there are readers of HN who never encountered the later "light pens." A photodiode picks up the raster on a CRT based monitor and with appropriate timing logic uses this to decide where to draw pixels. I had a cheap light pen on an 80s microcomputer before I ever got my hands on a mouse.

Ah, happy (but often frustrating) days...