ECE 5th SEMESTER

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ECE 5th SEMESTER

• Subject
• Consumer Electronics
• (CE)

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TOPIC

• TELEVISION CAMERA TUBES

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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.

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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
n-type layer sno2
Glass face plate
p type layer PbO
Scanning beam
Light
Intrinsic layer
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Plan of a colour television camera showing
generation of colour signals and Y matrix for
obtaining the luminance (brightness) signal.
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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.

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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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THANKS