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Title. Light Detectors. Characteristics. Sensitivity Accuracy Spectral Relative Response(R(  )) Absolute Sensitivity(S(  )) Signal-to-noise ratio --Noise equivalent input power. Characteristics. Intensity range Response time -effect of detector time constant Price.

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  1. Title Light Detectors

  2. Characteristics • Sensitivity • Accuracy • Spectral Relative Response(R()) • Absolute Sensitivity(S()) • Signal-to-noise ratio --Noise equivalent input power

  3. Characteristics • Intensity range • Response time -effect of detector time constant • Price

  4. Types of Detectors • Light Detectors can be classified int • Thermal Detectors --changes the temperature dependent properties of detectors --wavelength independent sensitivity --sensitivity depends on detector parameters --heat capacitance --thermal losses

  5. Thermal Detectors • Time constant of detector depends ratio of heat capacitance and thermal losses =H/G where H=heat capacity G=thermal losses --Sensitive to small values of G --time constant of detector limits the frequency of detector

  6. Thermal Detectors • Calorimeter add figure 4.73a-c

  7. Thermal Detectors • Bolometer consists of N-thermocouples in series Equations: Limitations: Input impedance of the amplifier should be larger than R for a change in current Current through bolometer should be kept Constant Temperature rise due to joule’s heating limits the maximum current through bolometer

  8. Golay Cell • Add figure 4.75

  9. Direct Photo detectors • Direct Photo detectors are based on  spectral based on emission of photoelectrons  changes in conductivity of semiconductors  voltage generated by the internal photo effect spectral response depends on work function or band gap

  10. Photodiode • Doped semiconductors • Can be either photovoltaic or photoconductive • P-n junction when irradiated generates photovoltage • Photoconductive elements change their internal resistance • Add figure 4.77 and figure 4.79

  11. Photodiode • Absorption coefficient is spectral dependent • Should be operated at low temperature in order to minimize thermal excitation of electrons For < 10 micrometers– liquid nitrogen For > 10 micrometers– liquid helium add figure 4.81 and 4.82

  12. Photoconductive diodes • When illuminated its electric resistance decreases • Time constant is dependent on diffusion time of electrons

  13. Photovoltaic detector • Add figure 4.83 • When illuminated generates electron-hole pairs • Add figure 4.84

  14. PIN Photodiode • At photon energies close to band gap absorption coefficient decreases • Add figure

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