After the terrorist attacks of 11th September 2001 pressure has been increased on technology improvement and development of procedures related to the general safety, in particular – safety of air travel, especially passenger travel. Other procedures increase safety, but this is achieved at the cost of lower throughput at the airport, increase of social unrest and appearance of many controversies.
Thus, the goal set by the consortium undertaking the project is devising a fast and safe monitoring technology for use in passenger air communication, based on an innovative method of passive detection of terahertz radiation (naturally emitted by all objects). This technology, in combination with advanced data processing, allows performing continuous scanning of selected space in order to quickly detect appearance of particular substances within this space (such as: explosives, illicit drugs and biologically active substances) – even if they are partially or completely covered.
This ambitious task has been divided into several smaller tasks, undertaken independently by separate teams:
It is the heart of the system. It comprises of a matrix of so called spectral pixels, each of which is made of a specific number of quantum dots with varied shapes. This variation makes each of the dots sensitive to a slightly different radiation wavelength.
A spectral pixel may not measure continuous spectrum, however, appropriate selection of quantum dots allows signature of substances to be determined.
The unique feature of the detector is combination of its passive character (only radiation naturally emitted by all bodies is measured, there is none, not even potentially dangerous artificial radiation „passing through” involved) with spectral possibilities – the radiation is measured „in colour”, in a sense.
The passive character of the detector requires it to work in temperatures below 1.5 K (-272.6 °C). Thus, an efficient cooling system is required, one which will ensure this temperature in adequately large working space, and additionally will meet requirements of field work, such as: robust framework, simple operation, low energy usage, low noise levels.
Within the project, an innovative, liquid-free cryogenic system has been created, which meets all these assumptions and requirements. It may find wide application also in other fields.
Requirements for optical system imaging in the THz band are very high. It has to transfer the image to the interior of the cryostat, to the detector space, at the lowest level of distortion. In addition, one may not accept too large heat losses through the cryostat window. The designed system is a hybrid combination of classical and diffraction optics, while appropriate filters block heat radiation with minimal losses of radiation energy in the terahertz part of the spectrum.
The task of cold (operating inside the cryostat) electronics is appropriate amplification of electric signals from the sensor matrix and their multiplication. It is based on an appropriate information processing in such a way that it may be read outside cryostat using the least number of signal cables. Too high number of these cables could result in excessive heat flow to the cooled area.
Data processing system
Analysis of data collected by the THz detector is a complex problem, far exceeding the framework of classical image analysis. These data, having character of an image, also carry spectral information, and their interpretation requires taking into account a whole array of external factors, such as: temperature distribution and distances in the field of observation current air humidity.
Both the data processing system itself as well as a part of recognition algorithms have been implemented in the LabVIEW environment.
Only after such preparation the information is processed by appropriate classifying algorithms. Their goal is to separate sought substances in the field of vision of the detector. This utilises spectral signatures of objects, measured by the detector, which is matched to spectra stored in the database. Depending on the situation, algorithms based on statistical methods, neural networks or fuzzy logic may be used. Their action is supported by a simplified model of radiation propagation, which also enables detection of substances hidden, e.g. under clothing.
A part of software modules comprising the data processing system has been designed and implemented by SKA Polska.
It should be functional, clear and easy to use and at the same time supplying such an amount of information that it optimally helps the operator with decision making and does not cause too many false alarms.
What is very important, only effects of measured radiation, overlaid on a picture from a visual camera, are visualised. The passive system is thus free of controversies related to looking „through the clothing”.
Within this project, SKA Polska was responsible for visualising algorithms and programming of the user interface prototype.
THz simulator and optimisation of quantum dots
During works on the detector it did not have full imaging capability yet. In order to enable development of the data processing system, recognition algorithms and used interface despite this drawback, a mathematical model (simulator) of terahertz radiation has been developed. Taking into account geometric distribution of objects, temperature differences, influence of the atmosphere and signatures of all defined materials, the simulator allows to determine image for a predefined scene, which would have been registered with a fully functional detector.
A simplified version of propagation model has also been used in a tool used for optimisation of quantum dot shapes in the detector. The used genetic algorithm selects dot sizes in such a way that it maximizes. The used genetic algorithm selects dot sizes in such a way that it maximises efficiency of recognition of a particular set of materials.
Both the optimisation software and the simulator itself have been devised by SKA Polska. The simulator has been used for virtual testing of different scenario of system use as well as an environment verifying correctness of operation of recognition algorithms.
The video clips presents user interface. On the left side – visualisation of “raw” data from the sensors (the green colour is the THz detector). Image on the right side shows what the user sees – analysis results are overlaid on image from a normal camera – probability of explosives detection. The whole scene has been generated using a THz simulator.
The project has been implemented by 21 companies and research institutes from eight countries.