Why do CRTs have 3 electron guns?

Question

I have been wondering about this for a while:

Since the phosphor will remain excited for a certain period of time, I could imagine a single electron gun could target the red, green and blue phosphors sequentially instead having 3 parallel beams. This would also solve all the convergence problems.

As the industry went with 3 beams and the tubes are designed by people a lot more knowledgeable than me, they obviously have a good reason to use 3 beams and I would like to know where the flaw is in my thinking.

Answer 1

The first color TVs were built entirely from analog components. It would have been extremely difficult to sequence three colors through a single electron gun with the technology available at the time.

Also, the separate guns allow the separate excitation of the corresponding sets of phosphor dots through the shadow mask precisely BECAUSE they are in physically distinct locations. It is the distinct angle of arrival that makes sure that each electron beam excites only the color that it is supposed to.

Remember, the phosphor dots are MUCH smaller than the diameter of the electron beam when it reaches the screen. If you had a single electron gun and no shadow mask, the phosphor dots would have to be somewhat larger than the beam diameter in order to prevent "bleeding" among the colors, which would make them objectionably large ("grainy") when viewed.

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That said, there was at least one experimental design that did use a single gun and time-division multiplexing of the colors. It used vertical stripes of phosphor, with an extra inward-facing stripe included in each group. This inward facing stripe produced bursts of light that were caught by a photomultiplier built into the CRT, and these pulses were used to keep the color multiplexing circuit in sync with the actual beam position.

Needless to say, it never caught on.

Answer 2

A monochrome TV has only one gun that paints lines across the screen. A colour TV needs to paint three colours on the screen.

A classical TV signal has the three color channels mixed into a single signal and time multiplexed. This information is separated to generate the red, green, and blue intensity levels for the beam as it tracks across.

Unfortunately in order to keep the colours crisp you do not want the red information painting over the green and blue, and vica-versa.

In order to do that the inventors of colour television came up with a clever trick of having three guns fire at the screen at a slight angle. The beams then must pass through a screen of holes. The screen effectively creates a shadow everywhere except where the appropriate coloured phosphor is. That is, the red gun can only shine on red phosphor, green on green, and blue on blue.

Note the gun is not painting pixels. The beam is larger than the holes in the screen. In fact the TV has no idea how many pixels are on the screen.

Could that be done today with a single gun and high frequency control over a single very tightly focussed electron beam, possibly, but it would not be a simple matter. With no feedback of where the beam is actually hitting the phosphor you are extremely sensitive to temperature changes in the tube and the electronics and mechanical variations.

You have to remember at the time colour TV was invented vacuum tubes were still the norm and transistorized TV's were still a pipe-dream. In fact it is quite remarkable that they managed to make CRTs as good as they did.

Of course modern non CRT TVs do not work this way and are actually pixel driven.

Answer 3

Not all color televisions have 3 electron guns!

I could imagine a single electron gun could target the red, green and blue phosphors sequentially instead having 3 parallel beams. This would also solve all the convergence problems.

You're describing how Sony's Trinitron picture tube works. It uses only one electron gun!

Quote from Wikipedia page:

The Trinitron design incorporates two unique features: the single-gun three-cathode picture tube, and the vertically aligned aperture grille.

See this excellent video by Technology Connections for an explanation of the Trinitron tube.

Off topic: Saw a Trinitron TV once, bought one when I could afford it, never went back. Also my first PC monitor was a small Trinitron.

Answer 4

Writing 3 colors with 1 beam has been tried, this is called a "beam-index tube". Using position feedback information, a narrow electron beam can be made to scan over 1 phosphor stripe. Repeat 3 times for 3 colors.

https://en.wikipedia.org/wiki/Beam-index_tube

Advantages are:

• 3 times higher efficiency due to absence of shadow mask.

• Thinner gun (only 1 cathode), thinner neck, lower volume of magnetic field, more efficient deflection.
• Opportunities for flat faceflate and/or shallow cone.

Disavantages are:

• An electron beam fundamentally cannot be narrow at high current, due to electron repulsion.
• Where to get the indexing information from ? A current sensor in the anode at 30 kV ? A light sensor, using invisible light ?
• How to accurately steer the beam ? Magnetic deflection is relatively slow.
• You also need to steer the beam at near-black. Are you willing to give up a deep black for a stronger feedback signal ?
• Triple bandwidth of the video signal needed. Problematic for HDTV.

It was a failed attempt to extend the life cycle of CRTs when plasma's and LCDs were already on the horizon. A shadow mask with all its complications is simpler.

Think of this: the color filters on an LCD panel are the equivalent of a shadow mask, they also absorb 2/3 of the light. Solving this should be much easier than indexing a CRT, yet nobody seems to be doing it. The display industry is very inert. The cost of change is so high.

PS The Sony Trinitron gun has 3 cathodes in 3 guns, sharing a single large main lens. 3 in-line guns is not unique to Trinitron, but it allows a shadow mask "aperture grid" consisting only of vertical wires. For practical purposes it is just another shadow mask, with some + and -.

PPS You can also use 1 B/W display with a cyclic color filter outside of it, this gives you "field sequential color". Most DLP (by TI) beamers do this. It saves you the 2 extra imagers, and they are fast enough to handle it.

Answer 5

The shadow mask CRT, instead of using one electron gun, uses 3 different guns placed one by the side of the other to form a triangle or a "Delta" .Each pixel point on the screen is also made up of 3 types of phosphors to produce red, blue and green colors This plate has holes placed strategically, so that when the beams from the three electron guns are focused on a particular pixel, they get focused on particular color producing pixel only.

This displays are also called refresh line drawing displays, because the picture vanishes (typically in about 100 Milli seconds ) and the pictures have to be continuously refreshed so that the human persistence of vision makes them see as static pictures. They are costly on one hand and also tend to flicker when complex pictures are displayed

Answer 6

I find it amusing that your question states "This would also solve all the convergence problems." by removing the mechanism for color separation and convergence. The resolution of the color mask happens to be semi-orthogonal to the resolution of the TV picture (which strictly speaking is defined exactly only vertically since horizontally the beam changes along with the analog signal): one "dot" is fuzzily bounded and represented by several red, green, and blue phosphor areas. Color adjustment makes sure that guns, mask, and phosphors cooperate in a manner where only colored dots of the right kind light up.

Triniton replaces the hexagonal grid with colored stripes, reducing the necessary amount of black between colors: the "mask" consists of vertical wires. To stabilize them, there are two horizontal wires weaved across that appear as slightly dark lines across the screen.

Either way, focus of the beam is wide enough to make the various lines on the screen cover a reasonably contiguous area, and that is quite smaller than the size of the color dots or stripes. The difference is borne out by the color mask and can be calibrated independently of general picture geometry which is quite less precise.