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MAYURBHANJ SCHOOL OF ENGINEERING � LAXMIPOSI ,BARIPADA,757107

  • DEPARTMENT- E&TC ENGG.
  • SEMISTAR-3rd
  • SUBJECT-MEASUREMENT
  • NAME OF TOPIC –LIGHT SENSOR
  • PREPARED BY - VISWA NATH BEHERA

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Basic types of light sensors

  • Light sensors indicate light intensity by measuring radiant energy of photons in a range of frequencies.

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  • There are 2 main categories of light sensors, each with its own operating principle(s)
  • Generates electricity when illuminated
    1. Photo-emissive
    2. Photo-voltaic
  • Changes electrical properties when illuminated
    • Photo-conductive

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Figure 1: Summary of light sensor operating principles

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Photo-emissive sensors

Operation:

  1. Photons hit a light-sensitive material, releasing free electrons
  2. Electrons are collected in anode, forming a current
  3. The current is measured, and is dependent on photon intensity and frequency

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Figure 2: Photo-emissive (phototube) operation

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Photo-voltaic sensors

Operation:

  1. Photons hit a light-sensitive semiconductor, producing electron-hole pairs
  2. Electrons move across the semiconductor’s p-n junction, producing a voltage across it
  3. The current produced by this voltage is measured

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Figure 3: Photo-voltaic cell operation

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Photo-conductive sensors

Operation:

  1. Photons hit a light-sensitive semiconductor, producing electron-hole pairs
  2. Instead of accumulating a voltage (as in the photo-voltaic process), this semiconductor reduces its resistance
  3. This increases the amount of current flowing through the semiconductor, which is ten measured

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Figure 4: Photo-conductive operation

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Light sensor characteristics: responses to light

  • Figure 5: Photo-voltaic cell (G = solar irradiance)
  • Figure 6: Photo-emissive (vacuum) phototube
  • Figure 7: Photo-conductive photoresistor

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Interface electronic circuits

  • Needs usually an amplifier
  • The output depends on the load resistor
  • Change in wavelength changes the sensitivity response
  • Input voltage is needed so the sensor is not passive
  • Low pass filter is required parallel with the load resistor to decrease the noise
  • Typically white light is used

Figure 8: Sample electronic configuration for an ambient light photo sensor

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Applications

  • Burglar alarm where the light is send from one to another sensor, if the light is interrupted, the alarm will go on
  • Barcode scanner where the scanner illuminates the barcode and collects information
  • Remote control with infrared light sensor
  • Telecommunication use light emitting diode to send information with the optical fiber
  • Mobile phones have many different light sensors

Figure 9: Barcode scanner and typical photoelectric sensor components

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