​​كاميرات التلفزيون .. الالكترونيات

Television cameras .. Electronics

TV cameras

TV cameras provide us with the image signal that is amplified and then sent. Due to the difficulty of converting the entire scene into electrical signals at once and at the same time, the image is scanned or covered by regular scanning lines. After that, the frame scanning of the image is done in a sequential manner and at a high speed that prevents it from shaking.

The television image is created from a very small point of light resulting from the cathode ray moving back and forth on the screen at a very high speed. As for the television camera, it does the reverse process, that is, it first converts the image into electrical signals whose intensity varies according to the brightness of the image and the degree of lighting and shadows on it. As for the photography done by spacecraft, it is stored on recording tapes in preparation for sending it later.

Figure (279) shows how the process of scanning the television image or taking small samples from it and collecting them together to form the complete image during the scanning of a single frame (see the section on television receivers, p. 88).

The camera lens gives an image of the scene to be sent on a film prepared for this purpose (280). Lenses with a variable focal length (zoom) or lenses that can be exchanged between them are also used to obtain a wide view of the image or to bring it closer or further away or to photograph its different sides from several directions.

279 - Scanning the image.
280 - A diverging lens forming the image by.

The camera called an OrTecon projects the image onto a photocathode (281) from one side of which electrons are emitted when light hits the other side. The number of electrons emitted varies according to the amount of incident light, and is large in the bright parts of the image. In addition, an electric field attracts these electrons towards the target plate, which is a plate of thin glass covered with a fine metal grid containing a large number of holes in each square. The collision of the photoelectrons with the glass leads to the emission of secondary electrons that leave behind positive charges, so that the positions and intensity of these charges become corresponding to the illuminated parts of the image. The beam emitted by the electron gun scans the back of the glass plate. When it passes over a patch containing positive charges, it gives up some electrons that neutralize these charges. However, if it passes over an uncharged patch, it only gives up a small number of them, and thus the current becomes corresponding to the illuminated parts of the image. An electronic multiplier amplifies the changes in the beam current itself. This is done using a series of electrodes with voltages that vary in value in a gradual manner (282) and secondary electrons are emitted from them as soon as they collide with the electrons. Then the amplified signal passes through other amplifiers or dark ones.

281 - Image Orthicon in a TV camera.
282 - Electronic multiplier.