Electronic Television

(Questions and Answers)

What is television?

Television is vision obtained of a distant object by means of various devices identified as the transmitting and receiving apparatus. The problem of television broadly is that of: 1, converting light signals into electrical signal^; 2, transmitting the electrical signals to a distant station; 3, converting the transmitted electrical signals back into light signals.

How is light converted into electrical energy?

By means of various light sensitive tubes generally known as photo-electric tubes or cells* The. cathode or light sensitive surface of such a tube consists of a certain amount of a light sensitive element such as rubidium, lithium, potassium, sodium or caesium.

When the cathode is illuminated, photo electrons or current is emitted—the emission varies in degree with the amount of light on the photo-sensitive surface of the tube. Figs. 1 and 2 shows two types of photo electric generators for converting light into electrical energy.

*For a detailed treatment on the various kinds of photo-electric tubes, and their practical employment in industrial applications, the reader is advised to study our book on Electronic Devices and Their Application.

When was the first television system constructed?

In 1875 it was first proposed to imitate the human eye by a mosaic consisting of a large number of selenium photocells arranged as shown in fig. 3. The selenium cells constitute the transmitter (pick-up camera) and a group of lamps each one

PHOTO-ELECTRIC CELL

LIGHT SOURCE

PHOTO-ELECTRIC CELL

LIGHT SOURCE

DEFLECTION DUE TO LIGHT

LIGHT-DEFLECTION

DEFLECTION DUE TO LIGHT

SENSITIVE CURRENT INSTRUMENT

NO LIGHT —NO DEFLECTION

FIG. 1—Illustrating how variations in light on the photo-electric cell causes deflection of the needle on a galvanometer or other sensitive current measuring device.

GALVANOMETER

GALVANOMETER

FIG. 2—To increase the amount of deflection (efficiency of light conversion) various methods of current amplification similar to that associated with current amplification in radio circuits is employed.

connected through an amplifier to its similarly positioned electro-magnet which opens a shutter connecting a light, which makes up the receiver (distant) end!

When the light-image to be transmitted is focused on the mosaic of photocells, an electric current will flow through the

FIG. 3—Diagram of an early television system. At the transmitting end light is converted into electrical energy which when amplified will energize a magnetic shutter or similar device at the receiving end causing a reproduction of the picture. Each photo-electric cell must be individually converted to its correspondingly remotely located light element. In the case under consideration therefore, approximately 80 wires will be necessary.

FIG. 3—Diagram of an early television system. At the transmitting end light is converted into electrical energy which when amplified will energize a magnetic shutter or similar device at the receiving end causing a reproduction of the picture. Each photo-electric cell must be individually converted to its correspondingly remotely located light element. In the case under consideration therefore, approximately 80 wires will be necessary.

circuits, conneeting those of the photocells with the lamps resulting in a reproduction of the subject, which will appear as an illuminated picture.

In a system of this kind, the amount of detail that can be transmitted is obviously limited by the actual dimensions of the individual photo-elements in the pick-up area.

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