In the mid 80's, several national news outlets showed people able to reproduce a computers monitors screen at least 100ft away. How were early computers compromised like this?
It was a timing attack — the electron gun in a CRT is instantaneously lighting only a single point, and the region lit is only that point plus a very short slither of the screen behind it where the phosphors are not yet fully dark again. But each of those is substantially less bright than the current position. You see a full 2d image through a mixture of persistence-of-vision and psychology.
So such an attack just samples the amount of light emanating from the monitor as a whole, e.g. through the amount falling on an operator's face or anything else in the room, at a very high rate and reconstructs the original scan from that. Some human intervention may be necessary as sync signals cannot reliably be detected — no electrons are fired during those periods so you could figure it out if the screen were otherwise entirely lit but anything like text on a black background spends a lot of time not generating light for reasons other than sync.
Modern monitors light the entire surface the whole time. So they're not susceptible to the same attack.