A television system consists primarily of two parts: picture transmission
and picture reception. A television camera used to photograph a
television program is similar in some ways to a still camera. Light bounces
off the subject being photographed and enters the lens at the front of the
television camera. The lens forms a clear image of the subject being photographed
on a screen, which is located behind the lens.
Transmission. The surface of the screen contains millions of tiny particles
of selenium or some other photosensitive (sensitive to light)
material. These particles act like tiny photocells. That is, when struck by
light, they emit a small electrical pulse. An electron gun at the back
of the television camera scans back and forth, up and down across the
screen at the front of the camera. As it scans, it detects electrical pulses
being given off by various parts of the screen. A bright region in the scene
being photographed will give off a lot of light. That light will be converted
by the selenium into a relatively large electrical pulse. The electron
gun will detect that electrical pulse as being greater than other pulses
around it.
The electrical pulses detected by the electron gun are then amplified
and sent to the broadcasting tower. In the broadcasting tower, the
electrical current from the television camera is converted into radio waves
and sent out through the air. The process is similar to the way in which
a radio program is transmitted except the frequency is different.
Television reception. At the receiving station, the above process is
repeated in reverse order. Radio signals are received, amplified, and then
fed into an electron gun in a television picture tube. The electron gun is
pointed at the back of a picture tube. It travels back and forth across the
picture tube tracing 525 lines on the tube 30 times every second. The back
of the tube is covered with a photosensitive material that gives off light
whenever it is struck by an electrical pulse. An intense beam from the
electron gun (corresponding to an intense beam originally seen by the
television camera) produces a strong burst of light. A weaker beam from
the electron gun produces a weaker burst of light.
What the electron gun in the picture tube is producing, then, is a series
of individual dots, one at a time, spread out across the screen at a
very rapid pace. This mass of dots appears as a coherent picture to the
human eye because of a phenomenon known as persistence. The term persistence
refers to the fact that a visual image projected onto the retina of
the human eye tends to remain there for a fraction of a second. Thus, what
our eye sees as the electron gun scans the picture tube is a collection of
millions of individual spots of light that, taken together, makes up a complete
picture. That picture is identical to the one photographed originally
by the television camera.
and picture reception. A television camera used to photograph a
television program is similar in some ways to a still camera. Light bounces
off the subject being photographed and enters the lens at the front of the
television camera. The lens forms a clear image of the subject being photographed
on a screen, which is located behind the lens.
Transmission. The surface of the screen contains millions of tiny particles
of selenium or some other photosensitive (sensitive to light)
material. These particles act like tiny photocells. That is, when struck by
light, they emit a small electrical pulse. An electron gun at the back
of the television camera scans back and forth, up and down across the
screen at the front of the camera. As it scans, it detects electrical pulses
being given off by various parts of the screen. A bright region in the scene
being photographed will give off a lot of light. That light will be converted
by the selenium into a relatively large electrical pulse. The electron
gun will detect that electrical pulse as being greater than other pulses
around it.
The electrical pulses detected by the electron gun are then amplified
and sent to the broadcasting tower. In the broadcasting tower, the
electrical current from the television camera is converted into radio waves
and sent out through the air. The process is similar to the way in which
a radio program is transmitted except the frequency is different.
Television reception. At the receiving station, the above process is
repeated in reverse order. Radio signals are received, amplified, and then
fed into an electron gun in a television picture tube. The electron gun is
pointed at the back of a picture tube. It travels back and forth across the
picture tube tracing 525 lines on the tube 30 times every second. The back
of the tube is covered with a photosensitive material that gives off light
whenever it is struck by an electrical pulse. An intense beam from the
electron gun (corresponding to an intense beam originally seen by the
television camera) produces a strong burst of light. A weaker beam from
the electron gun produces a weaker burst of light.
What the electron gun in the picture tube is producing, then, is a series
of individual dots, one at a time, spread out across the screen at a
very rapid pace. This mass of dots appears as a coherent picture to the
human eye because of a phenomenon known as persistence. The term persistence
refers to the fact that a visual image projected onto the retina of
the human eye tends to remain there for a fraction of a second. Thus, what
our eye sees as the electron gun scans the picture tube is a collection of
millions of individual spots of light that, taken together, makes up a complete
picture. That picture is identical to the one photographed originally
by the television camera.
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