Image orthicon camera tube was developed in 1945 by Radio Corporation of America (RCA). It is a sensitive tube and handling a wide range of light values and contrast. This tube makes use of the high photoemissive sensitivity obtainable from photocathodes, image multiplication at the target caused by secondary emission and an electron multiplier. It has three main sections:
- Image section
- Scanning section
- Electron gun-cum-multiplier section.
Table of Contents
A photosensitive surface, called photocathode, acting as main transducer in image orthicon camera tube. A very large negative potential i.e. -600v is given to the photocathode. It consists of a coating of a silver-antimony-cesium compound on the inside of the glass faceplate of the tube. When the light from an object focused on the photocathode surface by a lens system, the optical image is formed, photoelectrons, in proportion to the amount of incident light impinging, are emitted. The energy of the electrons emitted from different points of the cathode depends on the incident light energy. The number of electrons emitted at any point in the photocathode has a distribution according to the optical image. Since the photocathode is a conductor so it cannot store the charge. Thus the image formed at photocathode should move towards the target plate which is positioned at a short distance from it.
The target plate is a very thin sheet of glass of low resistivity, having a thickness of about 4 microns, which can store the charge. Most of the photoelectrons pass through the screen and hit the target plate. This target plate is maintained at about 400 volts more positive with respect to the photocathode. The electrons have a tendency to repel each other and this can result in distortion in the information which is available as charge image. This effect can be minimized by using a ‘long focus coil’ which generates an axial magnetic field. Thus the emitted electrons are focused on the target plate into a well-defined electron image of the original optical image.
The photoelectrons are accelerated to several hundred electron volts, they liberate several secondary electrons from the target plate surface. A wire mesh screen is provided to collect these secondary electrons and it is kept at higher potential with respect to the target plate. The wire-mesh screen has about 300 meshes 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 device. To achieve this, the target is made out of an extremely thin sheet of glass. The positive charge distribution builds up during the frame storage time (40 ms) and thus enhances the sensitivity of the tube. It should be clearly understood, that 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 a positive charge on the target plate.
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 camera tube.
The other side of the plate is now scanned by a beam of low-velocity electrons generated by an electron gun. The electron gun produces an electron beam of electrons that are accelerated towards the target. Positive accelerating potentials of 80 to 330 volts are applied to grid 2, grid 3, and grid 4. These grids are connected internally to the metalized conductive coating on the inside wall of the tube. The electron beam is focused on the target plate by applying a magnetic field of the external focus coil and by the voltage supplied to grid 4. The beam is deflected on the plate is vertical and horizontal directions and enables the electron beam to scan the whole plate.
The deflection of an electron beam is achieved by the magnetic fields of vertical and horizontal deflecting coils and these coils are mounted on yoke outside the tube.
The target plate is close to zero potential and therefore electrons in the scanning beam can be made to stop their forward motion at its surface and then return towards the gun structure. The grid 4 voltage is adjusted to produce uniform deceleration of electrons for the entire target area. Some of the total electrons in the beam, some get deposited on the target plate, while the remaining stop at its surface and turn back to go towards the first electrode of the electron multiplier. Because of low resistivity across the two sides of the target, the deposited negative charge neutralizes the existing positive charge in less than a frame time. The target can again become charged as a result of the incident picture information, to be scanned during the successive frames.
The number of electrons leaving from the cathode of the gun is practically constant, and out of this, some get deposited and remaining electrons, which travel backward provide a signal current that varies in amplitude in accordance with the picture information. On the contrary for high light areas, on the picture, there is a maximum loss of electrons from the target plate, due to secondary emission, and this results in large deposits of electrons from the beam and this reduces the amplitude of the returning beam current. The resultant beam current that turns away from the target is thus, maximum for black areas and minimum for bright areas on the picture.
It may be mentioned at this stage that since the beam is of low-velocity type, being reduced to near zero velocity in the region of the target it is subjected to stray electric fields in its vicinity, which can cause defocusing and thus loss of resolution. Also in contact with the target, the electrons would normally glide along its surface tangentially for a short distance and the point of contact becomes ill-defined. The beam must strike the target at the right angle at all points of the target, for better resolution.
These difficulties are overcome in the image-orthicon by the combined action of the electrostatic field because of potential on grid 4, and the magnetic field of the long focusing coil. The interaction of two fields gives rise to cycloidal motion to the beam in the vicinity of the target, which then hits it at a right angle no matter which point is being scanned. This very much improves the resolving capability of the picture tube.
An electron multiplier is located within the pickup tube for amplifying the electron density variation in the returning beam. If any conventional amplifier used instead of electron multiplier, it may cause the problems of signal to noise ratio because the signal amplitude is very low.
The returning stream of electrons arrives at the gun close to the aperture from which
Therefore secondary electrons are attracted to the dynodes at progressively higher positive potentials. Five stages of multiplication are used, each multiplier stage provides a certain gain in the electron multiplier. This is known as signal multiplication. The multiplication so obtained maintains a high signal to noise ratio. The secondary electrons are finally collected by the anode, which is connected to the highest supply voltage of + 1500 volts in series with a load resistance RL. The anode current through RL has the same variations that are present in the return beam from the target and amplified by the electron multiplier.
Field Mesh Image Orthicon
The tube described above is a non-field mesh image orthicon.
In some designs, an additional pancake-shaped magnetic coil is provided in front of the faceplate. This is connected in series with the main focusing coil. The location of the coil results in a graded magnetic field such that the optically focused photocathode image is magnified by about 1.5 times. Thus the charge image produced on the target plate is bigger in size and this results in improved resolution and better overall performance. Such a camera tube is known as a field mesh Image Orthicon.
Light Transfer Characteristics and Applications
During the evolution of image orthicon tubes, two separate types were developed, one with a very close target-mesh spacing (less than 0.001 cm) and the other with somewhat wider spacing. The tube, with very close target mesh spacing, has a very high signal to noise ratio but this is obtained at the expense of sensitivity and contrast ratio. This is a worthwhile exchange where lighting conditions can be controlled and picture quality is of primary importance. This is generally used for live shows in the studios. The other type with wider target-mesh spacing has a high sensitivity and contrast ratio with a more desirable spectral response. This tube has wider application for outdoor or other remote pickups where a wide range of lighting conditions have to be accommodated.