Figure 8-9.--Structure of image orthicon.

frequencies and separate and demodulate the video

(photocathode) by the camera lens. Electrons are

and audio frequencies. They must be separated to use

released from the photocathode in proportion to the

the sync pulses to reproduce the picture on the screen

intensity of the light striking it.

and sound at the speaker. How well each job is done

The released photoelectrons are directed on

depends largely on the design and quality of the TV

parallel courses from the photocathode through the

receiver.

target screen, to the two-sided target. This is done by

When the signal in CCTV is sent from the camera

an electrostatic field between the target screen and the

to the viewing unit by cables, there are no antenna

photocathode. Electrons are released from the target

problems. However, where the control unit contains a

by the impacting photoelectrons. This leaves a pattern

small oscillator, which furnishes an amplitude

of positive charges on the front of the target (fig. 8-9).

modulated video signal, the viewing unit must have a

The released electrons are collected by the target

tunable receiver.

screen, which has a slightly positive charge relative to

the target.

CAMERA TUBES

The back of the target is scanned by a beam from

an electron gun in the base of the tube. The beam is

The objective of television camera tubes is to

slowed to a near zero velocity at the target by the

convert an image into a continuous electrical signal.

decelerating ring. This is to avoid producing unwanted

The image can be a live scene, a series of

motion-picture frames, or a still picture. The signal

secondary electrons on the back side of the target. If

from the tube consists of a succession of voltages

the point on the target the beam approaches has a

proportional to the light intensity of each individual

positive charge, it extracts enough electrons from the

element encountered in the scanning process.

beam to become neutral. The remaining electrons in

the beam travel back toward the aperture. The aperture

The two types of camera tubes we will discuss in

is a good emitter of secondary electrons, thus

this chapter are the image orthicon and the vidicon.

providing amplification of the return beam. The

secondary electrons are directed into the electron

Image Orthicon

multipliers, where the signal is amplified, to a value

great enough to be fed into a video amplifier. The

The image orthicon tube (fig. 8-9) is an

focus, alignment, and deflection coils, along with

ultrasensitive camera tube used in modem television

special grids, keep the various streams in all parts of

systems. This tube picks up a light image from the

object that is focused on the light-sensitive face

the tube under proper control.

Figure 8-9.--Structure of image orthicon.

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