The performance of thermal electro-optic infrared sensors (EO/IR) may be limited in certain specific circumstances, particularly for the detection and identification of targets embedded in an isothermal scene, i.e. when there is insufficient thermal contrast between the targets against their surrounding background. Such situations generally occur at the beginning and end of the day, but can also happen at any time during the day. One way to cope with this limitation is to employ EO/IR sensors that are sensitive to the polarization states of light. With this intention, Defence Research and Development Canada (DRDC) has developed thermal infrared multispectral and hyperspectral polarimetric imaging systems and spectral algorithms to extract the polarized radiance components of targets of interest, and use this additional information to enhance detection and identification while reducing false alarm rate. This paper presents experimental results from measurements using ground-based multispectral and hyperspectral polarimetric imaging sensors to acquire the polarized radiance of targets set up at multiple orientation angles with respect to the sensors lineof-sight (LOS). The objectives of the experiments were to study the phenomenology of polarized surface radiance in the Long-Wave Infrared (LWIR) and assess the effect of different materials on the resulting s-polarized and p-polarized spectral components. Experimental results show the advantages of thermal multispectral and hyperspectral polarimetric imaging sensors over conventional unpolarized ones to discriminate targets against their background, particularly during thermal cross-over periods.
Nathalie Roy, Alexandre Vallières, Daniel St-Germain, Simon Potvin, Michel Dupuis, Jean-Claude Bouchard, André Villemaire, Martin Bérubé, Mélanie Breton, Guillaume Gagné
A novel approach is used to characterize and compare the performance of night vision systems in conditions more
representative of night operation in terms of spectral content. Its main advantage compared to standard testing
methodologies is that it provides a fast and efficient way for untrained observers to compare night vision system
performances with realistic illumination spectra. The testing methodology relies on a custom tumbling-E target and on a
new LED-based illumination source that better emulates night sky spectral irradiances from deep overcast starlight to
quarter-moon conditions. In this paper, we describe the setup and we demonstrate that the novel approach can be an
efficient method to characterize among others night vision goggles (NVG) performances with a small error on the photogenerated
electrons compared to the STANAG 4351 procedure.
Surveillance operations and search and rescue missions regularly exploit electro-optic imaging systems to detect targets
of interest in both the civilian and military communities. By incorporating the polarization of light as supplementary
information to such electro-optic imaging systems, it is possible to increase their target discrimination capabilities,
considering that man-made objects are known to depolarized light in different manner than natural backgrounds. As it is
known that electro-magnetic radiation emitted and reflected from a smooth surface observed near a grazing angle
becomes partially polarized in the visible and infrared wavelength bands, additional information about the shape,
roughness, shading, and surface temperatures of difficult targets can be extracted by processing effectively such
reflected/emitted polarized signatures. This paper presents a set of polarimetric image processing algorithms devised to
extract meaningful information from a broad range of man-made objects. Passive polarimetric signatures are acquired in
the visible, shortwave infrared, midwave infrared, and longwave infrared bands using a fully automated imaging system
developed at DRDC Valcartier. A fusion algorithm is used to enable the discrimination of some objects lying in
shadowed areas. Performance metrics, derived from the computed Stokes parameters, characterize the degree of
polarization of man-made objects. Field experiments conducted during winter and summer time demonstrate: 1) the
utility of the imaging system to collect polarized signatures of different objects in the visible and infrared spectral bands,
and 2) the enhanced performance of target discrimination and fusion algorithms to exploit the polarized signatures of
man-made objects against cluttered backgrounds.
Spectral sensors are commonly used to measure the intensity of optical radiation and to provide spectral information
about the distribution of material components in a given scene, over a limited number of wave bands. By exploiting the
polarization of light to measure information about the vector nature of the optical field across a scene, collected
polarimetric images have the potential to provide additional information about the shape, shading, roughness, and
surface features of targets of interest. The overall performance of target detection algorithms could thus be increased by
exploiting these polarimetric signatures to discriminate man-made objects against different natural backgrounds. This is
achieved through the use of performance metrics, derived from the computed Stokes parameters, defining the degree of
polarization of man-made objects. This paper describes performance metrics that have been developed to optimize the
image acquisition of selected polarization angle and degree of linear polarization, by using the Poincare sphere and
Stokes vectors from previously acquired images, and then by extracting some specific features from the polarimetric
images. Polarimetric signatures of man-made objects have been acquired using a passive polarimetric imaging sensor
developed at DRDC Valcartier. The sensor operates concomitantly (bore-sighted images, aligned polarizations) in the
visible, shortwave infrared, midwave infrared, and the long-wave infrared bands. Results demonstrate the improvement
of using these performance metrics to characterize the degree of polarization of man-made objects using passive
polarimetric images.
