The ingenuity of some people is beyond belief. Take a look at this article in the New Scientist.

I’ve heard of people spying on computer data by detecting the radio frequency emissions from the monitor and it is not real difficult for me to understand how that might be done.

But this one never occurred to me. It is possible to see what’s on someone’s computer monitor by just the light reflected off the wall or diffused through curtains, and this can even be done from a great distance via a telescope.

That this was possible never occurred to me before. The article did not elaborate on HOW this was done. Being familiar with how CRT displays work, after reading that was possible, it was immediately obvious to me how it was possible.

On a CRT display, an electron beam is swept from left to right and top to bottom. Thus the light hitting the wall, which appears to be a diffuse flickering glow, is actually rapidly modulating as the electron beams trace out the image on the monitors phosphors.

with a sensitive light detector at the focal point of a telescope, one need only determine the horizontal and vertical sweep rates, and this can be determined by the regular dimming during the fly back and vertical retrace periods, in order to reconstruct the image. The electron beam is swept from left to right, each successive line lower than the one before. When it gets all the way to the right, it rapidly “flys back”, and during this time it is turned off so it doesn’t make lines backwards across the display. That off period should be recognizable and usable as a sync period. A similar thing happens when the beam reaches the bottom of the screen and has to return to the top.

Essentially what you have is a three channel optical link. You need filters to separate red/green/blue, but for each channel the data is basically a serial optical link.