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Human perception of images
(2005)
Abweichend von der gängigen Vorgehensweise, mit einem Bleistiftstrahl
die Lage eines Bildpunktes festzulegen und die rückgestreute Strahlung großflächig aufzunehmen, bietet die neu entwickelte Kamera Unabhängigkeit von Einstrahl- und Blickrichtung. Ausgestattet mit einer 'Optik' für hochenergetische Strahlung, wie die bereits beschriebene Schlitzblendenkamera, ist die variable Gestaltung von Einstrahl- und Blickrichtung möglich. Damit eröffnet sich die
Chance, Strahlung absorbierende Details in einem Objekt sichtbar werden zu lassen, die sonst von einer streuenden Umgebung überstrahlt würden. Zudem wird gezeigt, wie sich das Erscheinungsbild von Objekten umgebungsbedingt im Rückstreubild ändern kann.
Für die öffentliche Sicherheit wird gesorgt, indem Gefahren rechtzeitig entdeckt werden. Deren Ursprung kann im technischen oder menschlichen Versagen liegen, womit im englischen Sprachraum zwischen 'Safety' und 'Security' unterschieden wird. Hinsichtlich der Folgen verschwimmt aber dieser Unterschied, obwohl für deren Verhinderung unterschiedliche Zuständigkeiten existieren. Bezüglich des gemeinsamen Interesses und auch überlappender Methoden und Vorgehensweisen würden beide Seiten gewinnen können, wenn es eine Brücke zwischen ihnen gäbe. Eine solche zu schaffen hat sich die EFNDT Arbeitsgruppe (Working Group) 5 zur Aufgabe gestellt. Gestaltet werden soll sie durch gemeinsame Veranstaltungen und Seminaren, die zur Bildung von Konsortien und der Durchführung gemeinsamer Projekte führen sollen.
Verleimte Holzproben wurden je nach Größe mit thermischen (ANTARES) bzw. schnellen Neutronen (NECTAR) an der Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) der Technischen Universität München in Garching untersucht. Entweder ließen die durchstrahlbare Schichtdicke oder die eingeschränkte Strahlgeometrie es nicht zu, das Probenteil allseitig zu durchstrahlen.
Um dennoch innere Strukturen wie Leimschichten zumindest mit einem eingeschränkten Winkelbereich tomographisch darzustellen, wurde mit Hilfe des Slice-Theorems erfolgreich rekonstruiert. Ein Brett von 2 cm Dicke wurde mit thermischen Neutronen erfolgreich untersucht, ein größeres mit schnellen.
Neutrons are absorbed particularly by hydrogen containing materials so
they can be used as a tool for visualising heterogeneous density distributions of organic materials. Penetration limits are set by the layer thicknesses and the neutron energies applied. In case of specimens with a flat shape the situation may be encountered that the object only could be penetrated in selected directions. In addition, the overall size may exceed the beam diameter and thus the viewing window if a certain region of interest should be studied by tomography without destroying the integrity of the specimen.
This study shows the capabilities and limits of thermal and fast neutrons to investigate flat wooden specimens such as boards and girders by neutron tomography under such circumstances. Taking projections was impaired either by the limits of penetrability or by the total size of the object. As a consequence, projections were included for reconstruction only from a limited angular range of 90°. It could be shown that an approach based on the slice theorem was capable to visualise structural features along the beam directions while simply omitting the perpendicular ones without causing additional artefacts.
Small samples with a thickness of up to 2 cm but several times broader could be studied with the ANTARES facility of the FRM II neutron source in Garching providing thermal neutrons while larger objects required a beam of higher energy as available in the NECTAR facility of the same institution. The fast (fission) neutrons (1.5 – 2 MeV) of this site allowed investigating an area of interest inside a girder with a cross section of 23.5 x 49 cm². Internal features such as inclusions could be detected as well as a heterogeneous density distribution in glue layers.
Reducing risks and confining them to an acceptable level can be regarded as an essential commitment of technical applications for safety purposes and the public security measures.
Rating the efficiency of such measures requires an assessment of the risk that is supposed to be reduced. However, approaching this subject means to face a plethora of existing literature and an ongoing discussion if and how this can be achieved. On one hand, it remains a rather uncertain estimate particularly if it comes to rare interrupting events. On the other hand, making a decision responsibly requires a certain rational base. Several approaches exist for assessing risks, from verbal to quantitative, depending on the individual case. The frequently quoted Delphi technique represents the verbal one, others such as the Bayesian statistics a quantitative one or the consequence/probability matrix in-between.
A problem remains with understanding probability. Even if an event is highly unlikely it may happen right now.
The ultimate purpose of digital image filtering is to support the visual identification of certain features expressed
by characteristic shapes and patterns. Numerous recipes, algorithms and ready made programs exist nowadays
that predominantly have in common that users have to set certain parameters. Particularly if processing is fast
and shows results rather immediately, the choice of parameters may be guided by making the image 'looking
nice'. However, in practical situations most users are not in a mood to 'play around' with a displayed image,
particularly if they are in a stressy situation as it may encountered in security applications. The requirements for
the application of digital image processing under such circumstances will be discussed with an example of automatic
filtering without manual parameter settings that even entails the advantage of delivering unbiased results.
Safety and security both entail freedom from danger, i.e. unacceptable risk, whatever the cause might be. This
entails an understanding of the term 'risk' that is broadly used in areas such as economics, health, insurance etc.
