1. Physics of thermography
Please use the dd month yyyy format for the date for example 11 January The main title can be one or two lines long.
Janice Dulieu-Barton

2. Infra-red thermography
Aim:
Introduce temperature measurement
Describe the physics of thermography
Understand factors that effect the measurement
Show some examples

3. Temperature measurement
The measurement of temperature yields diagnostic information.
In heat transfer processes, and in operations that involve the transport of media at elevated temperatures the temperature gives information about the system performance.
In mechanical and electrical machines, heat generation is often an early sign of failure.

4. Liquid expansion sensors
Mercury or alcohol in glass are commonly used, but are fragile.
Liquid in metal devices are available, but all these thermometers have a large contact area (typically a bulb) and are therefore unsuitable for surface temperature measurement.
Can be quite accurate (0.1°C).
Difficult to read
Cannot be interfaced easily with a data acquisition system.

5. Thermocouples
Two wires of different metal generate a voltage across the junction that is proportional to temperature.
To complete the electric circuit, two junctions are normally used, a "hot" and a "cold". One junction is kept at a known (usually room) temperature and the measured potential represents the difference in temperature between the two junctions.
Taking into account the sensitivity of the junctions and the accuracy of the meter, the devices can measure accurate to about 0.5°C.
Portable devices for thermocouple measurements are quite common.
Junctions can be as small as 0.5 mm across, so with careful use thermocouples can be used to measure surface temperatures.
Thermocouples can be used at up to 700°C.
These are very useful for monitoring purposes as they can be permanently attached to surface, and connected to a data acquisition system and information stored at regular intervals.

6. Resistance sensors
A metal or semiconductor changes its electrical resistance with temperature. Semiconductors are considerably more sensitive.
The temperature is limited to about 300°C and some drift may be experienced.
Thermistor (semiconductor) devices can be as small as 1.5 mm diameter and are used in portable instruments.
The key difference between thermocouples and resistance sensors is that the resistance sensor behaves linearly with temperature, whereas the thermocouple behaves in a non-linear manner.
The thermocouple is more suited to monitoring applications because it is less bulky.
Resistance sensors require a power supply to operate and both can be attached to data acquisition systems.

7. Passive thermography
Monitoring the emission of infrared radiation from the surface of an object using IR detector
No external stimulus added
Commonly used for:
Building surveys
Production lines
Electrical fault detection
Search and rescue
Security
Medical
Blue
Cold
Black
Pink/red
Hot
White
gatewayhomeinspection.com, maverickinspection.com, infraredcameras.net.au, optotherm.com, coastguard.dodlive.mil, coolcosmos.ipac.caltech.edu

8. Thermography overview
Infrared Thermography (IRT)
Pulsed and Pulsed Phase Thermography (PPT)
Thermoelastic Stress Analysis (TSA)

10. Modern IR cameras

11. Thermography
The surface temperature is often used to predict internal temperatures, because it is difficult to penetrate housings, vessels, pipes, or conductors.
The surface temperature of a body is a function of the sub-surface heat generation, and the emission of energy at the surface.
It may be convenient to place contact sensors on the surface, but often non- contact techniques are used to avoid touching dangerous components, e.g. those of high temperature or carrying a high voltage.
Non-contact temperature measurement uses infra-red radiation to predict surface temperature.
The radiation measured by an instrument includes emitted, transmitted and reflected energy.
The fraction of energy emitted is a property of the surface, and is called the emissivity, e.
For a perfect emitter or black body e is 1, but for engineering surfaces it can be in the range 0.02 and 0.97.

12. Physics
A body with a temperature above absolute zero will emit energy in the form of electromagnetic radiation/thermal radiation at its surface.
As the temperature increases the quantity of heat transferred by means of thermal radiation increases.
By using the amount of energy emitted in the form of electromagnetic radiation, accurate temperature measurements can be made over a wide range by means of infrared thermometry.
The spectral radiant emittance (λ,b) for a blackbody in a hemisphere in the wavelength range from  to  + can be found using Planck’s law, i.e.
where C1 is the first radiation constant = 2 c2 h (h is Planck’s constant, c is the speed of light and λ is wavelength), C2 is the second radiation constant = c h / k (k is the Boltzmann constant) and T is absolute temperature.

13. Emitted energy
The spectral radiant emittance for a blackbody at various temperatures can be plotted against wavelength
λ,b has a maximum, the wavelength of which this occurs is defined as max, and can be calculated from Wien’s displacement law.

14. Bolometers
An instrument that measures radiant energy by correlating the radiation-induced change in electrical resistance of a blackened metal foil with the amount of radiation absorbed i.e. a temperature-sensitive electrical resistor
Thermometer and absorber are connected by a weak thermal link to a heat sink
Incoming energy is converted to heat in the absorber
Temperature rise decays as power in absorber flows out to the heat sink
Temperature rise is proportional to the incoming energy

15. Bolometer response
The thermal time constant τth of a bolometer is determined by the thermal mass or heat capacity of the material, C, and by the thermal conductivity, G, between the detector and the thermal reservoir: 𝜏 𝑡ℎ = 𝐶 𝐺
This is the time it takes to discharge the heat and be ready for the next measurement
The response time is 3τth making monitoring a rapidly changing scene is difficult.

