> a keyboard hack that flashes the caps lock key imperceptibly fast but brightly enough so that a remote watcher can see the encoded keystrokes on your curtains

> a keyboard hack that flashes the caps lock key imperceptibly fast but brightly enough so that a remote watcher can see the encoded keystrokes on your curtains or ceiling, it's in that same paper you linked, page 20. Now the real beautiful result is the one where Kuhn deconvolves the diffuse light output (not an image, the total instantaneous light output, as would be collected by a fast photomultiplier tube or a photodiode) of a CRT with the impulse response of the phosphor -- to obtain the original image! https://www.cl.cam.ac.uk/~mgk25/ieee02-optical.pdf https://www.cl.cam.ac.uk/~mgk25/ieee02-optical.pdf