Beam indexing offered much brighter pictures than shadow-mask CRTs, reducing power consumption, and as they used a single electron gun rather than three, they were easier to build and required no alignment adjustments.
In conventional black-and-white (B&W) televisions, the CRT screen has a uniform coating of phosphor that emits white light when struck by electrons.
The beam from an electron gun at the back of the tube is deflected (most commonly) by the varying fields from magnetic coils so it may be directed at any point on the screen.
Electronic circuits known as time base generators pull the beam across the tube and down, creating the scanning pattern used in television signals.
In order to produce reasonable resolution similar to that of a black-and-white set, the phosphors have to be deposited in very small dots or stripes.
A number of solutions were attempted that used a single electron gun and some sort of electrical or magnetic field very close to the screen to provide the same result as the shadow mask.
Ernest Lawrence developed a similar system known as Chromatron, which used a grid of fine wires behind the screen to electrically deflect the beam, but it suffered from the same basic problem as RCA's approach.
[1] Several different arrangements of components, materials and electronics were used while researching this approach during a ten-year development period, during most of which time it was kept secret.
When the indexing beam hit the magnesium oxide, a shower of electrons was given off, which were collected by a conductive coating of carbon deposited on the inside of the tube.
In order to distinguish between them, the pilot beam was modulated with a varying signal timed so that it was at maximum power only when it would be in the approximate location of the index stripes.
[2] In order to ensure the positioning of the pilot and writing beams remained as constant as possible, the apple tube used a unique electron gun arrangement.
They were then magnetically focused so they crossed at a point just in front of the electron guns, where a single-slit aperture was used to clear up the signal to produce a sharp-edged elliptical beam pattern.
Their version used a new material that gave off ultraviolet light in place of the x-rays and replaced the scintillators with a single photomultiplier tube.
[5] Development of the system was also picked up by Sylvania and Thorn Electrical Industries in the UK, who published details on what they called the "Zebra tube" in 1961.
[6] They were apparently successful in their work, but as no color television standard effort was making headway in Britain at the time, no commercial versions were forthcoming from this development either.
[9] Sunstein produced a prototype Uniray system using an original Philco tube and new electronics, and started shopping the concept around in 1972.
[11] Ferranti in the UK also offered a 4-by-3-inch (102 by 76 mm) beam-index tube as the mapping display in the Panavia Tornado mid-life upgrade.
The optically indexed tube displayed images by lighting vertical stripes of colored phosphor arranged in a red-green-blue pattern.
A single electron gun was used to excite the stripes, and the beam strength is modulated in order to produce different colors.
Each RGB pattern was followed by a single stripe of UV phosphor on the inside face of the tube, where the light was not visible to the viewer.
This way even if the beam was advancing too fast or too slow, the index system would adjust the timing on the fly to ensure proper colors were produced.
In practice the wires were difficult to keep aligned with the phosphors, and gave off electrical noise that interfered with the radio receivers in a television application.
The other similar design is the Trinitron, which combined the vertical stripes of the beam-index and Chromatron tubes with a new single-gun three-beam cathode and an aperture grille instead of a shadow mask.