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

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

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

Thermal Detectors

Calorimeter

add figure 4.73a-c

Thermal Detectors

Bolometer consists of N-thermocouples in seriesEquations:Limitations:Input impedance of the amplifier should be larger than R for

a change in currentCurrent through bolometer should be kept Constant Temperature rise due to joule’s heating limits the maximum

current through bolometer

Golay Cell

Add figure 4.75

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

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

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

Photoconductive diodes

When illuminated its electric resistance decreases

Time constant is dependent on diffusion time of electrons

Photovoltaic detector

Add figure 4.83 When illuminated generates electron-hole

pairs Add figure 4.84

PIN Photodiode

At photon energies close to band gap absorption coefficient decreases

Add figure