1. Infra Red Thermal Imaging

2. 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)
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3. 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
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4. 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
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5. Building Block of Photo-detectors
Semiconductors:
Building Block of Photo-detectors
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6. 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
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7. 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
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8. 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
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9. 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
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10. 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
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11. Working principle of Photon Detectors
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12. 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
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13. 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.
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14. 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
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15. 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
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16. 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
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17. 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
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18. 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
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19. 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
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20. Photo detector Performance Metrics
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21. Responsivity Responsivity is defined as detector output per unit input
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22. Quantum Efficiency
Quantum efficiency: number of electron-hole pairs generated by the detector per incident photon
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23. 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
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24. 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
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25. 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
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26. Surveillance & Spy camera
Photodetectors: Commercial Imaging
Surveillance & Spy camera
Webcams
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27. Thank you.