"An image is created on the CRT surface by varying the electron beam intensity for each pixel. The room in which the CRT is located is partially illuminated by the pixels. As a result, the light in the room becomes a measure for the electron beam current. In particular, there is a little invisible ultrafast flash each time the electron beam refreshes a bright pixel that is surrounded by dark pixels on its left and right.
"So if you measure the brightness of a wall in this room with a very fast photosensor, and feed the result in another monitor that receives the exact same synchronization signals for steering its electron beam, you get to see an image like this."
"Tempest" refers to the ability to evesdrop on the contents of a monitor by listening to the radio frequency noise emitted by its electronics.
Here's another fun one from a few years back: Tempest for Eliza:
"A program that uses your computer monitor to send out AM radio signals. You can then hear computer generated music in your radio. [...] the song will be on the radio at multiple frequencies, but the best transmission is at the frequency you gave on the commandline."