Basic principle of operation:

• The Vidicon operates on the principle of photoconductivity, where the resistance of the target material shows a marked decrease when exposed to light. While Image Orthicon based on photoemission from photocathodes, image multiplication at the target caused by secondary emission and an electron multiplier.
  • Vidicon is a short tube with a length of 12 to 20 cm and diameter between 1.5 and 4 cm. Its life is estimated to be between 5000 and 20,000 hours.
  • The target consists of a thin photo conductive layer of either selenium or anti-mony compounds. This is deposited on a transparent conducting film, coated on the inner surface of the face plate. This conductive coating is known as signal electrode or plate.
  • Image side of the photo layer, which is in contact with the signal electrode, is connected to DC supply through the load resistance. The beam that emerges from the electron gun is focused on surface of the photo conductive layer by combined action of uniform magnetic field of an external coil and electrostatic field of grid No 3.
  • Grid No. 4 provides a uniform decelerating field between itself, and the photo conductive layer, so that the electron beam approaches the layer with a low velocity to prevent any secondary emission.

Charge Image: - The photolayer has a thickness of about 0.0001 cm, and behaves like an insulator with a resistance of approximately 20 MΩ when in dark. - With light focused on it, the photon energy enables more electrons to go to the conduction band and this reduces its resistivity. When bright light falls on any area of the photoconductive coating, resistance across the thickness of that portion gets reduces to about 2 MΩ.

• Thus, with an image on the target, each point on the gun side of the photolayer assumes a certain potential with respect to the DC supply, depending on its resistance to the signal plate.

• An area with high illumination may attain a potential of about + 39 V on the beam side. Similarly dark areas, on account of high resistance of the photolayer may rise to only about + 35 volts.

• Thus, a pattern of positive potentials appears, on the gun side of the photo layer, producing a charge image that corresponds to the incident optical image.

Applications: Earlier types of vidicons were used only where there was no fast movement, because of inherent lag. These applications included slides, pictures, closed circuit TV etc. The present day improved vidicon finds wide applications in education, medicine, industry, aerospace and oceanography. It is, perhaps, the most popular tube in the television industry.

Advantages:

-Vidicon is a short tube with a length of 12 to 20 cm and diameter between 1.5 and 4 cm.

-Its life is estimated to be between 5000 and 20,000 hours.

-Light in weight hence portable

-S/N is better than Image Orthicon

-Spectral Response is close to that of the eyes

-It does not require Photomultipler

-Sensitivity can be adjusted as per the illumination of scences

Difference with respect to other Camera tubes: