International projectionist (Jan 1943-Dec 1944)

TELEVISION TODAY IV. — The Iconoscope By JAMES FRANK, JR. THE SUCCESS or failure of a television system depends largely on the pick-up device which converts the light image into electrical signals, and the viewing arrangement transforming the electrical signals back into visible images. We have already mentioned that the RCA television system uses the Iconoscope for dissecting images. In this chapter we will explain the design and operation of the Iconoscope in its present form. Historically, the development of any form of television had to await a means of converting a light signal into a corresponding electrical impulse. This step became possible through the discovery of the photo-conductive properties of selenium in 1873. Within two years after this discovery, Carey proposed to make use of the properties of selenium in the solution of the problem of television. His suggestion was to construct a mosaic consisting of a great number of selenium cells, in a sense imitating the retina of the human eye. These cells were to be connected to shutters or lamps in corresponding positions on a viewing board (see Figure 7) . Although the suggestion was made in 1875, the device was not put into operation until 1906 when Rignoux and Fourhier used this arrangement to transmit simple patterns and letters. Their mosaic consisted of a checker-board of sixty-four selenium cells. Each cell was connected to a shutter on a viewing screen which was also made up of sixty-four elements in positions corresponding to those in the pick-up screen. When a picture was projected on the selenium cells the resistance of those illuminated decreased, allowing an electric current to flow which opened corresponding shutters on the viewing screen. A light behind these shutters made the reproduced picture visible. The idea of dividing the picture into elements, converting the illumination on each element into electric current and sending the signal from each over individual wires is practical for a small number of divisions or picture elements and for transmission over short distance, but is useless as a means of producing pictures of the standard required of television today. The next step was proposed by Nipkow in 1884. Instead of using individual wires connecting each picture element, he suggested sending the information from one element at a time over a single communication channel and then reassembling this information again at the viewing screen. This process was to be carried out at such a rate that the picture appeared continuous due to "persistence of vision." 1 Scanning Principle Is Sound The means proposed to accomplish this point-by-point transmission was the scanning disc. At the time of its invention the necessary technique of handling and amplifying small currents had not yet been developed so that it was a number of years before this scanning principle could be put to practicaluse. However, the principle was sound and the scanning principle has been the basis of all television systems since then. While this development represents a great step forward, it was only attained at considerable expense of available picture signal. The loss is due to the fact that each element only contributes to the picture a small fraction of the total time, whereas with the first system suggested each element operated continuously. To make this clear, consider again the simple sixty-four element mosaic used by Rignoux and Fournier. Each photoelectric element was connected to the viewing screen by a separate conductor and the picture to be transmitted projected continuously on all the elements, so that a signal current passed through every light sensitive element all the time. To reduce the scanning system to a comparable case, assume we have the same mosaic of sixty-four photosensitive elements, but that they are all connected to a common communication channel. The elements are covered with shutters (i. e. the scanning disc) which allow only the light from one element of the picture at a time to reach its corresponding photocell. These shutters are opened one at a time in rotation covering the entire picture twenty or thirty times a second. Thus, each light sensitive element is only operating for a fraction of the total time equal to one over the number of picture elements, in this case onesixty-fourth of the time. In order to regain this lost signal and yet retain the principle of scanning, the Iconoscope was developed. To illustrate the method of attack, consider again the sixty-four element array of photocells. Instead of scanning the elements with shutters, assume that each element is connected to the contact points of a switch which connects them in rotation to the main communication channel. Thus, the scanning is accomplished by means of a commutator switch. So far, we have gained nothing over the previous method of scanning, but now if a condenser is placed across each of the photocells in such a way that it accumulates or stores the entire electric charge released by the action of the light during the time the element is not connected to the communication channel, this charge can be used when the commutator switch again makes contact with this element. Therefore, photo-electric current is being released continuously by every element and the charge is stored in the condenser belonging to that element until it is needed at the end of a scanning cycle. The reduction of this principle to some practical form is obviously a difficult problem. The number of individual photocells and condensers for a television picture of suitable quality will be of the order of 250,000 units, and it is quite apparent that a screen composed of that many conventional photocells and condensers is out of the question. The Iconoscope consisting of an electron gun and photosensitive mosaic enclosed in a highly evacuated glass envelope is the solution to this problem (Figures 8 and 9). It not only applies 1 See p. 27. 0 VIEW TO 8E SENT IMAGE OF VIEW FALLS OM FRONT OF PHOTOCELLS footnote "Television Today." I. P., Nov. 1943, {ampT} BANK Of PHOTOCELLS each photocell is connected to its corresponoino lamp tnrou&h an amplifier ano seperate wires FIGURE 7. Elements of the Carey system. 20 INTERIVATIOXAL PROJECTIONIST