The motion picture projectionist (Oct 1927-Sept 1928)

18 The Motion Picture Projectionist September, 1928 How a Rotary Brush of Light Paints Pictures Afar* By Professor C. M. Jansky IN the preceding article were described the general principles of impressing varying light intensities on an electromagnetic wave. The photoelectric cell is the agent or means by which this is accomplished. It remains yet to explain more in detail how the varying intensities of light can be impressed on the photoelectric cell. It is obvious that if the whole picture, or lantern slide used for illustration in the preceding article, be interposed between the light source and the cell, the result will be merely one impulse and not a series of impulses which are essential for picture transmission. If the whole picture were interposed, the photoelectric current would be equivalent to the average illumination and so long as the picture did not change, the resulting current would be of constant intensity. The process or method to be used in the production of fluctuating currents are in principle the same for the transmission of pictures as for television, but as the physiological properties of the eye do not enter in picture transmission the processes of transmission and reception are much simpler than for television, hence radio transmission of pictures will be explained first. Transmitting Process The picture to be transmitted is made translucent, much the same as the ordinary film negative. It is then wrapped on a glass cylinder which is rotated by a synchronous motor. A beam of light is focused on the film by a system of lenses and the light passing through the picture enters a photoelectric cell. The arrangement of these elements is shown in Fig. 1, where L is the lamp; D is a condensing lens; A is a diaphragm; S is a projection lens; C is the rotating cylinder on which the photograph is mounted, and P is the photoelectric cell. As the cylinder is rotated it is moved endwise by a screw. The point of light incident on the translucent photograph describes a spiral or thread of the same pitch as that of the lead screw. It is obvious that the intensity of light incident on the photoelectric cell will vary as the light and dark parts of the film pass across the pencil. The fluctuating light will cause a fluctuating current in the photoelectric cell which, when amplified, is then, used to modulate the carrier electromagnetic wave, explained in earlier articles. This process of scanning the picture is analogous to the making of a picture by one continuous stroke of the pen. Some years ago there was on the market a pen picture of William B. McKinley. The pen artist began with the tip of the nose and by a spiral and continuous movement produced a very good likeness of the President. The features were delineated by Figure 1 heavy and light lines. Perhaps some of the readers remember seeing such a picture. The pen, of course, takes the place of the pencil of light and the heavy and light ink lines correspond to the more intense and less intense pencil of light as it passes through the translucent photograph. Picture Made by One Stroke At the receiving end the picture is reproduced by a pencil of light whose variations in intensity are exact counterparts of those at the sending end. The modulated electromagnetic waves are received in exactly the same manner as those which are translated into music by the diaphragm of the loud speaker. The translating equipment is again different. In a broadcasting station the sound waves produce fluctuating electric currents which at the receiving station, after being Fig. 2. — Left: The original photo. Right: The radio photo "Electrical Workers' Journal. tial and indispensable element of this apparatus is the neon tube lamp, same in principle but different in form as the many neon tube lamps now used in advertising where the name or word is formed by a amplified, reproduce the sound through the agency of the diaphragm of the loud speaker. Likewise, in picture transmission varying light intensities, through the agency of the photoelectric cell, produce varying electric currents which at the receiving end are translated into varying light intensities. Manner of Reception The apparatus at the receiving end is simpler than at the transmitting end. It consists of a radio receiver, a rectifier, a neon gas lamp, a revolving cylinder housed in a light tight box, and a synchronous motor geared to the cylinder. The essenglass tube emitting a pink or reddish light. The lamp consists of a glass tube into the ends of which are sealed two terminals to which a source of voltage is connected. This voltage sends a small electric current through the tube between the electrodes, and this current produces the familiar reddish light. The characteristic of the neon lamp that makes it an essential element of picture transmission is its sensitiveness to voltage variations. The slightest variationof voltage across the terminals is immediately followed by a change in the intensity of light. The lamp is mounted in front of the lens which focusses its light on the sensitized film mounted on the cylinder in the camera. The signal or electromagnetic waves actuate the radio receiver. They are then carried to a transformer, rectifier and amplifier, and then to the neon lamp. Every fluctuation or change in the electromagnetic wave actuating the receiving set is accompanied by a change in the light emitted by the neon lamp and incident on the sensitized film. Swiftly Transmitted As the cylinder rotates, the fluctuating pencil of light traces a screw or helical path on the sensitized film. As the chemical action of light varies with its intensity, it is obvious that the fluctuating pencil will produce more and less intense chemical action. When the film is developed the dark and light portions of the line traced by the pencil of light produce the picture in much the same manner as the penman mentioned above. A picture transmitted by radio is shown in Fig. 2. The fact that the cylinder at the receiving station must rotate in exact synchronism with the cylinder at the sending station is almost obvious. If it did not, the light and dark lines to form the nose, for example, would not be in proper juxtaposition and the developed picture would show a distorted nose. The process of transmitting pictures by radio has been improved to such an extent that a picture ¥/2 by 8 inches can be transmitted in one and one-half minutes.