July 1944 QST Article
iconoscope was an early form of television image capturing tube. Some
amateur radio operators were experimenting with slow scan TV even back
when the technology was relatively new to the world. When this article
was written in 1944, there were still large portions of the United States
that did not have television broadcast coverage. Of course I would argue
that at the time of my growing up in the 1960s and early 1970s a lot
of areas - even suburbs - were still not covered by TV signals, based
on how cruddy the reception at my parents' house was. But I digress.
The article mentions that because of the lack of TV coverage, many amateurs
did not even have television receivers (TV sets) in their homes to use
along with experimental television transmitters. With ubiquitous communications
we experience now in the early 21st century, it takes reviewing some
of these old articles to really get a feel for what it was like back
in the days when even finding a public pay phone was considered a great
July 1944 QST
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights (if any) are hereby acknowledged.
See all available
vintage QST articles.
A Brief Resume of Its Principles of Operation
By B. W.
amateurs have the urge to experiment with new developments in the field
of radio. There are those who will wish to explore the possibilities
in the use of television on the amateur bands after the war is over.
Since there are large parts of the country not covered by commercial
television signals, it will often be necessary for the amateur experimenter
to build his own television transmitter as well as a picture receiver.
QST has published articles on the construction of a video camera and
transmitter using a Type 1847 pickup tube for operation in the 112-Mc.
In this article an attempt will be made to give
the amateur experimenter a clear understanding of what happens within
the iconoscope tube. The word "iconoscope" comes from a combination
of two Greek words - eikon, meaning "image," and skopein, meaning "to
observe." Various types of iconoscope tubes have been manufactured.
Fig. 1 shows a sketch of a typical tube of this type.
The essential element in the evacuated
tube is the mosaic. The base of the mosaic is a flat mica plate which
is used because of its high electrical insulation, good surface and
its uniform thickness. The thickness of the mosaic plate is on the order
of about 1 mil (0.001 inch). One side of the plate is coated with a
thin, finely sifted coating of silver-oxide powder. After the mica has
been coated it is baked in an oven, which reduces the silver oxide to
pure silver. The silver congeals in the form of extremely minute globules
less than 0.001 inch in diameter. Each globule is separated and insulated
from its neighbors by the mica.
The silver globules are then
made photosensitive by the admission of cesium vapor to the tube and
by passing a glow discharge through the tube in an atmosphere of oxygen.
Before it is placed in the tube, the reverse side of the mosaic
is coated with a thin signal coat of colloidal graphite. This coating
serves as the electrode through which the signal is transferred to the
external circuits during the process of scanning. Silver plating sometimes
replaces the colloidal graphite as the signal coat.
is mounted in the iconoscope in such a position that the electron beam
strikes the photosensitized side at an angle of 30 degrees from the
normal, and the optical image to be transmitted is projected normal
to the surface on the same side. The scene to be transmitted is focused
through an optical lens onto the mosaic, as if the latter were the film
of an ordinary photographic camera.
The mosaic may be thought of as a great number of minute photocells,
each of which is coupled by an electrical condenser to a common signal
lead, as shown in Fig. 1. When the mosaic is illuminated these condensers
are positively charged, as a result of the, emission of photoelectrons
from its surface. The fundamental action of photoelectricity is in this
way performed, and the optical image is thus translated into an electrical
Fig. 1.- Sketch showing the basic construction of the iconoscope
and static fields between the elements of how an image of the object
being viewed is focused on the mosaic plate. the gun, as shown in
Fig. 2 - Sketch showing the principal points of the "electron gun."
There now remains
the task of dissecting the electrical image obtained on the mosaic into
an orderly series of horizontal lines. This is accomplished by means
of an electron gun; which is also contained within the iconoscope tube.
The electron gun produces a very narrow stream of cathode rays which
serve as a commutator for the tiny photocells on the mosaic. The gun
may be thought of as an electron projector which concentrates the electrons
emitted from the cathode of the gun in a very small spot on the mosaic.
