Beam-index Tube - Description

Description

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 paint 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. Light given off by this stripe was captured by a photomultiplier tube on the outside of the tube that was positioned over a clear window in the tube surface. Light from the UV stripes was amplified and sent into the color decoder circuit.

The color decoder electrically subtracted the signal from the photomultiplier from the existing color burst signal. This resulted in a phase difference that advanced or retarded the modulation of the single beam. 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 order to ensure there was enough time for the electronics to make this adjustment, early models used a second beam that swept the screen slightly in advance of the first. Later sets did not need this adjustment and were able to correct by indexing off the preceding UV stripe. In order to receive a signal strong enough to index with, the beam had to be left on at all times, which reduced contrast ratio in relation to conventional tubes.

The beam-index tube bears some resemblance to two other types of television tubes which also used vertical stripes of colored phosphor instead of dots or grids. The Chromatron used two sets of fine wires suspended behind the display area to electrically focus its single beam, one set of wires pulling the beam towards the red side and the other towards the blue. The grids were aligned so the beam would normally focus onto the green stripe in the middle, but by varying the relative voltage between the two the beam could accurately hit the colored stripes. 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. It saw some use in military settings, but no commercial television use.

The other similar design is the Trinitron. The Trinitron combined the vertical stripes of the beam-index and Chromatron tubes with a new single-gun three-beam cathode and an aperture grille shadow mask. The result was a design with the mechanical simplicity of the shadow mask design and the bright images of the beam-index system. Trinitron was a major product for Sony for several decades, representing the high-point of conventional color TV displays until the widespread introduction of plasma displays and LCD televisions in the 21st century.