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Generation of video signal by photoconductive effect...

  1. Jun 6, 2016 #1
    The thing which I am not getting is the role of scanning in generation of video signal by photoconductive way...the optical image is focused over the photoconductive material ...which decrease the resistance of the material according to the brightness of the scene being casted over the material...
    So exactly how does scanning cause a current to flow inside the material..??

    The text says that " An electron beam is used to pick-up the picture information now available on the target plate in terms of varying resistance at each point. "
    what does pick up refer here??

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  2. jcsd
  3. Jun 7, 2016 #2


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    You will see that there is a circuit formed by the battery (-), cathode, electron beam, photoconductive coatong, anode plate,load resistance and back to the battery (+). The current in the circuit will vary depending on the resistance of the coating. which is controlled by the light. The varying current causes a voltage drop across the load resistance which is used as the output and fed to an amplifier.
  4. Jun 7, 2016 #3
    Yes very well said..a complete circuit loop is formed in which current is regulated by the varying resistance of photoconductive material...but I wanted to ask that whats so special about the electron beam that is being used here...
    If the issue here is just to complete the circuit we could do it by any other means also....
    Then why only electron beam...??
  5. Jun 7, 2016 #4


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    How else would you determine the intensity of the image in just this one little spot? We need a focused beam that sweeps the entire image at an established rate. If you did not focus the beam and just 'showered' the target plate all we would get is a current based on the entire plates illumination and that really contains no useable information. Certainly not detail.
  6. Jun 7, 2016 #5
    first make it clear to me that from where we are getting this current.....is it the electron beam which is causing it....or the battery that is connected beneath the load resistance....??
    or would it be something like this...that a particular point on a photoconductive material with a definite resistance is brought into the circuit loop ((battery (-)--> cathode--->electron beam--->photoconductive coating---> anode plate --->load resistance and back to the battery (+)) when the electron beam is scanned over it..
    Last edited: Jun 7, 2016
  7. Jun 7, 2016 #6


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    Yes. The electron beam is just a very light conductive pointer which can be easily deflected to any position. The current originates from the battery. For TV, the electron beam scans a regular pattern over the photoconductive anode.
  8. Jun 8, 2016 #7
    Actually it would be partially true to say that the current originates from the battery..
    This is what I read further...
    "As the beam scans the target plate, it encounters different positive potentials on the side of the photolayer that faces the gun. Sufficient number of electrons from the beam are then deposited on the photolayer surface to reduce the potential of each element towards the zero cathode potential.The sudden change in potential on each element while the beam scans, causes a current flow in the signal electrode circuit producing a varying voltage across the load resistance RL."

    Now reading this gave me two queries...
    1. How positive potential occurs on the photoconductive material??
    2.Since the electron from the beam neutralises the positive potential towards zero cathode potential ..then how the current flow in the signal electrode circuit??
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