Image Orthicon


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image orthicon

[′im·ij ′ȯr·thə‚kän]
(electronics)
A television camera tube in which an electron image is produced by a photoemitting surface and focused on one side of a separate storage tube that is scanned on its opposite side by a beam of low-velocity electrons; electrons that are reflected from the storage tube, after positive stored charges are neutralized by the scanning beam, form a return beam which is amplified by an electron multiplier.

Image Orthicon

 

(in Russian, superortikon), a storage-type television camera tube with image transfer from a photocathode to a two-sided target, scanning (image readout from the target) by slow electrons, and signal amplification by means of a secondary electron multiplier. Image orthicons are among the most common camera tubes currently in use by the television industry (mid-1970’s). The image orthicon was developed by the American scientists A. Rose, P. Weimer, and H. Law in 1946. Its distinctive component is a two-sided target whose functional elements are a semiconducting membrane and a fine wire mesh screen. A similar target was proposed by the Soviet scientist G. V. Braude in 1939.

When the optical image of an object is projected onto the photocathode of an image orthicon (see Figure 1), light quanta cause the emission of photoelectrons from the cathode. The electrons are aimed by an accelerating field toward the target membrane, where they knock secondary electrons from the surface, forming a positive charge pattern that reproduces the light distribution at the photocathode. A scanning beam emitted from the opposite side of the target membrane by an electron gun passes in turn over each section of the membrane. Some of the electrons—a proportion that varies with the charge on a given area of the membrane—settle on the target; the rest are returned as a beam modulated by the charge pattern. When the return beam reaches the anode of the electron gun, it knocks out secondary electrons, which subsequently strike the electron multiplier. At the collector of the multiplier, the modulated current is 103–104 times larger than the return beam current, depending on the number of amplification stages. The output signal of an image orthicon—the video signal—is drawn off across a load resistor connected to the collector lead. The magnitude of the signal is determined by the pattern of the “electron image” on the target and, therefore, by the illumination of various sections of the photocathode—that is, by the brightness of the features of the televised subject.

Figure 1. Image orthicon: (1) object to be televised, (2) lens, (3) photocathode, (4) accelerator grid, (5) (low of photoelectrons, (6) target mesh, (7) target membrane, (8) decelerator grid, (9) focusing electrode, (10) focusing coil, (11) scanning electron beam, (12) return beam, (13) deflection coil, (14) barrel of electron multiplier, (15) alignment coil, (16) anode of electron gun (first dynode of electron multiplier), (17) dynodes of electron multiplier, (18) alignment electrode of gun, (19) hot cathode of gun, (20) collector of electron multiplier. Electron paths are indicated by arrows.

The image orthicon, most sensitive of the commonly used television camera tubes, operates reliably under a wide range of illumination levels. Some image orthicons are designed for highquality transmission from television studios and operate at signal-to-noise ratios of 100 and more when the photocathode is illuminated at a level of 0.1–1.0 lux. Other image orthicons are even more sensitive and can function in almost total darkness—that is, at a photocathode illumination level of 10–7–10–8 lux.

REFERENCES

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N. D. GALINSKII