The electron optical system consists of two electron lenses formed by
the cylindrically symmetrical electrostatic fields between the elements
of the gun, as shown in Fig. 2.
Details of the gun construction
are of considerable interest. The cathode is indirectly heated with
its emitting area at the tip of the cathode cylinder, which is mounted
with the emitting .area a few thousandths of an inch in front of an
aperture in the control grid. A long cylinder with three defining apertures,
whose axes coincide with that of the cathode and control grid, serves
to give the electrons their initial acceleration. This cylinder is known
as the first anode, or the accelerating anode. A second cylinder, of
somewhat greater diameter than the first add mounted along the same
axis, serves as a second anode which gives the electrons their final
velocity. The second anode generally is formed by applying a metal coating
to the neck of the iconoscope bulb.
The electron beam is aimed initially at the extreme
upper left-hand corner of the image and is then moved horizontally,
from left to right, across the upper edge of the picture, to trace out
the first scanning line. As it passes over each silver globule of this
line the beam contributes electrons to each globule in succession, thereby
cancelling the positive charge created by illumination and restoring
for an instant the charge to the value it possessed before illumination
- the equilibrium charge. This change in charge results in the generation
of a minute voltage across the small capacity between the globule and
the signal plate. This voltage is then transferred to the signal terminals
and amplified to the necessary degree for modulation. As each charge
is restored the image plate potential changes, resulting in the potential
of the plate assuming a rapid succession of different values, each value
depending upon the amount of charge restored at that particular instant.
The deflection of the electron beam for scanning the mosaic is accomplished
by means of deflection coils arranged in the form of a yoke which slips
over the neck of the iconoscope.
As the electron beam completes
its motion across the first scanning line, it is blanked out and instantaneously
returned to the left-hand edge of the picture. During the scanning and
return motions the beam is moved vertically downward at a comparatively
slow rate, so that its position is somewhat below the initial starting
position of the previous line. The beam then traces out a new scanning
line across the mosaic, parallel to the preceding one but separated
from it by the width of one line. The beam therefore scans the mosaic
in a succession of alternate. lines. The empty space between lines is
later filled in by a second interlacing field.
When the beam reaches the bottom of the mosaic,
the slow vertical motion is stopped. The beam is then extinguished and
returned while in that state to the top of the picture. Here the beam
again begins its scanning motion, but this time it is positioned to
scan the spaces between the lines previously scanned, thus filling in
the gaps in an interlacing fashion. When the beam again reaches the
bottom of the picture it has covered every point on the mosaic in two
series of alternate lines.
The picture mosaic is scanned at
the rate of thirty complete pictures per second. There are various methods
of scanning, but the interlaced method just described has been adopted
as standard in the United States.
A picture element has a height
equal to the distance between centers of adjacent scanning lines. The
number of picture elements depends upon the number of lines by which
a complete picture is scanned. The greater the number of lines, the
greater the number of picture elements, and hence the higher degree
of definition obtainable.
In the Type 1847 iconoscope the inner
signal electrode (the conductive film on the mosaic) is a band of conductive
material on the inner surface of the tube. Another band of conductive
material is placed on the external surface of the tube, directly over
the internal band. The capacitance between the two bands, in series
with the capacitance between the signal electrode and mosaic, provides
the coupling between the signal-electrode terminal and the mosaic.
Storage vs. Non-Storage Types
Image pick-up tubes may be divided into two groups; namely, storage
pickups and non-storage pick-ups. In the storage type, which is the
one described in this article, the photoelectric current from an element
of the picture charges an individual condenser for a period of time
equal to the scanning time of one complete picture. This condenser is
discharged once during the scanning time of a complete picture, the
time of discharge being only the time of scanning of one picture element.
In the non-storage pickup the current from the photoelectric cell flows
only during the time of scanning, does not charge a condenser, and therefore
no storage of the charge caused by the photoelectric effect takes place.
Widespread use of television
promises to be one of the earlier postwar developments. The experimentally
inclined ham therefore should have more than ordinary interest in this
explanatory discussion of the "eye" of the television transmitter -