Maybe worth noting in the context of HN: There are two types of electron emissions in a CRT. The primary emission, which produces the visible image, and the secondary emission, which causes an electron well around the excited spot, as free electrons take up positions of the electrons emitted by the primary emission. It's the latter that was utilized in the CRT memory Williams-Kilburn tubes (the first viable random access memory), which detected that local depletion of electrons in the phosphor in the read cycle.<p>Moreover, there were also dark-trace CRTs or Skiatrons (developed just before WWII and used for Radar during the war), producing a dark-on-light image, which were also based on the secondary emission, rather than the primary one. (Technically, this was based on the displacement of free electrons in an alkali-halide crystal lattice, which made the CRT locally opaque.)