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Learn about the characteristics, sensitivity, accuracy, and types of light detectors including Thermal, Direct Photo, Photodiode, and more. Explore their capabilities, limitations, and operational considerations for optimal performance in various applications.

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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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