1 topic television camera tubes akshay jilowa gpcg jalandharpunjab edusat society

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TOPIC

TELEVISION CAMERA TUBES

Akshay Jilowa GPCG Jalandhar Punjab EDUSAT Society

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OUTLINE OF LECTURE

CAMERA TUBES BASIC PRINCIPLE TYPES OF CAMERA TUBES


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TELEVISION CAMERA TUBES An 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. Akshay Jilowa GPCG Jalandhar Punjab EDUSAT Society

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BASIC PRINCIPLE Photoconductive effect Photoemission 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.


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Photoconductive principle

In 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.


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Types of Camera Tubes

Image Orthicon tube Vidicon Plumbicon


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Image Orthicon tube


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Image Orthicon tube

Principle : Photoemissive It has three section: Image section Scanning section Electron gun –cum-multiplier

section


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Image section Glass plate : coated with Silver-antimony serve

as photocathode Lens assembly : light from the scene to be

televised is focused on the photocathode surface by lens system

Photocathode 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 .


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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


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Scanning section

Fine electron beam Focus ,alignment coil Deflection coil 1. Horizontal deflection 2. Vertical deflection


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Electron gun assembly and multiplier

Electron gun Multistage electron multiplier


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Vidicon

Principle : Photoconductive It has three section: Target section Scanning section Electron gun


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Target section

Glass plate Lens assembly


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Scanning section

Fine electron beam Focus ,alignment coil Deflection coil 1. Horizontal deflection

2. Vertical deflection


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Electron gun assembly Electron gun


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Principle of operation


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Application of vidicon Close circuit TV system Earlier 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


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Target plate

Glass face plate

Light

Intrinsic layer

n-type layer sno2

Scanning beam

p type layer PbO


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


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.


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.


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.


1 topic television camera tubes akshay jilowa gpcg jalandharpunjab edusat society

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