A new passive polarimetric imaging system collecting polarization
signatures in the visible and infrared bands
Daniel A. Lavigne*a, Mélanie Bretonb, Georges Fourniera, Mario Pichettea, Vincent Rivetb
a
Defence Research and Development Canada – Valcartier
2459 Pie-XI Blvd. North, Quebec, Qc, Canada G3J 1X5
b
AEREX Avionics Inc., 324 Saint-Augustin avenue, Breakeyville, Qc, Canada G0S 1E2
ABSTRACT
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.
Keywords: Polarimetric imaging sensor, infrared bands, target discrimination, polarized light, polarization
1. INTRODUCTION
Electro-optical imaging sensors are commonly used to collect spectral and geospatial data of numerous kinds of
scenarios ranging from civilian (e.g. rescue missions) to military operations (e.g. C4ISR related ones). Both civilian and
military communities can employ a broad range of imaging sensors, each with his own spatial and spectral
configurations. The addition of polarization to traditional spectral imaging devices makes it possible to increase the
target discrimination performance of man-made objects considering the fact that they depolarize light in different
manners than natural backgrounds.
Indeed, it is well known that the electro-magnetic radiation emitted and reflected from a smooth surface observed near a
grazing angle becomes partially polarized in the visible and infrared wavelength bands1. Therefore, polarimetric imaging
can be used for characterizing materials and observing contrast levels that are not detectable in conventional intensity
images2. Analyzing the backscattered light from targets of interest may reveal important features that may not be
discernible in intensity images. It has been reported that such polarimetric images are independent of the spatial nonuniformity of the illumination, since they are normalized by the local total intensity3.
*daniel.lavigne@drdc-rddc.gc.ca; phone 1 418 844-4000 ext.4157; fax 1 418 844-4511; www.valcartier.drdc-rddc.gc.ca
Infrared Imaging Systems: Design, Analysis, Modeling, and Testing XX, edited by Gerald C. Holst
Proc. of SPIE Vol. 7300, 730010 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.819011
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Numerous Stokes-vector imagers operating in diverse spectral bands have been developed and commercialized over the
years. These sensors generally operate using one single spectral band at a time, limiting considerably the polarization
information collected across a scene over a predefined specific spectral range. In order to improve the understanding of
the phenomena arising in polarimetric signatures of man-made targets, a new passive polarimetric imaging system was
designed to collect polarization signatures simultaneously using four broad-band cameras in the visible and infrared
spectral bands: the Visible Infrared Passive Spectral Polarimetric Imager for Contrast Enhancement (VIP SPICE).
2. PASSIVE POLARIMETRIC IMAGING SENSOR
This section presents the hardware setup and the data acquisition process of the VIP SPICE system. The metrics used to
perform the contrast enhancement of the targets are also presented.
2.1 Hardware System
The Visible Infrared Passive Spectral Polarimetric Imager for Contrast Enhancement (VIP SPICE) operates a suite of
four cameras concomitantly in the visible (VIS), the shortwave infrared (SWIR), the midwave infrared (MWIR), and the
long-wave infrared (LWIR) bands. Additionally, a seven-position filter wheel containing six different filters (400 nm,
436 nm, 490 nm, 530 nm, 550 nm, 690 nm) is mounted in front of the visible camera. The sensor is made of four
synchronously-rotating polarizers mounted in front of each of the four cameras. A single belt links all the polarizers so
their rotation is synchronized. This setup enables the acquisition of a scene at different polarization angles. Linear
polarizers are oriented successively at 0, 45, 90, and 135 degrees and along specific time intervals. The four-sensor suite
is mounted on a motorized pan & tilt platform device. The calibration, data acquisition, and data processing are fully
automated. The computer controls the entire capture process: from aligning the pan & tilt toward the target to the data
acquisition. The capture process includes the calibration, the capture and the display of the images. The cameras are
plugged individually to an input frame grabber card (DFG/MC4). For every capture, five images are acquired at each
polarization angle by all cameras. Then, the polarizers are flipped up and five unpolarized images are acquired. The
entire capture process takes less than 2m30s to perform. Precise geolocation of the sensor and the targets are achieved
using a GPS receiver and a range finder, respectively. A 30 degrees visible camera provides a global view of the scene.
