I believe this is a well known method of spying on remote networks — look with high speed optical sensors through windows at the LEDs on network routers. Good routers will lowpass filter the LED signal.
On Apr 9, 2018, at 8:12 PM, Henry Baker <hbaker1@pipeline.com> wrote:
I'm surprised that no one used the CRT+light pen to take pictures of the computer operator in the 1950's. (See the previous thread about one-pixel cameras.)
The computer could display time-varying "stuff" on the screen and record the time-varying response from the single-pixel light sensor on the light pen simply sitting calmly on the desk. With enough time and enough resolution, one could conceivably take a picture of the (static) room, if not the operator him/herself.
--- Human eyes can't distinguish flickering much above 60Hz, but LED's can reliably transmit bits into the MBits/sec range (remember those IR ports on older PC's?). The human operator may not be able to tell what the computer's doing from the console blinking lights, but another computer looking at the computer console through a window -- even from quite far away -- might be able to figure it out.
At 12:46 PM 4/9/2018, Mike Beeler wrote:
Nowadays, even if the PC were easily available, instruction rate is way too fast for a CRT to keep up with, but perhaps a pseudo-random sampling would accomplish the same effect.
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