Optical System

8. The optical system of a television camera includes a fixed lens that is used to focus the scene onto the front of the pickup device. Color cameras also have a system of prisms and mirrors that separate incoming light from a scene into the three primary colors: red, green, and blue. Each beam of light is then directed to its own pickup device. Almost any color can be reproduced by combining these colors in the appropriate proportions. Most inexpensive consumer video cameras use a filter that breaks light from an image into the three primary colors.

9. The pickup device takes light from a scene and translates it into electronic signals. The first pickup devices used in cameras were camera tubes. The first camera tube used in television was the iconoscope. Invented in the 1920s, it needed a great deal of light to produce a signal, so it was impractical to use in a low-light setting, such as an outdoor evening scene. The image-orthicon tube and the vidicon tube were invented in the 1940s and were a vast improvement on the iconoscope.

They needed only about as much light to record a scene as human eyes need to see. Instead of camera tubes, most modern cameras now use light-sensitive integrated circuits (tiny, electronic devices) called charge-coupled devices (CCDs).

10. When recording television images, the pickup device replaces the function of film used in making movies. In a camera tube pickup device, the front of the tube contains a layer of photosensitive material called a target. In the image-orthicon tube, the target material is photoemissive - that is, it emits electrons when it is struck by light. In the vidicon camera tube, the target material is photoconductive - that is, it conducts electricity when it is struck by light. In both cases, the lens of a camera focuses light from a scene onto the front of the camera tube, and this light causes changes in the target material. The light image is transformed into an electronic image, which can then be read from the back of the target by a beam of electrons (tiny, negatively charged particles).

11. The beam of electrons is produced by an electron gun at the back of the camera tube. The beam is controlled by a system of electromagnets that make the beam systematically scan the target material. Whenever the electron beam hits the bright parts of the electronic image on the target material, the tube emits a high voltage, and when the beam hits a dark part of the image, the tube emits a low voltage. This varying voltage is the electronic television signal.

12. A charge-coupled device (CCD) can be much smaller than a camera tube and is much more durable. As a result, cameras with CCDs are more compact and portable than those using a camera tube. The image they create is less vulnerable to distortion and is therefore clearer. In a CCD, the light from a scene strikes an array of photodiodes arranged on a silicon chip. Photodiodes are devices that conduct electricity when they are struck by light; they send this electricity to tiny capacitors. The capacitors store the electrical charge, with the amount, of charge stored depending on the strength of the light that struck the photodiode. The CCD converts the incoming light from the scene into an electrical signal by releasing the charges from the photodiodes in an order that follows the scanning pattern that the receiver will follow in re­creating the image.

13. In color television, the signals from the three camera tubes or charge-coupled devices are firs; amplified, then sent to the encoder before leaving the camera. The encoder combines the three signals into a single electronic signal that contains the brightness information of the colors (luminance). It then adds another signal that contains the code used to combine the colors (color burst), and the synchronization information used to direct the television receiver to follow the same scanning pattern as the camera. The color television receiver uses the color burst part of the signal to separate the three colors again.

III. After-text Exercises

1. Give equivalents of the following words and expressions:

електромагнітне випромінювання; широко розповсюджений;

дистанційне керування; обробляти; той, що передається безпосередньо в ефір; монтувати, збирати; слідкуючий пристрій; відповідне співвідношення; відеошукач; світлочутлива інтегральна схема; пристрій із зарядовим зв'язком.

2. Say whether the following statements are true or false:

a) In a typical television station, the signals from live and recorded features, including commercials, are put separately in a master control room to provide the station's; continuous broadcast schedule.

b) The waves excite weak electric currents in all television-receiving antennas within range.

c) When recording television images, the pickup device doesn't replace the function of film used in making movies.

d) The beam of electrons is produced by an electron gun at the back of the camera tube.

e) A charge-coupled device (CCD) can be much bigger than a camera tube and much more durable.

3. Answer the following questions on paragraphs 9-12:

а) What was the first camera tube used in television?

b) What devices are called charge-coupled devices (CCDs)?

c) Where are the target materials photoemissive and photoconductive?

d) How is the beam of electrons produced and controlled?

e) What arc the main characteristics of a charge-coupled device (CCD)?

4. Translate paragraphs7,8,13 in writing.