Figure 5-32.-SEC camera tube target operation.
electron striking the target are emitted and stored
(fig. 5-32).
The photoelectrons possess sufficient energy to
penetrate the thin, metallic signal and support plate.
As they travel through the porous potassium-chloride
layer, many secondary electrons are emitted as the
beam strikes the interlined particles. These
secondary electrons either escape to the positive
collector screen or they travel through the spaces of
the porous layer to the positive collector plate. This
loss of electrons produces a positive charge on the
scanned side of the target. Several hundred secondary
electrons are emitted for each incident electron,
producing a substantial gain at the target.
The video signal is developed from the target by
the scanning beam discharging the positively charged
areas of the target in the same manner as in a vidicon
tube. This charging current, flowing out of the signal
plate connection, is then amplified by an external
amplifier.
Figure 5-33.-Color picture tube.
The SEC tube has applications in extremely
low-light, nighttime military TV systems where a
high internal amplification and a fast speed or
response to moving images are important.
Picture Tubes
A monochrome picture tube is a specialized form
of the cathode-ray tube. An electron gun in the tube
directs a beam of electrons toward a fluorescent
material on the screen. The screen glows when struck
by the electrons. Between the gun and the screen are
deflection coils that deflect the beam horizontally and
vertically to form a raster. The brightness of the
picture is controlled by varying the grid-bias voltage
with respect to the cathode voltage. This bias can be
changed by varying either the cathode voltage or the
grid voltage.
Color picture tubes operate on the same basic
principle as monochrome picture tubes. The
difference between the two systems is the types of
phosphors that coat the screen. The different types of
phosphors produce colors when struck with the
electron beam. Three basic or primary colors are used
in combination to produce all the other desired colors.
These primary colors are red, green, and blue. In a
three-gun color picture tube (fig. 5-33), there is a
separate gun for each of the color phosphors. The
tube’s screen consists of small, closely spaced
phosphor dots of red, green, and blue. The dots are
arranged so a red, green, and blue dot form a small
triangle. The shadow mask provides a centering hole
in the middle of the triangle of dots. The convergence
electrode causes the three separate electron beams to
meet and cross at the hole in the shadow mask.
Figure 5-34.-Color picture tube. A. Side view. B. Front view.
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