*TOPIC

TELEVISION CAMERA TUBES*SRKR

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*OUTLINE OF LECTURE

CAMERA TUBESBASIC PRINCIPLETYPES OF CAMERA TUBES*SRKR

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*TELEVISION CAMERA TUBESAn electron tube use a light-sensitive material/receptor that converts an optical image into an electrical television video signal. The tube is used in a television camera to generate a train of electrical pulses representing the light intensities present in an optical image focused on the tube. Each point of this image is interrogated in its proper turn by the beam in tube , and an electrical signal corresponding to the amount of light at that point of the optical image is generated by the tube. This signal represents the video or picture portion of a television signal. *SRKR

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*BASIC PRINCIPLEPhotoconductive effectPhotoemission effect

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*Photoemission principle 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 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.

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*The number of electrons which can overcome the potential barrier and get emitted ,depend on the light.Alkali metal are used as photocathode because they have very low work function.Cesium silver / bismuth silver cesium oxides are preferred as cathode surface because they are sensitive to incandescent light and have spectral response very close to the human eye.*SRKR

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*Photoconductive principleIn some metal , when light photon fall on it then its conductivity / resistivity change.Semiconductor metals like selenium, tellurium and lead with their oxide have this property known as photoconductivity.The variation of resistance at each point across the surface of the material is utilized to develop a varying signal by scanning it uniformly with an electron beam. *SRKR

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*Types of Camera TubesImage Orthicon tubeVidicon Plumbicon*SRKR

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*Image Orthicon tube*SRKR

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*Image Orthicon tubePrinciple : Photoemissive It has three section:Image sectionScanning sectionElectron gun cum-multiplier section

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*Image sectionGlass plate : coated with Silver-antimony serve as photocathodeLens assembly : light from the scene to be televised is focused on the photocathode surface by lens systemPhotocathode surface is semitransparent and light rays penetrate it to reach its inner surface where electron emission takes place .Target plate : when electron emit from cathode ,it can not store charge being a conductor .*SRKR

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*For this reason the electron image is made to move towards the target plate located at short distance from it.The target plate is made of a very thin sheet of glass and can store charge received by it. This is maintained at about 400v more +ve w.r.t photocathode When electron move from cathode to target plate there is chance that it may repel each other ,this can result in distortion of the information available as charge image.

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*To prevent this divergence effect an axial magnetic field generated by focus is employed This magnetic field impart helical motion to electrons and focus them on target plate .The image side of target plate has coating of cesium that emit secondary electron.these secondary electron is collected by wire mesh which is located at front of target plate .The +ve charge appeared on target plate is proportional to light intensity distribution on photocathode*SRKR

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*Scanning sectionFine electron beamFocus ,alignment coilDeflection coil 1. Horizontal deflection2. Vertical deflection *SRKR

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*Electron gun assembly and multiplierElectron gunMultistage electron multiplier*SRKR

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*VidiconPrinciple : Photoconductive It has three section:Target sectionScanning sectionElectron gun*SRKR

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*Target sectionGlass plate Lens assembly*SRKR

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*Scanning sectionFine electron beamFocus ,alignment coilDeflection coil 1. Horizontal deflection2. Vertical deflection

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*Electron gun assembly Electron gun*SRKR

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*Principle of operation *SRKR

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*Application of vidiconClose circuit TV systemEarlier type of vidicon were used only where there was no fast movement , because of inherent lag

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*Plumbicon Camera Tube lower image lag.It has fast response and produce high quality pictures at low light level.It has small size and light weight and has low power operating characteristics .It is similar to vidicon tube except small change in target plate *SRKR

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*Target plate Glass face plateLightIntrinsic layern-type layer sno2Scanning beamp type layer PbO*SRKR

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*Plan of a colour television camera showing generation of colour signals and Y matrix for obtaining the luminance (brightness) signal.*SRKR

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Figure shows a simple block schematic of a colour TV camera. It essentially consists of three camera tubes in which each tube receives selectively filtered primary colours. Each camera tube develops a signal voltage proportional to the respective colour intensity received by it.**SRKR

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Light from the scene is processed by the objective lens system. The image formed by the lens is split into three images by means of glass prisms. These prisms are designed as diachroic mirrors.A diachroic mirror passes one wavelength and rejects other wavelengths (colours of light).Thus red, green, and blue colour images are formed.

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The rays from each of the light splitters also pass through colour filters called trimming filters. These filters provide highly precise primary colour images which are converted into video signals by image-orthicon or vidicon camera tubes. Thus the three colour signals are generated. These are called Red (R), Green (G) and Blue (B) signals.

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