单选题
The Television Camera
The television camera is rather like the human eye. Both the eye and the camera have a lens, and both produce a picture on a screen. In each case the picture is made up of millions of spots of light.
Let us see how the eye works. When we look at an object, a person, a house, or whatever it may be, we do not see all the details of the object in one piece. We imagine that we do, but this is not the case. In fact, the eye builds up the picture for us in our brain, which controls our sight, in millions of separate parts, and although we do not realize it, all these details are seen separately.
This is what happens when we look at something. Beams of light of different degrees of intensity, reflected from all parts of the object, strike the lens of the eye. The lens then gathers together the spots of light from these beams and focuses them on to a light-sensitive plate, the retina, at the back of the eyeball. In this way an image of the object is produced on the retina in the form of a pattern of lights.
The retina contains millions of minute light-sensitive elements, each of which is separately connected to the brain by a tiny fiber in the optic nerve. These nerve fibers, working independently, pick out minute details from the image on the retina and turn the small spots of light into nerve impulses of different strengths. They then transmit these impulses to the brain. They do this all at the same time.
All the details of the image are fed to the brain, and, so we have taught our brain to add them together correctly. We see a clear picture of the object as a whole.
Television, which means vision at a distance, operates on a similar principle. A television picture is built up in thousands of separate parts.
Beams of light reflected from the subject being televised strike the lens of the television camera, which corresponds to the lens of the eye. The camera lens gathers together the spots of light from these beams and focuses an image of the subject on to a plate, the surface of which is coated with millions of photo-electric elements sensitive to light.
The spots of light forming the image on the plate cannot be transmitted as light. So they are temporarily converted by an electronic device into millions of electrical impulses; that is, into charges of electricity.'
These electrical impulses are then sent through space on a wireless wave to the homes of the viewers. They are picked up by the aerials and conveyed to the receivers the to television sets. There, they are finally converted back into the spots of light that make up the picture on the television screen.