Image pickup tube

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An image pickup tube is an electron tube that converts an optical image into electrical signals . Image pick-up tubes can basically only generate signals for single-color images; color television requires upstream optics with color filters or a strip filter with corresponding electronic switching.

Image pick-up tubes have largely been replaced by CCD sensors .

function

An image pickup tube basically consists of

  • a photocathode onto which the image to be converted is projected optically,
  • from a scanning device which converts this charge image of the photocathode , which is equivalent to the exposure, into a time-discrete signal.

Types of image pickup tubes

The above-described conversion of the optical image into a charge image is common to all image pickup tubes. The differences can be found in the construction details.

Iconoscope

In this type, the charge image is generated by an arrangement of small capacitor cells made of light-sensitive material ( cesium ). This layer combines the photocathode and storage of the charge image. This charge image is scanned line by line using an electron beam . The different charging of the capacitors results in a different current during the charge reversal of the capacitor cells and thus the electrical signal.

Due to its mechanical design, the iconoscope is quite sensitive to the effects of secondary electron emission. The trickle iconoscope compensated for these interference effects by sprinkling the capacitor cell plate with separately generated electrons.

Scheme of a supericonoscope
Super iconoscope without deflection and focusing coils

Super iconoscope

The supericonoscope differs from the normal iconoscope in that the photocathode and storage layer are separated. The photocathode is arranged here in front of the capacitor cell layer , the charge image generated in this way is projected electronically onto the storage layer. This increases the sensitivity due to the possibility of intensifying (accelerating) the electron image by external fields .

Probe tube

The probe tube developed by Philo Farnsworth also generates a charge image on a photocathode. Using suitable electron optics, this charge image is guided past the entrance of a secondary electron multiplier completely line by line . The pre-amplified signal can be picked up there.

Orthicon

Like the iconoscope, the orthicon also has a capacitor arrangement made of light-sensitive material. However, this arrangement is semi-transparent to light and is used from both sides. The optical image is projected onto this arrangement from the front and thus generates the charge image. An electron beam scans this charge image from the rear side in the manner known from the iconoscope.

In contrast to the iconoscope, this electron beam is slowed down to very low speeds by suitable measures so that no secondary electrons are knocked out of the photocathode. After the capacitor cells have been scanned, this electron beam is deflected again and returns to an anode in the vicinity of the cathode. The signal can be picked up there.

Superorthikon

This image recording tube, known outside of the German-speaking world as Image Orthicon, separates the photocathode and storage layer, similar to the supericonoscope. In addition, a type of grid is arranged between the photocathode and the storage layer, just in front of the storage layer, which is intended to prevent the formation of secondary electrons.

A thin glass plate forming the rear side of the storage layer, the charge image is formed by electrostatic induction sampled by this plate away also by a slow and then reverse deflected electron beam. Instead of a normal anode, this returning, modulated electron beam is directed into a secondary electron multiplier.

This means that the Superorthicon has a sensitivity that is around four hundred times greater than that of the Ikonoskop and thus allows images to be taken even in cloudy weather or in normal lighting.

Vidicon and sub-types

Vidicon from Japanese production
"Comet tail" effect of a Saticon camera

The translucent photocathode in the Vidicon is made of semiconducting material, the resistance of which changes when exposed to light. If the scanning electron beam now strikes this semiconductor cathode, its current fluctuates due to the different conductivity of the more or less strongly exposed areas. In its classic design, introduced in 1951, the Vidicon works with a photocathode made of antimony trisulfide .

Further developments have made it possible to considerably improve the properties of this type of image pick-up tube. Introduced in 1962, the Plumbicon with a semiconductor layer made of lead oxide (PbO) brought a dramatic improvement in image quality, which made the further use of the large and expensive superorthicon unnecessary. The saticon , a Japanese development introduced in 1973 that uses a semiconductor layer made of selenium with additions of arsenic and tellurium, was also widely used for radio purposes. The Newvicon was widely used in amateur and surveillance cameras. It is characterized by its particularly high light sensitivity, but does not match the image quality of Plumbicon and Saticon.

More types

  • The isoscope , very common in the medical field,
  • the graph works with two electron beams, a scanning beam and an erasing beam, which coat a semiconducting layer on both sides,
  • the eriscope , a tube related to the iconoscope.

history

TV cameras with image pick-up tubes based on the iconoscope were first implemented in practice in 1926 .

Individual evidence

  1. See literature, p. 50, paragraph 2. This information comes from 1952 and is possibly out of date.
  2. ^ B. and W. Heimann: TV camera tubes - properties and applications. (PDF) In: Special print from “TV and Cinema Technology”. 1978, accessed on June 25, 2015 (German).

literature

  • Heinz Richter: television for everyone. Franckh'sche Verlagshandlung Stuttgart, 1952, pp. 40ff.

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