Television Camera Tube-Basic Principle
The purpose of a TV pick-up tube is to sense each element independently and develop a signal in electrical form proportional to the brightness of each element. Light from the scene is focused on a photosensitive surface known as the image plate, and the optical image thus formed with a lens system represents light intensity variations of the scene. The photoelectric properties of the image plate then convert different light intensities into corresponding electrical variations.
- The two photoelectric effects used for converting variations of light intensity into electrical variations are (i) photoemission and (ii) photoconductivity.
- Certain metals emit electrons when light falls on their surface.
- These emitted electrons are called photoelectrons and the emitting surface a photocathode.
- Light consists of small bundles of energy called photons.
- When light is made incident on a photocathode, the photons give away their energy to the outer valence electrons to allow them to overcome the potential-energy barrier at the surface.
- The number of electrons which can overcome the potential barrier and get emitted depends on the light intensity.
- Alkali metals are used as photocathode because they have very low work-function.
- Cesium-silver or bismuth-silver-cesium oxides are preferred as photo emissive surfaces because they are sensitive to incandescent light and have spectral response very close to the human eye.
- The second method of producing an electrical image is by photoconduction, where the conductivity or resistivity of the photosensitive surface varies in proportion to the intensity of light focused on it.
- In general the semiconductor metals including selenium, tellurium and lead with their oxides have this property known as photoconductivity.
- The variations in resistance at each point across the surface of the material are utilized to develop a varying signal by scanning it uniformly with an electron beam.
Image Storage Principle
- Television cameras developed during the initial stages of development were of the non-storage type, where the signal output from the camera for the light on each picture element is produced only at the instant it is scanned.
- High camera sensitivity is necessary to televise scenes at low light levels and to achieve this, storage type tubes have been developed.
- In storage type camera tubes the effect of illumination on every picture element is allowed to accumulate between the times it is scanned in successive frames.
- With light storage tubes the amount of photoelectric signal an be increased 10,000 times approximately compared with the earlier non-storage type.
The Electron Scanning Beam
- In camera tubes magnetic focusing is normally employed.
- The electrons must be focused to a very narrow and thin beam because this is what determines the resolving capability of the camera.
- The diameter of the beam determines the size of the smallest picture element and hence the finest detail of the scene to which it can be resolved.
- Any movement of electric charge is a flow of current and thus the electron beam constitutes a very small current which leaves the cathode in the electron gun and scans the target plate.
- The scanning is done by deflecting the beam with the help of magnetic fields produced by horizontal and vertical coils in the deflection yoke put around the tubes.
- The beam scans 312.5 lines per field and 50 such fields are scanned per second.