Editing Cathode-ray tube (section)

===Charactrons===
{{Main|Charactron}}
Some displays for early computers (those that needed to display more text than was practical using vectors, or that required high speed for photographic output) used Charactron CRTs. These incorporate a perforated metal character mask ([[stencil]]), which shapes a wide electron beam to form a character on the screen. The system selects a character on the mask using one set of deflection circuits, but that causes the extruded beam to be aimed off-axis, so a second set of deflection plates has to re-aim the beam so it is headed toward the center of the screen. A third set of plates places the character wherever required. The beam is unblanked (turned on) briefly to draw the character at that position. Graphics could be drawn by selecting the position on the mask corresponding to the code for a space (in practice, they were simply not drawn), which had a small round hole in the center; this effectively disabled the character mask, and the system reverted to regular vector behavior. Charactrons had exceptionally long necks, because of the need for three deflection systems.<ref>{{cite web |url=https://patents.google.com/patent/US2735956|title=Cathode ray apparatus|access-date=4 October 2009}}</ref><ref>{{cite web |url=https://patents.google.com/patent/US2824250|title=INPUT|access-date=4 October 2009}}</ref>