July 1944 QST Article
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.
July 1944 QST
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A Brief Resume of Its Principles of Operation
By 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 pickup 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.
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.
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.
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.
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 - the iconoscope.