Infra Red Thermal Imaging

Electromagnetic Waves Water and sound waves transfer energy from one place to another They require a medium through which to travel. They are mechanical waves EM wave is emitted whenever an electric charge is accelerated Matter is largely composed of electrically charged particles that are continuously in motion, hence EM radiation is continuously emitted by all objects EM waves do not require a medium to travel. They travel as vibrations in electrical and magnetic fields Speed of electromagnetic waves =3x108 meters/second (Light takes 8 minutes to move from the sun to earth {150 million miles} at this speed) 32/78

Energy of wave proportional to A2 Light Waves Classical theory describes Light as a Wave characterized by: Amplitude (A) Frequency (n) Wavelength (l) Energy of wave proportional to A2 Quantum theory describes light as a particle called a photon having an energy given by: E = h n = hc/ This is known as the dual nature of light (or the duality of light) Wave Particle OR 33/78 3

So is light a wave or a particle ? On large scales, we can treat a large number of photons as a wave For subatomic phenomenon, we often deal with a single or few photon. In this case, light is treated as a particle For explaining operation of photo-detectors we use the particle nature of light 34/78 4

Building Block of Photo-detectors Semiconductors: Building Block of Photo-detectors 35/78

From energy level to energy band Band Theory of Solids Solids are crystalline in nature Energy levels of single atoms do not apply to solids When a large number of atoms (as in solid crystals) are brought together, these levels split into nearly continuous energy bands There may or may not be an energy gap between energy bands Electrons occupy these bands based upon the energy they possess From energy level to energy band 36/78

Band Theory of Solids The last filled or partially filled band is called the valance band (VB) The empty band above VB, across the energy gap, is called the conduction band (CB) The difference in energy between the valence and conduction bands is defined as the band gap (Eg) for a particular material Conduction Band Eg Valence Band Electrons 37/78 7

Conductors, Semiconductors & Insulators Conductors Semiconductors Insulators VB is partially filled (VB and CB overlap) VB is full and the gap between the filled VB and the CB is small (Eg~1 eV) The gap between the filled VB and the CB is large (Eg~ 5 eV) Electrons free to move in partially filled VB Electrons jump from VB to CB when they get enough energy from photons or thermal energy Electrons cannot move unless given large amount of energy to jump from VB to CB Always conduct electricity Conduct small amount of electricity at room temperature Do not conduct electricity Example: metals Example: Ge, Si Example: Carbon Valence band Eg Eg Conduction band 38/78 8

Types of Semiconductors Semiconductors can be classified into two types: Intrinsic semiconductor It is pure semiconductor crystal with no impurities Such semiconductor have no charge carriers at T = 0 K => zero conductivity at 0 K At higher temperature or due to illumination, electron are excited and cross the band gap to the conduction band => finite conductivity at room temperature Extrinsic semiconductor Extra carriers created in semiconductor by introducing impurities into the crystal => doping When dopant creates extra electron (negative charge) it is called n – type semiconductor When dopant creates extra hole (positive charge) it is called p – type semiconductor By doping, a crystal can be altered so that it has a predominance of either electrons or holes 39/78 9

Detector Classification IR Detectors Thermal or Uncooled Photon or Cooled Pyroelectric Thermopile Photo diode Photo voltaic Bolometer Photo conductive Low sensitivity Slow response No cryogenic cooling required High sensitivity Fast response Require cryogenic cooling 40/78

Working principle of Photon Detectors 41/78

Photoconductive Detectors Fabricated from single type of semiconductor When photons are absorbed by the semiconductor it generates electron and hole pair (charge carriers) The increased number of charge carriers leads to an increase in the electrical conductivity of the semiconductor The change in electrical conductivity leads to an increase in the current flowing in the circuit, and hence to a measurable change in the voltage drop across the load resistor => photon gets detected 42/78