Electro-optical imaging systems are frequently employed during surveillance operations and search and rescue missions
to detect various targets of interest in both the civilian and military communities. By incorporating the polarization of
light as supplementary information to such electro-optical imaging systems, it may be possible to increase the target
discrimination performance considering that man-made objects are known to depolarize light in different manners than
natural backgrounds. Consequently, many passive Stokes-vector imagers have been developed over the years. These
sensors generally operate using one single spectral band at a time, which limits considerably the polarization information
collected across a scene over a predefined specific spectral range. In order to improve the understanding of the
phenomena that arise in polarimetric signatures of man-made targets, a new passive polarimetric imaging system was
developed at Defence Research and Development Canada - Valcartier to collect polarization signatures over an extended
spectral coverage. The Visible Infrared Passive Spectral Polarimetric Imager for Contrast Enhancement (VIP SPICE)
operates four broad-band cameras concomitantly in the visible (VIS), the shortwave infrared (SWIR), the midwave
infrared (MWIR), and the longwave infrared (LWIR) bands. The sensor is made of four synchronously-rotating
polarizers mounted in front of each of the four cameras. Polarimetric signatures of man-made objects were acquired at
various polarization angles in the four spectral bands. Preliminary results demonstrate the utility of the sensor to collect
significant polarimetric signatures to discriminate man-made objects from their background.
Electro-optic (EO) imaging systems are commonly used to detect civilian and military targets during surveillance
operations and search and rescue missions. Adding the polarization of light as additional information to such active and
passive EO imaging systems may increase the target discrimination performance, as man made objects are known to
depolarized light in different manner than natural background. However, while the polarization of light has been used
and studied in the past for numerous applications, the understanding of the polarization phenomenology taking place
with targets used in cluttered backgrounds requires additional experimentations. Specifically, the target contrast
enhancement obtained by analyzing the polarization of the reflected light from either a direct polarized laser source as
encountered in active imagers, or from natural ambient illumination, needs further investigation. This paper describes an
investigation of the use of polarization-based imaging sensors to discriminate civilian and military targets against
different backgrounds. Measurements were carried out using two custom-designed active and passive imaging systems
operating in the near infrared (NIR) and the long-wave infrared (LWIR) spectral bands. Polarimetric signatures were
acquired during two distinct trials that occurred in 2007, using specific civilian and military targets such as cars and
military vehicles. Results demonstrate to what extent and under which illumination and environmental conditions the
exploitation of active and passive polarimetric images is suitable to enable target detection and recognition for some
events of interest, according to various specific scenarios.
Rocket detection over a wide field of view is an important issue in the protection of light armored vehicle. Traditionally, the detection occurs in UV band, but recent studies have shown the existence of significant emission peaks in the visible and near infrared at rocket launch time. The use of the visible region is interesting in order to reduce the weight and cost of systems. Current methods to detect those specific peaks involve use of interferometric filters. However, they fail to combine wide angle with wavelength selectivity. A linear array of volume holographic elements combined with a curved exit slit is proposed for the development of a wide field of view sensor for the detection of solid propellant motor launch flash. The sensor is envisaged to trigger an active protection system. On the basis of geometric theory, a system has been designed. It consists of a collector, a linear array of holographic elements, a curved slit and a detector. The collector is an off-axis parabolic mirror. Holographic elements are recorded subdividing a hologram film in regions, each individually exposed with a different incidence angle. All regions have a common diffraction angle. The incident angle determines the instantaneous field of view of the elements. The volume hologram performs the function of separating and focusing the diffracted beam on an image plane to achieve wavelength filtering. Conical diffraction property is used to enlarge the field of view in elevation. A curved slit was designed to correspond to oblique incidence of the holographic linear array. It is situated at the image plane and filters the diffracted spectrum toward the sensor. The field of view of the design was calculated to be 34 degrees. This was validated by a prototype tested during a field trial. Results are presented and analyzed. The system succeeded in detecting the rocket launch flash at desired fields of view.
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