Acquired data are remotely accessible and process on a standalone workstation. Figure 1 illustrates the setup of the VIP
SPICE sensor.
6-filters
wheel
Front
p. el
Polarizers
p. el
p,vjp,
(35 jim)
Visible
(0.4-0.9 jim)
LWIIR
(8-12 jim)
TSPWfl
(1 .0-1 .9 jim)
Rang
Finder
Visible
oo
GPS
Pan&Tilt
Fig. 1. Setup of the VIP SPICE sensor. The sensor operates a suite of four cameras concomitantly in the visible (VIS), the
shortwave infrared (SWIR), the midwave infrared (MWIR), and the longwave infrared (LWIR) bands.
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The VIP SPICE’s visible camera is a CCD camera imager (LM165 from Lumenera) that operates in the 400 to 900 nm.
The camera has 1392 x 1040 pixels resolution and a field of view (FOV) of 5 degrees. The polarizer used is a linear
polarizer from Edmund Optics. The shortwave infrared camera is a solid-state InGaAs imager (SU320MSW-1.7RT from
Sensors Unlimited) that operates in the 0.9 to 1.7 µm. The camera has 320 x 256 pixels with a FOV of 20 degrees. The
SWIR band uses aluminum microwires on a glass substrate as a polarizer from Versalight. The midwave infrared camera
is an IR-M700 from Mitsubishi. It is a focal plane array (platinum silicide) that operates from 1.2 to 5.9 µm. It has a
focal plane array of 801 x 512 pixels with a FOV of 12 degrees. The polarizer used is a wire-grid ZnSe from Medway
Optics. The longwave infrared camera used is an uncooled microbolometer E6000 Thermal Imager from Nytech that
operates in the 8 to 12 µm spectral range and has a 640 x 480 pixels array infrared module, with a FOV of 10 degrees.
The gain and the offset can be set to a manual mode. The Non-Uniformity Correction (NUC) is done by closing the front
panel before the acquisition. The field of view is around 10°x10°. The polarizer used is a wire-grid ZnSe from Reflex
Analytical Corp. positioned directly in front of the lens.
Figure 2 shows a close-up view of the VIP SPICE sensor.
Fig. 2. Close-up view of the VIP SPICE sensor.
Dissimilarities in spectral bands and sensor resolutions generate the problem of registering together pixels of a same
object feature of the scene according to each sensor’s field of view and spatial/spectral resolutions. This is particularly an
issue if image fusion algorithms are expected to be applied afterward. Table 1 shows a summary of the main
characteristics of the four cameras used within the VIP SPICE sensor system.
Table 1. Main characteristics of the cameras embedded within the VIP SPICE sensor system
Bands
Camera
Company
Sensor
Spectral Band
VIS
SWIR
MWIR
LWIR
LM165
SU320MSW-1.7RT
IR-M700
E6000 (DRS)
Lumerera
Sensors Unlimited
Mitsubishi
Nytech
CCD
InGaAs
Focal plane array
(platinum silicide)
Microbolometer
uncooled
400 nm – 900 nm
0.9 – 1.7 µm
1.2 – 5.9 µm
8 – 12 µm
(4.0 – 5.9 µm)
Sensor size
1392 x 1040
o
320 x 256
801 x 512
o
Lens FOV
5
20
Polarizer
Glass polarizer
Microgrid
12
o
Microgrid ZnSe
640 x 480
10o
Microgrid ZnSe
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2.2 Acquisition Process
The acquisition process can be divided in two steps. The first one is the calibration stage where unpolarized images are
acquired and where the NUC is made on the thermal sensors. The second step is the acquisition of polarized images. The
automation and the fine control of this optical bench are done using embedded softwares within the VIP SPICE system.
