The ICONOSCOPE

Wireless World, September ist, 1933 197 The ICONOSCOPE

cell of average sensitivity, say, ro micro- America's amperes per lumen, a single picture element would produce a current of the order of 1.43 x ro -11 ampere. In the time Latest available this would produce a charge of 1 x 10-'7 coulomb, corresponding to about Television 63 electrons. All this means that tre- mendous amplification is necessary if an The Iconoscope, consisting of cathode ray tube containing the mosaic screen, and outdoor scene is to be transmitted success- the external deflecting coils. Favourite fully. The trouble with the scanning disc system of taking the picture apart is that the N interesting step towards practical according to the flow of current, and the Ight from a single point of the scene television at a picture is thus translated into an electro- was announced affects the photocell for such a short space recent convention held in Chicago static screen made up of many thousand of time. If the photocell could see all of by the Institute of Radio Engi- electrical charges, each condenser being the picture all of the time, as is true of neers ; the ` ` Iconoscope of charged according to the intensity of the " Vladimir the human eye, the efficiency or light Zworykin relating use of a light falling upon its photocell. to the cathode gathering power compared to that of a ray tube in place of a disc. The Within the same glass bulb as the scanning disc scanner would increase as the num- Iconoscope is to be several thou- mosaic is the electron gun which throws a claimed ber of picture elements. Thus with 70,000 sand times as efficient as disc beam of electrons at this screen and which the scanning elements the Iconoscope, which accom- . even when, it is working far below its is made to sweep across the screen hori- plishes this seeing -all -at -once, is theoretic- theoretical maximum efficiency. It makes zontally and vertically by means of deflect- ally 70,000 times as efficient as the scan- it possible to or ing coils, as in an ray transmit twenty pictures, ordinary cathode ning disc and hence the amplification may more, per second, with a detail corre- tube. Whenever an electron hits a photo- be reduced to a vast degree. Actually the sponding to as as lines per cell it neutralises of charge on the many 25o part the efficiency of the Zworykin device does not square inch. The most feature associated condenser. This discharge cur- important reach this figure, but it is stated to be of the device, however, is its power of rent is picked up, amplified, and trans- several thousand times as imitating mitted to receiving good as the the human eye in regarding a the cathode ray beam scanning disc. picture continuously and in possessing a which is moving across a fluourescent certain degree of retentivity. It replaces screen in synchronism with the scanning mechanical scanning and several stages of beam. The varying discharge currents MOSAt C SIGNAL PLATE amplification ; it is entirely electrical and modulate this receiving beam and hence ELECTRON BEAM P has no moving parts. the screen at the receiver. The Iconoscope consists of two devices -a photoelectric mosaic on which the Increase in Light Values ,z,n, picture is focused by a lens system, and ; Á a cathode ray gun which fires at this The Iconoscope apparently solves one screen a stream of electron projectiles. of the worst problems of television, that The elements of' the device are shown in of getting sufficient light from each point the drawing of. Fig. 1. The signal plate so that the resulting photocurrent can be upon which the picture is focused may effectively amplified. for use at the re- ELECTRON GUN be about 4 x 5 inches in dimensions and ceiver. In his description Dr. Zworykin DEFLECTING COILS on its surface are millions of small photo- states that a desirable picture would in- cells, each consisting of a minute silver volve some 70,000 picture elements. It Fig. I.-Elements of the Iconoscope device. globule sensitised by caesium. These there are twenty pictures per second, the minute photocells like a mosaic are de- time of transmission of each element The mosaic of photocells has the entire posited on an insulating plate, such as a would be 1 /1,400,000 of a second. This picture focused upon it, resulting in the thin sheet of mica, and the back of this is unimportant provided that the light in- continuous emission of electrons accord- plate is made conductive by a metal coat- tensity is sufficient to actuate the photo- ing to the degree of light and shade in the ing. Since both the photocell and the cell in this small time. This is where the picture. once in each repetition of the metal coating are conductors, and they are difficulty lies, however, for the photocell picture the charge on the condenser ac- insulated from one another by mica, each delivers a current depending upon the pro- quired by each element of the mosaic is individual photocell forms a small con- duct of the light intensity and the time released by the cathode ray beam. denser with the metal plate. This capa- during which this light acts. If the time The fact that the transmitting screen is city amounts to several hundred micro- be decreased, the current is reduced. 4 x 5 inches in size is no indication that microfarads per square centimetre, and With a scanning disc the current falls as the receiving screen is limited to this size. the sensitivity of the photocells is equal to the square of the number of picture ele- In fact, Zworykin claims methods of pro- that of the modern caesium oxide type ments because the time decreases for any jecting the picture from the fluourescent cells. increase in the number of holes. It has end of the cathode ray tube upon a larger been virtually impossible, therefore, to external screen by lenses and mirrors. How the Image is Translated televise outdoor scenes because of the diffi- Dr. Zworykin's paper, when available culty of getting enough light. for publication, will contain many most Now, suppose a scene, say, of a well - Again following Dr. Zworykin's reason- interesting facts with reference to the lighted outdoor landscape is focused ing, suppose it be desired to televise this composition of the mosaic, the size of the upon this plate. Since there are varying outdoor scene. A camera with a lens cathode ray beam when it hits the screen, degrees of light and shade, there are vary- working at a speed of F 4.5 would focus the detail secured, etc. In the opinion of ing currents flowing from the individual on a plate about r / 10 lumen. Now, if a well-known American engineers it marks photocells. These currents charge the scanning disc were used to take this another landmark in the history of tele- condensers associated with the photocells picture apart and transmit it into a photo- vision.