IMAGE ORTHICON
It has three main sections: Image section, Scanning section and Electron gun-cum-multiplier section
Image Section Light from the scene to be focused on the photocathode surface by a lens system and the optical image thus formed results in the release of electrons. 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 (thickness close to 0.004 mm)
Image Section This is maintained at about 400 volts. Secondary electrons are collected by a wire-mesh screen, which is located in front of the target on the image side.
Scanning Section The electron gun structure produces a beam of electrons that is accelerated towards the target. This accelerating potentials of 80 to 330 volts are applied to grid 2, grid 3 and grid 4 . Deflection of electron beams to scan the entire target plate is accomplished by magnetic fields of vertical and horizontal deflecting coils. These coils are fed from two oscillators, one working at 15625 Hz, for horizontal deflection and the other operating at 50 Hz, for vertical deflection.
Electron Multiplier The multiplication is obtained to maintains a high signal to noise ratio. The secondary electrons are collected by the anode, which is connected to the highest supply voltage of + 1500 volts in series with a load resistance R L .
Electron Multiplier Voltage across R L is the desired video signal; the amplitude of which varies in accordance with light intensity variations of scene. With R L = 20 K-ohms and typical dark and high light currents of magnitudes 30 μA and 5 μA respectively, the camera output signal will have an amplitude of 500 mV peak-to-peak.
CHARGE PATTERN
Next… VIDICON…
Vidicon General use in the early 50’s and gained immediate popularity because of its small size and ease of operation. It functions on the principle of photoconductivity. Target plate consists of a thin photo conductive layer of either selenium or anti-mony compounds.
Vidicon camera tube cross-section
Vidicon 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. Charge Image The photo layer has a thickness of about 0.0001 cm, and behaves like an insulator with a resistance of approximately 20 MΩ when in dark. When bright light falls, the resistance across the thickness of that portion gets reduces to about 2 MΩ.
Circuit for output signal from a Vidicon camera tube
Leaky Capacitor Concept
Vidicon Storage Action Light from the scene falls continuously on the target, each element of the photo coating is scanned at intervals equal to the frame time. This results in storage action and the net change in resistance, at any point will be depends on the time, which elapses between two successive scanning and the intensity of incident light. Since storage time for all points on the target plate is same
Vidicon Storage Action Light from the scene falls continuously on the target, each element of the photo coating is scanned at intervals equal to the frame time. This results in storage action and the net change in resistance, at any point will be depends on the time, which elapses between two successive scanning and the intensity of incident light. Since storage time for all points on the target plate is same
Light Transfer Characteristics Each curve is for a specific value of ‘dark’ current, which is the output with no light. Typical output for the vidicon is 0.4 μA for bright light with a dark current of 0.02 μA A: 40-60v…. B: 25-40v…C:10-20v
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. It is, perhaps, the most popular tube in the television industry. 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.
Next… PLUMBICON…