The VIP SPICE’s acquisition process is illustrated in figure 3a. In automatic mode, once the capture button is pressed,
the motorized pan & tilt is initially oriented toward the target. If the acquisition mode is manual, the calibration starts
right immediately. The front panel facing all sensors is closed and the polarizer panel is raised. The LWIR and MWIR
sensors are then calibrated. Then, the front panel is raised, the auto-gain parameter is turned off and five unpolarized
images are captured. The seven-position filter wheel in front of the visible camera is rotated iteratively to the next filter
position, and unpolarized images saved. Once the calibration is done, the front and the polarizers’ panels are lowered,
and the LWIR and the MWIR are calibrated with the polarizer facing the sensors. When the polarizers are lowered
down, the four-state polarizations are captured (fig. 3b). The seven-position filter wheel of the visible camera is then
rotated and polarized images are captured. At the end of the acquisition process, metadata such as the date, time, the pan
& tilt orientation, the GPS data, the range finder data and the sensors setting are all saved into a log file. The total
acquisition time for one single target is 2m30.
Move Filter to position p
Orient Fan&Tilt
1.
olove Folarisers to n degrees
Calibrate
Yes
(a)
(b)
Fig. 3. The VIP SPICE’s (a) Acquisition process and (b) the Capture process details.
3. METHODOLOGY
This section presents the methodology used to collect and process the polarimetric images used in this research.
3.1 Performance metrics
To study the phenomenology and the potential of polarimetric images to increase target / background discrimination
performance of man-made objects, the VIP SPICE sensor collected polarimetric signatures using linear polarized
oriented at 0, 45, 90 and 135 degrees. Each sequence of four polarimetric images acquired simultaneously is used to
determine the first three parameters of the Stokes vector at each image pixel. They are used to describe the intensity of
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radiation (I) of the wave whose geometrical parameters characterize the electromagnetic wave polarization, the
amplitudes of the electromagnetic waves in mutually perpendicular directions ( Ax , Ay ), the intensity ( A 2 ), the phase
angle between Ax and Ay ( γ ) . They are illustrated in Equation 1 in which
2
2
⎡
⎡ I ⎤ ⎢ Ax + Ay
⎢Q ⎥ ⎢ A 2 − A 2
x
y
F=⎢ ⎥=⎢
⎢U ⎥ ⎢ 2 A A cos γ
x y
⎢ ⎥ ⎢
⎣V ⎦ ⎢ 2 Ax Ay sin γ
⎣
indicates time averaging.
⎤
⎥ ⎡S0 ⎤
⎥ ⎢ S1 ⎥
⎥=⎢ ⎥
⎥ ⎢S 2 ⎥
⎥ ⎢S ⎥
⎥⎦ ⎣ 3 ⎦
(1)
The Stokes parameters are all related by I 2 = Q 2 + U 2 + V 2 , so only three of them are independent. Q is the difference
in radiant intensity between the mutually orthogonal x and y directions used to specify Ax and Ay . I and Q can be
obtained by passing light waves through linear polarizers at 0 and 90 degrees respectively, while U indicates the excess
of radiation in the +45o direction over that in the +135o direction relative to the plane of vision. V is the circularly
polarized component of the radiation.
Using the Stokes parameters, additional metrics can be computed to describe the behavior of polarized light and yield
additional information about the roughness and material of the target surface. For instance, the degree of linear
polarization (DoLP) can be calculated using only the linear information and normalizing it to the intensity:
DoLP =
S1 + S 2
(2)
S0
The polarization angle (PA) represents the polarizer angle where the intensity should be the strongest.
PA =
⎛S
1
tan −1 ⎜⎜ 2
2
⎝ S1
⎞
⎟⎟
⎠
(3)
In this research, both metrics were computed using the first three Stokes parameters (S0, S1, and S2), obtained from the
VIP SPICE sensor. These metrics were used to assess the polarization of light as a way to discriminate some targets of
interest.
