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January 1966 Radio-Electronics
 [Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
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In "How We See Color,"
Eric Leslie explores the mechanics behind the color television picture tube, a revolutionary
leap from black & white sets in 1966, driven by a trio of innovations: a three-electron-gun
assembly, a shadow mask with over 300,000 holes, and a phosphor screen dotted with
about a million red, green, and blue color points. Each gun, arranged in an equilateral
triangle at the tube's neck - blue atop, green lower left, red lower right - fires
a beam modulated by color and brightness signals, guided through the shadow mask’s
precise apertures to strike only its matching phosphor dots, ensuring accurate color
reproduction. The intricate convergence process, both static (via neck magnets)
and dynamic (via yoke circuitry), aligns the beams to produce pure white when overlapping
correctly or vivid hues when distinct, despite challenges like beam stray and mask
energy loss. Mr. Leslie underscores the engineering marvel of the shadow-mask
tube - bolstered by chemical advances like europium-enhanced red phosphors.
How We See Color - Cover Story
Read how a curved glass plate coated with tiny, precise dots
of three different kinds of phosphor gives you sharp, full-color pictures
By Eric Leslie
What makes color television? Obviously, the picture tube. Yes, special circuitry
is needed, too, but it's those color dots on the face plate that make the difference
between black-and-white and "living color."
Three things make a color tube different from an ordinary black-and-white picture
tube:
The gun assembly, with three electron guns instead of one.
The metal shadow mask, positioned just behind the screen, and containing over
300,000 holes, one hole for each trio of red, green and blue dots.
The three-color phosphor-dot viewing screen, composed of about a million red,
green and blue dots.
The beam from each of the three electron guns is modulated by one of the color
signals, as well as by a portion of the brightness signals (the brightness signal
is applied to the cathode of each gun, color to the grid) and sends electrons out
toward the phosphor screen. On their way they pass through the holes of the mask,
assuring that the beam of any of the three colors will fall on the dot of the same
color. These dots on the phosphor screen, coated onto the back of the face plate
of the tube, give us our final picture.
The real action isn't quite as simple as that. The beam can - and will - stray
from its appointed course unless prevented. Various structures inside the tube,
magnets outside it, some windings inside the yoke, and special circuits in the set
assure that each beam strikes dots of only its own color. But more of that later.
Fig. 1 - The three guns are arranged at the vertexes of an
equilateral triangle.
The three guns are mounted so that their bases form an equilateral triangle.
As seen from the rear, the blue gun is at the top, the green gun at lower left,
and the red gun at lower right (Fig. 1). The guns are not parallel. The front end
of each is tilted a little bit, about 1° toward the center.
The guns are alike. In each, the cylindrical control grid (No. 1 grid) is closest
to the cathode. As in black-and-white tubes, the control grid cylinder's end is
closed except for a small hole at the center for the electron beam.
This view of a color-tube gun assembly, though it shows only
two of the guns, shows all the principal parts.
Each gun has a separate cylindrical screen grid (grid No.2, see photo), the voltage
on that grid can be adjusted by the red, blue or green screen controls (depending
on which gun is being adjusted). The screen voltages may vary from 130 to 370. The
focusing electrodes (grid No.3) for all three guns are connected.
The last electrode is the accelerating anode, composed of three shorter cylinders.
The accelerating anode is operated at the regulator voltage, usually around 25,000
volts. The pole pieces through which the neck magnets on the outside of the tube
concentrate and converge the beams are also attached to the accelerating anode.
These magnets and how to position them are explained fully in our articles on converging
color sets.
Fig. 2 - With small permanent magnets on the neck of the
tube, beams can be shifted for perfect static convergence.
Fig. 3 - How the beams converge in a hole in the shadow mask,
then diverge just enough to strike the proper dots without overlap.
Why Convergence?
It is the convergence of these beams that makes the difference between a good
and a poor color (or black-and-white) picture. If all three beams fall with equal
intensity on adjoining dots we have white light. If they stray a little, the color
is marred, or color may appear on a black-and-white program.
Convergence comes in two kinds: static and dynamic. The beams are converged statically
(that is, at the center of the screen, with no deflection) by external magnets and
the structures in the tubes. Magnets on the neck make it possible to move each beam
axially (toward the center or the edge of the tube). Thus the green or the red beam
can be moved diagonally, as in Fig. 2, and the blue beam moves up and down.
So it should be possible to focus and aim the three beams so that all fall on
the same trio of dots - red, green and blue - on the face of the tube and produce
white light. But a look at the drawings shows that while any two of the beams can
theoretically be made to cross over each other, the third one's path might pass
the other two at some point other than where they converge. Therefore one of them
- the blue beam - has an additional magnet so it can be moved sideways as well as
up and down. (Fig. 2, again.) With this extra magnet (the blue lateral adjustment),
the blue beam can be converged exactly with the other two. The blue lateral adjustment
pole pieces are mounted near the center of the focusing electrode.
Now, with the help of the magnets, we can converge the beam so that, at the center
of the screen, we have exact convergence. However at the left or right end of the
scan, the distance from the electron guns to the face of the tube is a little longer
than at the center. Therefore, if nothing else were done, the beams would converge
before they reached the shadow mask, rather than in one of the holes in the mask,
and we would have impure color at the edges of the screen.
To take care of this, dynamic convergence circuits supply voltages to the yoke
which "pull" the beams to move the point of convergence farther from the gun as
the beam sweeps farther from the center of the tube. Thus a properly converged tube
shows colors faithfully out to the very edges. Dynamic convergence is more the job
of the yoke and the circuitry than of the tube. See the article "Convergence in
Basic English" on page 46 of this issue.
The Shadow Mask
The mask is the element that causes the red beam to strike only red dots, the
green beam to strike only green dots, etc. (after the beams have been properly converged).
As viewed from the tube axis, the center of each hole of the mask is exactly the
same distance from the center of each phosphor dot in the color trio just beyond
it. As seen from a gun, however, each hole is directly ahead of the dot that is
the target of that particular beam (Fig. 3). Thus when properly converged and lined
up, the red beam strikes red dots only, and the blue and green beams reach only
dots of their own color.
Drawings and explanations do not always make it clear that the electron beam
may not be confined to one hole in the mask. It may be large enough to cover several.
Thus a large number of electrons strike the mask between holes, and are drained
off as shadow-mask current. This is the basis for one of the criticisms leveled
against the shadow-mask tube: that the greater part of the electrical energy is
wasted in shadow-mask current rather than used to pro-duce color by striking the
screen.
The shadow-mask holes are smaller than the phosphor dots on the plate, thus making
it easier to focus a beam on a particular dot. A color screen has been hailed as
a triumph of engineering. Each set of color dots has to be laid down in perfect
relationship to the other two sets, and so transferred to the face plate of the
tube that the dots of one color do not contaminate those of another. (One of the
most ingenious tubes, the Tri-chromoscope, or Geer tube, was invented because of
the near impossibility of laying down the three separate colors on the face of the
tube.)
Chemical breakthroughs are improving phosphor quality and brightness. Until very
recently the green and blue phosphors were much brighter than the red. Therefore
the red control was usually run "wide open" and the green and blue adjusted for
proper color balance. Recent europium compounds have tremendously increased the
efficiency of the red phosphor, and therefore the brightness of the tube in general.
There have been numerous attempts to design better color display devices, ranging
from the Chromatron, which is finally coming into commercial use, through the Apples
and Bananas to tubes using X-rays, and even revolving tubes. But, from the amount
of money being invested by the larger companies in plants for manufacturing the
shadow-mask tube, it is apparent that the color TV industry does not expect it to
be replaced in the near future.
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