Infrared Detectors for Radiation Thermometry

This work presents an investigation into the potential next generation of infrared detectors for radiation thermometry. Motivations for this work include increased detector sensitivity, development of minimally cooled mid-wave infrared (MWIR) arrays and longer wavelength multi-colour ratio radiation thermometers. The high sensitivity of the Si avalanche photodiode (APD) is highly attractive for radiation thermometry. A Si APD is shown to offer increased sensitivity by measuring a lower temperature than a Si photodiode in order to satisfy specific threshold voltages. The Si APD is also shown to offer improvement in the signal-to-noise ratio (SNR) and temperature error. By combining the Si APD with a phase sensitive detection (PSD) method, further improvement is achieved. The MWIR InAs/GaSb type-II superlattice (T2SL) offers the potential development of minimally cooled MWIR arrays for radiation thermometry, as well as longer wavelength multi-colour detectors for ratio radiation thermometry. An uncooled T2SL detector on a GaSb substrate is demonstrated for measurement of a target temperature of 25 oC with SNR > 1. Cooling improves the detector’s performance, allowing operation at the thermo-electric cooler compatible temperature of 200 K. Further characterisation of a T2SL detector on a GaAs substrate demonstrates similar temperature dependence, suggesting challenges to the material growth for improved detector performance. Other potential challenges with T2SL development are identified and discussed. The quantum dot infrared photodetector (QDIP) also offers the potential for longer wavelength multi-colour detection. Combination with an infrared algorithmic spectrometer (IRAS) offers flexibility for development of a versatile ratio radiation thermometer. The QDIP-IRAS combination is demonstrated to successfully reproduce arbitrary filter shapes from blackbody photocurrent measurements. Ratios computed using the IRAS correspond well with ratios computed from the arbitrary target filters.

An InGaAlAs-InGaAs two-colour detector, InAs photodiode and Si SPAD for radiation thermometry

This work aims to develop infrared detectors and to introduce a new measurement technique for infrared radiation thermometry. It consists of two-colour detectors for ratio thermometry, InAs photodiode for 3.43 m narrow band thermometer and photon counting thermometer using a Si single photon avalanche photodiode (SPAD). In addition to research in these detectors, a Monte Carlo model for modelling impact ionisation in Si was also developed. InGaAlAs is attractive material for multi-colour detection at wavelengths up to 1.7 m, as it is lattice matched to InP substrate. InGaAlAs-InGaAs two-colour detector was evaluated as a ratio thermometer. When compared to a commercial Si-InGaAs detector, the InGaAlAs diode produces slightly higher (lower) output than Si at temperature below (above) 500 °C, while the InGaAs diode in this work also produces slightly higher output than that in the commercial Si-InGaAs detector. The InGaAlAs and InGaAs diodes detect blackbody temperatures as low as 275 and 125 oC, respectively, with signal to noise ratios (SNRs) above 10. As a ratio thermometer, the two-colour InGaAlAs-InGaAs photodetector achieves a temperature error of 12.8 °C at 275 °C, but this improves with temperature to 0.1 °C at 450 °C. If the maximum temperature error of 2 °C is defined, the InGaAlAs-InGaAs is capable of detecting an object temperature down to 325 °C. These results demonstrate the potential of InGaAlAs-InGaAs two-colour photodetector for development of high performance two-colour array detectors for radiation thermometry and thermal imaging of hot objects. The InAs photodiode offers huge potential for infrared sensing applications at wavelengths above 1.7 m. The performance of InAs photodiode was evaluated for use in radiation thermometry at wavelengths beyond InGaAs photodiode. For uncooled InAs, it successfully measured a blackbody temperature of 50 oC with an acceptable error of 0.17 oC. In order to evaluate its performance as a 3.43 m narrow band thermometer, measurements were repeated with a narrow band filter. InAs was demonstrated to have lower temperature error than a commercial PbSe detector. The temperature error was 1.88 oC for InAs at 50 oC compared to 3.78 oC for PbSe. This suggests that InAs is ideally X. ZHOU III suited for applications requiring 3.43 m operating wavelength. Further improvement was achieved by cooling InAs to 200 K. It was found that a temperature as low as 37 oC, with an error of less than 0.5 oC, can be measured indicating its potential for human body temperature sensing. An alternative to using a photodetector with longer wavelength response is to increase the sensitivity of the photodetector via internal gain mechanisms such as impact ionisation. By employing a very high internal gain in SPAD, the photon counting technique was evaluated for radiation thermometry. Photon induced avalanche pulses were successfully measured at temperature as low as 225 oC with an error less than 2 oC using Si SPAD. This is significantly lower than the lower temperature limit of 400 oC in conventional Si photodiode based radiation thermometer. The photon counting technique is therefore demonstrated to be a feasible technique to achieve lower temperature sensing

The Study of Fire Spread on an Inclined Wooden Surface by Multiple Spectrum Imaging Systems and Diagnostical Techniques

Fire disaster, as an unavoidable threat around the world, caused millions of loss of living beings and properties. To minimise the damage that fire disaster causes, the prediction of fire spread is significant. Due to the burning of wood is a series of complicated processes, the study of fire spread along the wood surface is far from complete. To comprehensively understand the mechanism of wood combustion as well as the fire spread, a systematically study is necessary. With the development of digital cameras and computer science, the vision systems based on the digital camera become a useful tool for measurement and visualisation. In this work, the fire spread on inclined wooden rod surface is systematically studied based on the vison systems. An original designed imaging system that synchronises visible, schlieren, and thermal imaging systems is developed. The various diagnostical techniques, including the temperature measurements by two-colour method and thermal imaging, optical flow motion estimation, and selective enhancement technique are developed along with the imaging system. Rely on the imaging system that developed in this study, the burning ability, temperature of the wooden surface and flame, the dim blue flame and the invisible hot gas flow can be visualised simultaneously. The first contribution of this work is the developed pyrometers which are based on two methods: the first one is the two-colour method, which relies on the response ratio between two selected wavelengths. This method is used for measuring the soot flame temperature. The second pyrometer is based on thermal imaging, which uses a narrow band wavelength thermal image with known emissivity. It is used for reading the temperature of the wood surface. With the instruments developed in this work, the flame temperature and surface temperature of the burning wood can be monitored at the same time. Based on the imaging system and diagnostical techniques, the fire spread on wooden rods surface that inclined at various angles is investigated. It is first found in this work both the flame and fire plume would have geometry change at the inclination surface. Besides, with the help of the optical flow method, the minor fluctuate (<2mm/s) of the flame and hot flow can be detected and used to analyse the burning phenomena. Another finding is the essential role of underneath preheating on sustaining the burning and spreading the fire. The pyrolysis zone, as well as the preheating zone, have been illustrated and visualised by synchronising the blue flame with the schlieren image by the first time. Furthermore, with involving the thermal imaging system, the preheating length underneath the burning rod is calculated and shows a monotonically increase with the increasing angle. Moreover, this work introduces a novel method to measure the flame attachment phenomenon quantitatively by using enhanced thermal image. The effects of cross-wind on the burning of wood are investigated systematically with the imaging system. The main finding is that under the low speed of cross-wind, the burning and fire spread are enhanced, while they are decreased under the high speed of the wind. With the help of the multiple spectrum imaging, the mechanisms of the wood combustion under wind condition has been studied and visualised. This new imaging system with developed diagnostical techniques is a useful tool for investigating the burning of wood and fire propagation. The findings in this works could help enhance the understanding of the fire protection and optimise the strategy both in fire protection and fire extinction