3.2 Data collection
The VIP SPICE imaging sensor was deployed for the first time in a dry prairie grassland environment in Alberta
(Canada) during Summer 2008. The sensor was mounted aboard a Genie boom and the images were acquired from nadir
to 45 degrees oblique views, at different elevation (from ground to 20 meters). Metallic plates, used as targets of interest,
were laid previously on the ground facing south-west. Each 60 cm x 60 cm aluminum targets has particular surface
characteristic in terms of color, coating, and roughness. They have a slight angle of 30 degrees wrt ground. Figure 4
shows the metallic plate layout.
The results presented in the next section use the aluminum metallic plate with tape strips shown as the second plate from
the bottom right in fig. 4. Part of this target was into the boom shadow for the duration of the two and half minute
acquisition period. It was located at a distance of 15 meters from the imaging sensor with a 25 degrees off-nadir angle.
The measurements were acquired late afternoon (5:30pm). The air temperature was 33oC and the sky was clear.
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Gray
Sandblasted
White (mat)
White (glossy)
Aluminum
Green (mat)
Golden
Blue (glossy)
Red (glossy)
Aluminum
(vertical)
Anodized
natural
Anodized black
(glossy)
Aluminum
(horizontal)
Aluminum with
tape
Anodized black
(mat)
Fig. 4. Metallic plates used as targets of interest. Each plate has different color, coating, roughness and surface characteristics.
Figure 5 illustrates the acquisition of polarimetric images of this target in all four bands of the VIP SPICE sensor,
according to their respective field of view. A slight portion of the target is in shadow, generated by the Genie boom. The
contrast enhancement between the target and the background is achieved using the degree of linear polarization and the
polarization angle4.
(a)
(b)
(c)
(d)
o
o
Fig. 5. Target sample as acquired in (a) the visible (FOV = 5 ), (b) shortwave infrared (FOV = 20 ), (c) midwave infrared
o
o
(FOV = 12 ), and (d) longwave infrared (FOV = 10 ) bands. Some portion of the plate is in the shadow originating
from the Genie boom.
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4. PRELIMINARY RESULTS
This section presents some preliminary results using the VIP SPICE sensor to collect polarimetric signatures of metallic
plates.
Figure 6 shows the image obtained in the longwave spectral band (LWIR). In the intensity image (S0), the metallic plate
is easily discriminated from the background (fig. 6a). The tape strips have a different emissivity than the plate. Edge
noise due to misregistration issues can be observed in the degree of linear polarization image (fig. 6b). The aluminum
metallic plate itself has a low degree of linear polarization. However, one can see differences in the textural information
in the region of interest of the plate. This has the potential to be used to discriminate the target from the background. The
polarization angle of the target is also different from the natural background (fig. 6c). The boom shadow seems to have
no effect on the three images.
(a)
(b)
(c)
Fig. 6. (a) Intensity image (S0) in the longwave infrared (LWIR) band, (b) degree of linear polarization (DoLP), and (c) angle of
polarization.
Figure 7 shows the image obtained with the midwave infrared (MWIR) camera. The tape strips and some shadow can be
observed in the intensity image (fig. 7a). The target is also well discriminated from the background. The DoLP image
shows that the upper right corner of the target is unpolarized (fig. 7b). As with the LWIR images, the edge noise due to
misregistration can be observed both in the DoLP and the polarization angle images (fig. 7c).
(a)
(b)
(c)
Fig. 7. (a) Intensity image (S0) in the midwave infrared (MWIR) band, (b) degree of linear polarization (DoLP), and (c) angle of
polarization.
Figure 8 shows the image obtained with the shortwave (SWIR) camera. This time, the metallic plate is almost
completely lost within the background in the intensity image, although some shadow information reveals the target
presence in the scene (fig. 8a). In the degree of linear polarization image, the metallic region is less polarized than the
environment and a smoother texture can be perceived (fig. 8b). Another interesting feature is that the region shadowed
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by the boom also shows less polarization than the background. The angle of polarization shows no relevant information
(fig. 8c).