16. An uncooled thermal sensor
Microbolometers
An uncooled thermal sensor
An array of pixels, each pixel being made up of several layers.
A silicone substrate and a readout integrated circuit (ROIC).
Electrical contacts are deposited and then selectively etched away.
A reflector is created beneath the IR absorbing material.
Light is able to pass through the absorbing layer, the reflector redirects this light back up to ensure the greatest possible absorption.
A sacrificial layer is deposited so that later in the process a gap can be created to thermally isolate the IR absorbing material from the ROIC – later removed
A layer of absorbing material is then deposited and selectively etched so that the final contacts can be created.
There is no cooling, so the absorbing material must be thermally isolated from the bottom ROIC and the bridge like structure allows for this to occur.

17. Stefan-Boltzman relationship
Integration of Planck’s law between 0 and  gives the well- known fourth-power Stefan Boltzmann relationship for evaluating the radiant emittance over all wavelengths b
where B is the Stefan-Boltzmann constant 5.69 x W/m2K4.

18. Photon Detectors

19. Photon detectors
Sensitive Infra-red thermography devices are ususally photon detectors – semi conductors
They count the total number of photons per unit area and time (Nb),
Obtained by dividing the energy in each wavelength interval by the energy carried by each photon and integrating between 0 and , to give
The quantity 0.370B/k = 1.52 x 1015 photons s-1 m-3 sr-1 K-3 and can be regarded as the Stefan-Boltzman constant for photon detectors.

20. Atmospheric windows
For thermal imaging, there are two atmospheric windows of interest, one is located from 3 to 5 μm and the other is from 8 to 12μm.

21. Temperature calibration
Calibration has been conducted using a cavity black body.
Water is circulated around the inner and outer cones to regulate the black body temperature.
Schematic of Black Body

22. Radiometric calibration

23. Emissivity
When radiation impinges on a body it is either transmitted through the body, absorbed by the body or reflected away from the body, so that
where  is the transmissibility, a is the absorptivity and r is the reflectivity.
Engineering materials are usually opaque in the infra-red region, even if they are transparent to visible light, e.g. glass. The transmitted energy is therefore zero. So
The reflectivity is then 1 - , so a portion of incident radiation is reflected back to the detector. Care must be taken to avoid anomalous readings from reflections from heaters and the sun. The absorbed energy is therefore equal to the emitted energy so

24. Effect of emissivity
For electrical systems and components e is often taken as 0.90 to 0.95, but many materials are different.
Often a layer of matt black paint is applied to the surface of a material to create an enhanced and uniform emissivity.

25. Emissivity

26. Comparison of Infrared (IR) Cameras
Photon Detector
Microbolometer
IR Camera
FLIR SC5500
FLIR A655sc
Detector material
InSb (Indium Antimonide)
VOx (Vanadium Oxide)
Working principle
Count photons  electrons
Radiation  thermal energy  electrical signal
Cost
~£100,000
~£15,000
Cooling
Yes No
Integration time
10 – 20,000 µs – variable
8 ms (thermal time constant) – fixed
Response time
10 – 20,000 µs
24 ms (3 x thermal time constant)
Sensitivity
<20 mK
<30 mK
Resolution
320 x 256 pixels
640 x 480 pixels
Input load
Weight
3.8 kg
0.9 kg
Size
310 x 141 x 159 mm
216 x 73 x 75 mm
The heat conductance Gth contains all heat exchange mechanism of the detector as conduction, convection and radiation.
The transduction of the temperature rise to an electrical output signal is done by using a temperature-dependent physical property, for the bolometer case: temperature dependence of the electrical resistance.
Essentially a photon detector counts the photons that impinge on the sensor and transform them to electrons
Thermal detectors convert the absorbed electromagnetic radiation into thermal energy causing a rise of the detector temperature that is transformed to electrical signal by means of electrical resistance.
Photon detectors are used for TSA as they respond quickly to changes of temperature and have a good thermal resolution. [8]. The IR camera system used in the present work is FLIR SC5500, which has a minimum integration time of ~5 𝜇s, i.e. the time available to collect electrons. Nevertheless, due to the high costs of the equipment, the introduction of low-cost bolometer IR cameras for TSA is currently under development [9, 10]. In this research, the FLIR A655sc IR camera system considered has a Vanadium Oxide (VOx) semiconductor bolometer. It works by changing the electrical resistance of the sensor when the infrared radiation is absorbed, so an electrical signal is obtained. Nonetheless, the limitation of this bolometer is the thermal time constant that is ~8 ms, which means that ~24 ms are required to perform an accurate reading [11]. This is a significant challenge when observing continuously varying temperature fields, such as for TSA.
Mention order of magnitude of the cost of the cameras: about £100,000 for the photon detector and about £10,000 for the bolometer
TSA is performed by using thermal cameras, therefore it is relevant to describe the two types of detectors used:
- Photon detectors, which convert the absorbed electromagnetic radiation into a change of the electronic energy distribution in a semiconductor. It needs to be cooled using a cryocooler to reduce thermal noise.
- Thermal detectors, which convert the absorbed electromagnetic radiation into thermal energy causing a rise in the detector temperature. Then, a corresponding change in some physical property of material produces the electrical output. There are two different approaches to this:
o Ferroelectric detectors, which use the ferroelectric phase transition in certain dielectric materials.
o Bolometers, which is a specific type of resistor, the IR (infrared) radiation changes the electrical resistance of the material and it is converted to electrical signals.

27. Our new camera!!