Trip wire detection using polarimetric IR

ABSTRACT
In this paper we present a method for passive stand off detection of trip wires. The concept described has the ability to detect trip wires camouflaged in a wide variety of natural backgrounds. The system includes an infrared camera using the 35 micron band and a polarising filter. A simple software algorithm is used to dewarp the trip wire in the scenes.
Therefore, to progress more effective clearance operations there is an obvious need for research in systems that
automatically (and safely) detect trip wires. The challenge is to develop an innovative and robust approach for detecting trip wires camouflaged in a wide variety of backgrounds including vegetation. The approach must possess a low false alarm rate and can lend itself to be implementable in real-time. In this paper we present a method which can be used either in an autonomous detection system, or in conjunction with other sensors to passively detect trip wires from a stand-off position. The idea is to exploit the polarimetric JR features of the light scattered and emitted in a scene. In general, man-made objects have geometric regularities and smother surfaces, which
result in polarisation features different from most natural backgrounds. By features it is
demonstrated to be possible to pick out the trip wires in a complex background, even when portions of the wires should be obscured by foliage.
fragmentation mines is a serious potential hazard to personnel operating in the field. For the operational deminer, the
detection of the trip wires is every bit as important as the detection of the minimum-metal mines, and yet attracts only a tiny fraction of the research efforts'.
sources that may be reflected or scattered off of the surface into the camera's field of view.
A complete description of a beam of light including intensity and the state of polarisation is given by the conventional Stokes vector representation, a four element real intensity vector.
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2. THEORY
Polarimetry is the measurement of the complete state of polarisation including any unpolarised component. The polarisation
effects are mainly dependent on the roughness and the geometry of the surfaces in the scene, the sensor, and any light
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Correspondence:
Email: jffi?ddre.dk; Telephone: +45 3915 1766; Fax: +45 3929 1533
In Detection and Remediation Technologies for Mines and Mine/ike Targets V, Abinash C. Dubey, James F. Harvey, J. Thomas Broach, Regina E. Dugan, Editors, Proceedings of SPIE Vol. 4038 (2000) • 0277-786X/00/$1 5.00
The current deminer method for detection and localisation of trip wires is by use of hand or a trip wire feeler (often a thin rod), which repeatedly is inserted parallel to the ground and raised slowly upwards until above all vegetation. Following the trip wire search, vegetation is cut down to about 5 cm above ground. However, in dense vegetation the trip wire search and cutting are performed in several steps. In manual clearance operations the search for trip wires and removal of vegetation is the most time consuming part and roughly estimated takes up about 60%-80% of the deminer's operational time.
(1)
5=
-1
so
S2
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The first element of the Stokes vector 5 give the intensity of the beam. The other three elements of the Stokes vector
describe the polarisation state for horizontal, +45°, and circular polarised light. The degree of linear polarisation, DoLP, is found from:
Keywords: Trip wire, polarisation, infrared, landmine detection
1. INTRODUCTION
Today, most research institutes dealing with landmine detection as a topic and even within many mine action communities there is a stereotype landmine image, where the only challenge is considered to be the detection of the small low- or nonmetallic anti-personnel blast mine. Indeed, this plastic cased pressure operated mine is difficult to detect. However, to the well protected deminer adhering strictly to standard operating procedures, blast AP mines are not the main threat. This mine type requires a direct and often substantial pressure to go off, and further the plastic casing creates a limited fragmentation hazard, which fortunately seldom result in serious injury or death. The presence of trip wires assisted fragmentation mines (stake mines, bounding mines and directional mines of the Claymore type) do comprise a substantial higher lethal risk to the deminer. It could from a naive point of view seem, peculiar, that these mines being visible (many of them are placed above ground) and further are easily detectable due to the high metallic content, should give reason to any particular concern. However, these mine types do not, in contrast to most blast mines, require a direct contact to the mine itself to initiate a detonation, and do in addition have a lethal range, which by far exceeds the length of the trip wire. A deminer's accidental "tripping" the wire of a fragmentation mine will for sure result in a severe bulleting. In general and reality, trip wire initiated