(a)
(b)
(c)
Fig. 8. (a) Intensity image (S0) in the shortwave infrared (SWIR) band, (b) degree of linear polarization (DoLP), and (c) angle of
polarization.
Figure 9 shows the image obtained with the visible camera. In the intensity image, the target is well defined, some strips
are seen on the plate and there is natural distinction between the shadowy and illuminated regions (fig. 9a). As in the
midwave infrared band (MWIR), the same unpolarized part of the target (upper right corner) can be observed (fig. 9b).
However, there is a marked difference between the polarization angle of the shadowed and the remainder of the target
(fig. 9c), which is not the case with the MWIR.
(a)
(b)
(c)
Fig. 9. (a) Intensity image (S0) in the visible band, (b) degree of linear polarization (DoLP), and (c) angle of polarization.
4. CONCLUSION
In order to improve the understanding of the phenomenology associated with polarization signatures of man-made
targets, a new polarimetic imaging system has been designed and developed at DRDC Valcartier. The VIP SPICE sensor
collects polarization signatures using four broad-band cameras from the visible to longwave infrared band. It has been
deployed successfully in the field for the first time during Summer 2008. The objectives of the experiments conducted
were to confirm that this new imaging system was operational and able to collect relevant polarimetric signatures of
different man-made objects. Fifteen diverse metallic plates have been used as surrogates of man-made objects. All
targets were aluminum made with different top coating, color, surface characteristics and roughness. To analyze the
behavior of polarized light and yield additional information about the target surface characteristics, the degree of linear
polarization and the polarization angle have been used as performance metrics.
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Preliminary results were showed for a single aluminum plate covered with strips of tape. A small portion of the target
was located in shadow area. In the visible band, the target was easily detectable in both shadowed and illuminated
regions of the plate. A small portion of the plate was unpolarized, similarly to the results obtained in the midwave
infrared band. However, contrary to the MWIR, there is a marked difference in the visible band between the shadowed
and the remainder regions of the target. For the image of the plate acquired by the SWIR camera, almost no polarization
information was valuable to discriminate the target from its background. However, some shadow information reveals the
presence of the target in the SWIR. Moreover, using the degree of linear polarization, the target seems to be less
polarized and to have a smoother texture than the neighborhood environment. In the MWIR band, the target, the shadow
and the tape defect are all well discriminated from the background. The degree of linear polarization (DoLP) image
showed that some portion of the target is unpolarized in the MWIR. Finally, as it is the case in the LWIR band, the edge
noise due to misregistration can be observed both in the DoLP and polarization angle images in the MWIR band.
The polarization signatures collected by the VIP SPICE sensor were successfully used to enhance the contrast level
associated with the polarization state of man-made targets against their background. Additional backgrounds and targets
will be considered in future field trials. The development of additional performance metrics is also mandatory in order to
use the polarization of light as additional information to improve the discrimination performance of target detection
algorithm.
REFERENCES
[1]
[2]
[3]
[4]
Cremer, F., Schwering, P.B.W., de Jong, W., Schutte, K., and de Jong, A.N., “Infrared polarization measurements of
targets and backgrounds in a marine environment”, Proc. SPIE 4370, 169-180 (2001).
Tyo, J.S., Rowe, M.P., Pugh, E.N. and Engheta, N., “Target detection in optical scattering media by polarizationdifference imaging”, Appl. Opt. 35, 1855-1870 (1996).
Solomon, J.E., “Polarization imaging”, Appl. Opt. 20, 1537-1544 (1981).
Lavigne, D.A., Breton, M., Fournier, G., Pichette, M., Rivet, V., “Development of performance metrics to
characterize the degree of polarization of man-made objects using passive polarimetric images”, Proc. SPIE 7336,
7336-48 (2009).
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