Editing Shadow mask (section)

===Rear-gun efforts===
A more practical system would use a single gun at the back of the tube, firing at a single multi-color screen on the front. Through the early 1950s, several major electronics companies started development of such systems.

One contender was [[General Electric]]'s [[Penetron]], which used three layers of phosphor painted on top of each other on the back of the screen. Color was selected by changing the energy of the electrons in the beam so that they penetrated to different depths within the phosphor layers. Actually hitting the correct layer proved almost impossible, and GE eventually gave up on the technology for television use, although it went on to see some use in the [[avionics]] world where the color gamut could be reduced, often to three colors, which the system was able to achieve.<ref>David Morton, [https://books.google.com/books?id=rABggQmp31MC&pg=PA87 "Electronics: The Life Story of a Technology"] {{Webarchive|url=https://web.archive.org/web/20220915133926/https://books.google.com/books?id=rABggQmp31MC&pg=PA87 |date=15 September 2022 }}, Johns Hopkins University Press, 2007, p. 87.</ref>

More common were attempts to use a secondary focussing arrangement just behind the screen to produce the required accuracy. [[Paramount Pictures]] worked long and hard on the [[Chromatron]], which used a set of wires behind the screen as a secondary "gun", further focusing the beam and steering it towards the correct color.<ref>[https://patents.google.com/patent/US2692532 U.S. Patent 2,692,532], "Cathode Ray Focusing Apparatus", Ernst O. Lawrence, University of California/Chromatic Television Laboratories (original Chromatron patent).</ref> [[Philco]]'s [[Beam-index tube|"Apple" tube]] used additional stripes of phosphor that released a burst of electrons when the electron beam swept across them, by timing the bursts it could adjust the passage of the beam and hit the correct colors.<ref>Richard Clapp et al., "A New Beam-Indexing Coor Television Display System", ''Proceedings of the IRE'', September 1956, p. 1108–1114.</ref>

It would be years before any of these systems made their way into production. GE had given up on the Penetron by the early 1960s. Sony tried the Chromatron in the 1960s, but gave up and developed the [[Trinitron]] instead. The Apple tube re-emerged in the 1970s and had some success with a variety of vendors. But it was RCA's success with the shadow mask that dampened most of these efforts. Until 1968, every color television sold used the RCA shadow mask concept,<ref name=g80/> in the spring of that year [[Sony]] introduced their first Trinitron sets.<ref>John Nathan, "Sony: The Private Life", Houghton Mifflin Harcourt, 2001, p. 48.</ref>