1938: Television

Descriptive Definition.

Television is that physical process whereby a specially constructed transmitter may have optical images and scenes thrown upon it and transformed into electrical impulses for transmission by wire or by radio. At a distant point these electrical impulses are taken by a receiver and retransformed into a faithful reproduction of the original images and scenes, and made visible to an observer. If sound effects also are transmitted, the more realistic becomes the whole process for the observer.

The modern telephone is an analogous procedure. The telephone is a device whereby a specially constructed transmitter may have audible sounds thrown upon it and transformed into electrical impulses for transmission by wire or by radio. At a distant point these electrical impulses are taken by a receiving device (receiver) and retransformed into a faithful reproduction of the original sounds and made audible to a listener. If visual effects could be added to the telephone's transmission of sound, the whole process would be more realistic.

Legacies of Science.

In order that the transmitter and the receiver for illuminated scenes may be better understood, it will be necessary to describe several discoveries and inventions whereby such transmissions are made possible.

It has been known for some time that when light falls upon layers of certain different kinds of metals, an electrical potential difference is set up between these layers of metal which varies with the quantity of light falling upon the bounding surfaces of the metals. Such a device is known as a photoelectric cell. This furnishes the basic principle for the transformation of the light reflected from objects and scenes into electrical impulses. By a process known as scanning, an object — for example, the face of an individual — has a pencil of light rapidly swept over its surface so that in a minute interval of time every little section of the surface has been touched in regular order by the delicate wand of light. Each successive area as illuminated reflects its light onto a photoelectric cell. Inasmuch as some sections of the surface will reflect less than others, there follows in rapid succession, as the scene is scanned, a series of electrical impulses from the photoelectric cell which are directly proportional to the quantity of light reflected from the section illuminated. These electrical impulses, into which light from the various areas of the illuminated object has been transformed, may now be sent into a radio amplifier and these stepped-up electrical impulses transmitted by wire or by radio as are other radio signals.

The Process.

At the receiving end, these electrical impulses must be transformed back into visible light. There are various forms of receivers for such electrical impulses, but the simplest to understand is the one based on the neon lamp. This form of light source consists of a glass tube from which all gas has been exhausted except a minute amount of neon gas. Into the ends of such a glass tube have been sealed metal wire leads. When a small potential difference is applied to the terminals of such a tube, a conduction of electrical charges occurs, and the neon gas emits its characteristic light, the amount of which will depend upon the potential difference applied to the terminals of the tube. Hence, if the varying electrical impulses which have been transmitted by wire or by radio from the transmitting set are applied to the neon tube, its illumination will fluctuate in just the same way as did the light falling originally on the transmitter. By this means a series of varying light and dark spots could be transmitted by electrical impulses and at the receiving set retransformed into a faithful reproduction of the original dark and light areas which were scanned in the transmitter.

But how can one receive a scene or the image of a complete object which has a large area of surface? To do this, the light of varying quantity which is reproduced at the receiving end must be fashioned into a pencil or wand of light similar to the one with which the original object was scanned. These two wands of light must now be synchronized so that they will move together in direction at all times and not get out of step with each other. If the sending pencil moves rapidly, so must the receiving pencil. Consequently, as the receiving pencil of light moves synchronously in speed and direction with the original scanning beam of light, this wand or brush of light at the receiving end will paint on a screen a series of light and dark spots which correspond precisely with the quantity of light reflected from the series of light and dark spots originally illuminated on the objects at the sending station.

The receiving pencil of light paints its successive spots so rapidly that, by the persistence of vision which belongs to one's eyes, we seem to see the whole image or scene at once, and yet its various parts have been laid down in an extremely rapid sequence.

Mechanics of Television.

These pencils of light are formed by drilling in a circular disk a spiral of holes which are separated by equal angles between the various radio vectors which connect the center of the disk and the holes. A broad beam of light falls on the upper part of the disk as it rotates. Each hole, as it moves in front of a definite opening, which serves as a frame to the picture and is situated between the disk and the source of illumination, will allow a pencil of light to pass through and is then swept across the object or scene in a horizontal direction, making a narrow, illuminated band across the object or scene. The next hole, being nearer the center of the disk, will, as it moves across the object, illuminate the same with a band which lies just below the one made by the preceding hole. Thus, for succeeding holes, the same thing will occur until every minute section of the entire surface of the object has been momentarily illuminated in rapid succession, one after the other.

At the receiving end is another disk exactly like the one just described, which is used in scanning the object at the transmitting station. Both are run on synchronous motors, so that the two disks are exactly in step. The light coming from the neon lamp goes through the holes in the disk as a pencil of light; and so, as each hole passes across the opening in the 'framing' of the picture, the pencil peculiar to each hole draws its own band of light across a screen in which each band corresponds precisely with the bands of illumination thrown on the object at the transmitting station. Due to persistence of vision, one sees the whole as a faithful reproduction of what was illuminated at the transmitter.

Other Systems.

Various systems have been devised besides the one just described. Instead of a pencil of light, one system employs a pencil of cathode rays as a sweep system, and the cathode particles excite a fluorescent screen to give the image of the distant illuminated object. Details will vary, but in most of these systems the scanning process seems to be a necessary one.

The year 1938 will stand out as a landmark in the history of the development of television because in that year it was decided actually to put television receiving sets on the market and in this way gradually to iron out some of the imperfections which still remain in the mechanisms for the electrical transmission of images of illuminated objects.