According a rather popular definition used in the economical sciences for a long time (KNIGHT), risk has a lot
in common with uncertainty while the former is regarded quantifiable and the latter one is not. However, it has
to be taken into account that there are other understandings of 'uncertainty'. Particularly encountering rare
events never experienced before and recent contemporary definitions are weakening Knightfs differentiation.
This is reflected in some recent standard definitions. Attempts have been made to define a risk as common as
possible covering not only societal, ecological and financial areas but also natural and technical, ones. This is of
particular concern in non-destructive testing. This raises the question how far this can be achieved in a common
understanding, i.e. the discussion on this term seems not at all finished yet.
It is a common ambition to lower the risk by several actions including detection technologies. This entails that a
risk could be estimated somehow. The existence of numerous approaches indicates the complexity of this question.
The EFNDT (European Federation for Non-Destructive Testing) Working Group 5 took a commitment
also to tackle this central aspect of safety and security in its understanding as a bridging forum between these
two areas.
Some of the most perfidious remnants of war are mines threatening not
only combating parties but also innocent civilian lives. Moreover, they make large
areas inaccessible that otherwise could be used economically. This threat exists
even long after cessation of hostile actions. Up to now, the range of tools for humanitarian
demining is confined to mine detection dogs, metal detectors, prodders,
manual excavation tools and mechanical clearance machines (flails and tillers). In
1999, this gave rise for the Board of Directors of the EFNDT to suggest whether
detection methods commonly applied may also be helpful in detecting mines and the
like. It was proposed to transfer existing NDT technology to these areas. As a consequence,
Working Group 5 Antipersonnel Landmine Detection (APLD) was
founded to support this concept. As soon as the group became aware of the fact that
mines were encountered not only in the soil but also elsewhere the name was
changed to Antipersonnel Mine Detection (APMD).
It now is becoming more and more obvious that vicious devises such as mines
and bombs threatening lives everywhere represent an increasing risk to everybody.
In contrast, the particular interest in antipersonnel mine detection is decreasing in
favour of general security and safety. However, all these problems have in common
that such devices need to be detected in time, i.e. before they become effective. This
gives reason to extend the scope of the existing Working Group 5 accordingly. It is
of advantage to exchange existing knowledge and there is a chance to keep the
awareness of the mine problem alive by this way.
For the radiographic investigation of large cargo Containers the energies of conventional Xray tubes are inadequately for certain layer thicknesses. In that case the high energy radiation sources like electron accelerators and gamma radiators (60CO) are adequate for the non invasive inspection of large Containers because of the high penetration through thick materials. Multiple imaging even enables to distinguish between different materials. The main challenging task in air and sea cargo Container inspection is to improve the detectability of contraband and dangerous materials which are hidden in the heterogeneously packed Containers by detailed analysis of cluttered radiographic images. So it is to be expected that objects of organic substances like certain explosives or their precursors are hard to identify behind thick walls of heavy metal objects such as engine-blocks with flat bottom holes. The primary aim here is to investigate the detectability of dangerous materials (typically of light elements) in cargo Containers using high energy X-ray digital radiography.
Zur Durchstrahlung großer, dichter Objekte reichen die Energien, die mit normalen Röntgenröhren erzielt werden können, ab gewissen Schichtdicken nicht mehr aus. Als hochenergetische Strahlenquellen stehen Kobalt-60 und Elektronenbeschleuniger zur Verfügung. Als Einschränkung der Hochenergieradiographie ist der geringere Kontrast besonders beim Vorhandensein leichterer Objekte umgeben von Körpern bestehend aus schweren Elementen anzusehen. Es ist daher zu erwarten, dass Objekte aus organischen Substanzen hinter dicken Schwermetallwänden oder in Bohrungen von Metallblöcken schwer zu erkennen sind. Im Gegensatz zu einem Gammastrahler wie Kobalt-60 mit den beiden Spektrallinien um 1,3 MeV besitzt die Bremsstrahlung aus einem Beschleuniger einen wesentlichen Anteil an niederenergetischer Strahlung, der fließend zu höheren Energien übergeht. Es wird hier untersucht, welche Signaturen von leichten Materialien in einer Umgebung aus Schwermetall bis zu welcher Dicke und bis zu welchem Komplexizitätsgrad erkennbar sind. Mit einem Betatron (JME X-ray Betatron 7,5 MeV) und einem Matrixdetektor (Perkin Elmer XRD 1621) wurden Aufnahmen von unterschiedlichen leichten Objekten angefertigt, die zunehmend in eine Umgebung aus Schwermetallen gestellt wurden. Mit unterschiedlichen Energieeinstellungen wurde untersucht, inwieweit eine Materialerkennung hinter welcher Abschirmung möglich ist. Die experimentellen Ergebnisse werden mit Simulationen verglichen, die mit einer Software zur Modellierung von Durchstrahlungsverfahren (aRTist) erzeugt wurden. Dabei wird dem Problem der Aufhärtung bei Anwendung von Bremsstrahlung Rechnung getragen. Die Ergebnisse können sowohl zur Erkennung von Fremdkörpern in Maschinen oder Pumpen als auch zur Überprüfung von Frachtladungen im Bereich der öffentlichen Sicherheit und bei Zollkontrollen dienen.