July 1944 QST
 Table
of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
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An 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 convenience.
The Iconoscope
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.
A Brief Resume of Its Principles of OperationBy
B. W. Southwell (W6OJW/2)
Most 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 pick-up tube for operation in the
112-Mc. band.
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.
Mosaic
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.
The mosaic 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.
Fig. 2 - Sketch showing the principal points of the "electron
gun."
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 image.
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 construc-tion 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.
Scanning
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.
Interlacing
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 obtain-able.
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
capaci-tance 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 dis-cussion of the "eye"
of the television transmitter - the iconoscope.
Posted June 14, 2019
(updated from original post
on 3/31/2011)
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