Television Camera Tube

an electronic device that converts an optical image into a sequence of electric pulses, or video signal. The first (input) element of the television transmission chain, it picks up the image being transmitted. It is the principal component of the television camera.

The operation of all television camera tubes is based on the photoelectric effect. When the photoemissive effect is made use of, the converting light-sensitive unit is a photocathode, which emits electrons upon being struck by light. When the photoconductive effect is utilized, the converting unit is a photosensitive target that changes its electric conductivity when light falls upon it. The electric charge image on the light-sensitive unit is usually “read” by an electron beam that scans one after another all sections of the unit’s surface. The scanning sequence is such that the image is broken up according to the accepted television standard into several hundred lines, which form a raster. Each line can be regarded as a succession of picture elements, or elemental areas of the image.

Television camera tubes are classified according to the method used in producing the video signal as either nonstorage or storage tubes. In nonstorage tubes, the magnitude of the electrical signal corresponding to a given picture element is proportional to the intensity of the light striking the elemental area of the light-sensitive unit at the moment the area is read. In storage tubes, the magnitude of the signal is proportional to the integral value of the light intensity in the time between successive scannings. During this time, the photoelectric effect causes a charging of the miniature capacitors formed by the separate elemental areas of the light-sensitive unit and the signal plate. The electron beam of the scanning system discharges these capacitors and thus causes the current of the video signal to flow through the circuit of the signal plate.

Television camera tubes of any kind must have several properties: (1) a sufficiently high sensitivity, which is defined as the light intensity sufficient to produce a video signal with a satisfactory (≥10:1) signal-to-noise ratio; (2) a certain spectral characteristic of the light-sensitive unit, particularly in the case of color television tubes; (3) the ability to transmit a sufficient number (~ 10) of brightness gradations, or halftones; (4) a high resolution, for example, 500–600 lines in broadcast television; (5) a short response, usually not exceeding the frame scanning period and low enough to permit the formation of images of moving objects without visible distortions; and (6) a certain type of relationship between the amplitude of the output signal and the light level of the object—the light-transfer characteristic. In addition, a television camera tube must satisfy such requirements as uniformity of background and absence of spurious signals.

Nonstorage television camera tubes generate only a low-magnitude photocurrent from each elemental area of the light-sensitive unit. They thus are not sensitive enough to produce a satisfactory video signal at acceptable levels of illumination of the light-sensitive unit. Sensitivity increases markedly if an electron multiplier is used in the camera tube; this effect is exemplified in the dissector.

The use of the charge-storage method should theoretically increase the sensitivity of a television camera tube by a factor of several hundred thousand—for example, by a factor of ~ 5 X 10⁵ for a 625-line television raster. The theoretical value, however, proved to be several tens of times greater than the sensitivity of the first storage tube—the iconoscope. The discrepancy was due mainly to the nonsaturation of the photocurrent and to the use of a beam of fast electrons for scanning the image. The electrons had an energy greater than 1 kiloelectron volt (keV) and caused substantial secondary emission. A satisfactory signal was obtained when there was an illumination of several tens of lux on the photocathode.

The orthicon provided a severalfold increase in sensitivity owing to a more complete collection (saturation) of the photo-current and to scanning by a beam of slow electrons with an energy <0.5 keV normal to the surface of the light-sensitive unit. This successor of the iconoscope could produce a satisfactory image at an illumination of ~ 10 lux.

A further increase in sensitivity was obtained by transferring the electron image in an accelerating electric field (with focusing by a longitudinal magnetic field) from the photocathode to a target at some distance from the photocathode. The target had a secondary emission coefficient > 1. In this arrangement, the charge stored on the target was greater than that on the photocathode, and a satisfactory signal could be obtained at a lower illumination of the photocathode. These results were implemented in the image iconoscope and the image orthicon. In addition, the signal in the image orthicon is amplified by electron multiplication, thus permitting a satisfactory signal to be obtained at an illumination of 10^–3–10^–4 lux on the photocathode.

A relatively high sensitivity is possessed by storage tubes with a target made of a semiconductor that changes its electric conductivity upon illumination. An example of such a tube is the vidicon, which yields a satisfactory signal at an illumination of several lux. The drawbacks of the vidicon include a long response and the dependence of the tube’s characteristics on temperature. The use of a semiconductor target with p-n junctions and having a high photosensitivity and a comparatively short response has permitted the development of additional types of television camera tubes: the Plumbicon and the silicon diode tube. In these tubes, a satisfactory signal is produced at a target illumination of the order of 1 lux. These tubes and the image orthicon are used for the transmission of both black-and-white and color images.