Photovoltaic Detectors Based on Photovoltaic effect across a p-n junction No bias voltage is required If the junction is short-circuited, current will flow in the circuit when the junction is illuminated. 43/78

Photodiode Detectors A pn junction with reverse bias voltage is termed a photodiode Applying a reverse bias across the photodiode increases its speed of response compared to photovoltaic detector However, the dark leakage current of the photodiode tends to increase with applied reverse voltage resulting in an increase in the amount of shot noise generated by the photodiode In general, a pn junction is operated in the photovoltaic mode when low nose is of prime concern, and under applied reverse bias when maximum speed is needed 44/78

Materials Materials commonly used for photoconductive & photodiode detector: Silicon Si is used in the visible, near ultraviolet, and near infrared Germanium Ge and Indium Gallium Arsenide InGaAs in the near infrared Indium Antimonide InSb, Indium Arsenide InAs, Mercury Cadmium Telluride MCT, and germanium doped with elements like copper and gold in the longer-wavelength infrared 45/78

Working principle of Thermal detectors Thermal insulation Absorber: Absorbs IR radiation & converts it into heat energy or temperature change Transducer: Senses temperature change and converts it into some measurable parameter Thermal Insulation: temperature rise in absorber/transducer IR Absorber ΔT change Transducer Measurable property 46/78

Thermopile Detector Based on Seebeck Effect When two dissimilar metals/semiconductors are joined together at each end and both junctions are at different temperatures, a thermoelectric EMF is generated causing a current to flow in the circuit Thermopile consists of large number of thermo- couples Change in temperature Thermocouple Change in voltage 47/78

Pyroelectric Detectors These detectors employ pyroelectric effect to sense the photons Among the materials that reveal the pyroelectric effect are those whose molecules have dipole moment such as triglycine sulfate (TGS), lithium tantalate (LiTaO3) and polyvinyl fluoride (PVF2) Advantages: Pyroelectric detectors have faster response as compared to other thermal detectors Disadvantages: Pyroelectric detectors do not have a DC signal and hence the surface temperature must be modulated in time by means of a chopper placed in front of the detector Applications: Pyroelectric detectors are used as temperature change sensors and in infrared imaging Change in temperature Change in polarization Pyroelectric 48/78

Bolometer detector structure for Thermal Imaging Bolometers use the phenomenon of change in resistance of a material when its temperature changes When IR radiation is incident onto the bolometer, its temperature changes which in turn causes change in its resistance The change in resistance is sensed by an electrical bias to measure the energy of the incident photon The bolometer is in a circuit in series with a voltage source, so that current flows through it and, as the resistance changes, the voltage drop across the element changes, providing a photon energy sensing mechanism Change in temperature Change in resistance Bolometer Bolometer detector structure for Thermal Imaging 49/78

Photo detector Performance Metrics 50/78

Responsivity Responsivity is defined as detector output per unit input 51/78

Quantum Efficiency Quantum efficiency: number of electron-hole pairs generated by the detector per incident photon 52/78

It is size of a single detector element of the imaging array Pixel Size It is size of a single detector element of the imaging array More pixel size means more collection of optical energy (light) => Higher Sensitivity But lower resolution => lesser details Typically 3-10µm for VIS & NIR detectors 20 μm for cooled MWIR & LWIR detectors 17 μm and 25 μm for uncooled LWIR detectors Single pixel 53/78

Fill Factor = Active area Ratio of active area to total area of single detector element Percent of pixel area that captures photons Typically 25% to 100% Fill Factor = Active area Total area Active area Total area 54/78

Ability of each pixel to collect the photo-generated electrons Well Capacity Ability of each pixel to collect the photo-generated electrons “Saturation charge” = 45 to 100 K electrons Depends on the pixel size Limits dynamic range Once detector well is filled up, electrons can overflow into neighboring pixels => blooming Blooming Effect 55/78

Surveillance & Spy camera Photodetectors: Commercial Imaging Surveillance & Spy camera Webcams 56/78

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