A sectional view of an image orthicon is shown in the below figure. It has three main sections: image section, scanning section and electron gun-cum-multiplier section.
Image section:
• The inside of the glass face plate at the front is coated with a silver-antimony coating sensitized with caesium to serve as photocathode.
• Light from the scene to be televised is focussed on the photocathode surface by a lens system and the optical image thus formed results in the release of electrons from each point on the photocathode in proportion the incident light intensity.
• Photocathode surface is semi-transparent and the light rays penetrate it to reach its inner surface from where electron emission takes place.
• Since the number of electrons emitted at any point in the photocathode has a distribution corresponding to the brightness of the optical image, an electron image of the scene or picture gets formed on the target side of the photo-coating and extends towards it.
• Though the conversion efficiency of the photocathode is quite high, it cannot store charge being a conductor.
• For this reason, the electron image produced at the photocathode is made to move towards the target plate located at a short distance from it.
• The target plate is made of a very thin sheet of glass and can store the charge received by it.
• This is maintained at about 400 volts more positive with respect to the photocathode and the resultant electric field gives the desired acceleration and motion to the emitted electrons towards it.
• The electrons, while in motion, have a tendency to repel each other and this can result in distortion of the information now available as charge image.
• To prevent this divergence effect, an axial magnetic field generated in this region by the long focus is employed.
• This magnetic field imparts helical motion of increasing pitch and focuses the emitted electrons on the target into a well-defined electron image of the original optical image.
• The image side of the target has a very small deposit of caesium and thus has a high secondary emission ratio.
• Because of the high velocity attained by the electrons while in motion from photocathode to the target plate, secondary emission results as the electrons bombard the target surface.
• These secondary electrons are collected by a wire-mesh screen, which is located in front of the target on the image side and is maintained at a slightly higher potential with respect to the target.
• The wire-mesh screen has about 300 metres per cm2 with an open area of 50 to 75 percent so that the screen wires do not interfere with the electron image.
• The secondary electrons leave behind on the target plate surface a positive charge distribution corresponding to the light intensity distribution on the original photocathode.
• For storage action, this charge on the target plate should not spread laterally over its surface during the storage time since this would destroy the resolution of the image.
• To achieve this, the target is made out of extremely thin sheet of glass.
• The positive charge distribution builds up during the frame storage time (40ms) and thus enhances the sensitivity of the tube.
• The light from the scene being televised continuously falls on the photocathode and the resultant emitted electrons on reaching the target plate cause continuous secondary emission.
• This continuous release of electrons results in the building up of positive charge on the target plated.
• Because of the high secondary emission ratio, the intensity of the positive charge distribution is four to five times more as compared to the charge liberated by the photocathode.
• This increase in charge density relative to the charge liberated at the photocathode is known as image multiplication and contributes to the increased sensitivity of image orthicon.
Scanning section:
• The face of the target remote from the photocathode is scanned by an electron beam emanating from a triode electron gun, the potentials being so adjusted that the beam approaches the target with a substantially zero velocity and is therefore unable to produce unwanted secondary electrons.
• The beam is focused at the target by the magnetic field of the external focusing coil and the electrostatic field of the wall coating (grid no 4) and deflection is accomplished by transverse magnetic fields produced by external deflection coils.
• The beam is aligned with the focusing magnetic field by means of a small transverse magnetic field produced by external coils located at the gun end of the focusing coil.
• The target end of the wall coating (grid no 4) is closed by a fine mesh screen called the field correcting mesh.
• This is maintained at a potential a few volts positive with respect to the wall coating.
• In addition to the improving the landing characteristics of the beam at the target, the presence of the field correcting mesh reduces the intensity of the white edging typical of pictures produce by earlier versions of the image orthicon.
• The edge field at the end of the beam focusing electrode is controlled by adjusting the potential of the decelerator (grid no 5) which is situated between grid no 4 and the field mesh. This adjustment helps to improve the landing characteristics of the beam.
• As a precaution against light leakage which has been shown to cause spurious results, the gun end of the tube is coated with opaque enamel.
Multiplier section:
• The return beam comprising electrons which are not required for neutralising the charge on the target travels back along approximately the same path as the outgoing electron beam and is directed into a five stage electron multiplier where it is amplified to become the output video signal.
• In order to reduce the intensity of the image of the first dynode which will be super-imposed on the transmitted picture, the whole of the multiplier section assembly is mounted off-centre.
• The multiplier gives an overall multiplication from the five stages of between 500 and 2000. This is sufficiently high to bring the random noise of the electron beam well above that of the input stage of the camera head amplifier and is therefore the limiting noise factor in the use of the tube.
• The multiplier also permits the use of an external amplifier of lower gain.
• It will be appreciated that when the beam moves from a less positive portion of the target to a more positive portion, the signal output voltage across the load resistor changes in the positive direction. Hence for highlights in the scene, the grid of the first video amplifier valve swings in the positive direction.
and 4 others joined a min ago.
teamques10
★ 68k