As a part of the Swedish mine detection project MOMS, an initial field trial was conducted at the Swedish EOD and
Demining Centre (SWEDEC). The purpose was to collect data on surface-laid mines, UXO, submunitions, IED's, and
background with a variety of optical sensors, for further use in the project. Three terrain types were covered: forest,
gravel road, and an area which had recovered after total removal of all vegetation some years before. The sensors used in
the field trial included UV, VIS, and NIR sensors as well as thermal, multi-spectral, and hyper-spectral sensors, 3-D laser
radar and polarization sensors. Some of the sensors were mounted on an aerial work platform, while others were placed
on tripods on the ground. This paper describes the field trial and the presents some initial results obtained from the
subsequent analysis.
KEYWORDS: Bidirectional reflectance transmission function, Polarization, Scattering, Reflectivity, Monte Carlo methods, Solid modeling, Ray tracing, Sun, Systems modeling, Geometrical optics
Model calculations have been done on 3-dimensional objects. The descriptions of the objects are in terms of facets that make it possible to use ray tracing geometrical optics as a technique to calculate the scattering from the objects. The Bidirectional Reflectance Distribution Function (BRDF) for different polarization states have been used to describe the reflectance at a certain facet. By using Monte Carlo calculations, the scattering from all the facet surfaces of the object is added to a total reflectance. Thus the total reflectance has been obtained for different polarization states and for different angles of reflectance throughout the whole scattering hemisphere. The model has been applied to rough surfaces of color and polarization contrast measures, for example the Degree Of Linear Polarization, has been calculated. Also, a simple background has been applied, in order to obtain contrast measures between the object and the background. Comparison is made with field measurements of a military vehicle.
The infrared reflectance and emittance of a double layer of silicon and silicon dioxide have been investigated by optical multilayer calculations and spectral and wavelength-integrated measurements. Low emittance in the interval 0.2 to 0.4 can be obtained simultaneously in both thermal atmospheric windows: 3 to 5 and 8 to 13 µm. These results are relevant for IR signature control. The sample consisted of a 0.9-µm Si and a 2.45-µm SiO2 layer on a Si wafer. The layers were grown by standard microelectronic chemical vapor deposition techniques. The key mechanism for lowering the emittance is the interaction between the SiO2 molecular reflectance band, around 9 µm, and interference effects in the double layer. Interference gives one peak in the 3- to 5-µm window, and a widening and strengthening of the SiO2 molecular reflectance band in the 8- to 13-µm window. The calculated spectra are in very good agreement with measured near-normal incidence reflectance spectra in the range 2.7 to 12.5 µm. The emittance of the samples heated to 61 °C was determined in the atmospheric windows using two heat cameras filtered for the respective intervals and equipped with polarizers. Emittance values for the sample in the two windows and the two main polarizations were determined as a function of emission angle from 10 to 60 deg. Qualitative agreement with values calculated from tabulated optical constants was obtained.
Infrared polarization has been used to investigate how well a partly covered object can be detected. It has earlier been shown that a covered object could be detected even though it was not at all possible to see the object measured in a conventional way without a polarizer. Also a measure of how much better an object is seen in polarization than without using polarization has been defined. This has earlier been applied to trip wires and partly covered surface laid personnel mines. In this work polarization measurement has been performed on realistic minefields, that has been setup by SWEDEC in Sweden. The goal has been to investigate how many mines and trip wires can be detected with polarization measurements in the LW region. The method is working well, but the present equipment are not very effective in finding mines and trip wires.
Polarization measurements in the infrared region have earlier been done on cenosphere surfaces. These surfaces have been constructed so that they have almost completely depolarized reflected and emitted radiations. It was also possible to construct the surface so that a desired normal incidence reflectivity and emissivity was achieved. The measurements were performed by measuring emissivity as a function of the emissive angle, from 5° to 85°, and for different polarization angles. From these angle measurements the polarization parameters such as Degree of Linear Polarization (DoLP) and θ can been calculated. In this work, model calculations of scattering from a rough surface consisting of cenospheres are performed, using Monte Carlo calculations on a geometrical optical model. The model calculations have given good results and can explain both the depolarization and the possibility to determine the emissivity, from the cenosphere surface, by changing the amount of gold deposited on the surface.
Polarization measurements in the IR region, especially in the 8-12 μm wavelength region (LW), are useful to detect man-made object in a natural environment. An example is surface laid mines partly covered by grass, dirt or sand. It has been shown that polarization measurements have improved the possibility to detect partly covered objects. This makes reconnaissance and surveillance sensors and warners more efficient. The equipment used in the measurements reported here is a IR Thermovision camera, in the region 8-12 μm. By applying a linear polarizing filter in front of the camera, it is possible to measure the Stokes parameters, “Degree of Linear Polarization” (DoLP) and “Theta”. The circular polarization component is regarded as small and is being neglected in these measurements. This article reports the results of surface scattering properties measurements on personal mines by using polarization. Measurements and simple model calculations performed on covered surface landmines in natural environment have been reported earlier. A more systematic investigation is reported here. The results indicate that IR polarization is a powerful tool to detect mines. Further, the emissivity as a function of emission angle has been measured for different coverage of the mine, and the DoLP has been calculated for different emission angle.
Camouflaged objects in a background, which are not possible to observe with conventional IR measurements without polarization, can be seen in polarization measurements. In this paper will be shown that polarization measurements increase the possibility to detect covered objects. The denial of polarization measurements of a covered object has earlier been achieved by construction of a surface covered with cenospheres. The emissivity as a function of angle of incidence has also been investigated on this newly developed surface, which can be designed to have emission properties decided in advance. The results indicate that it is possible to use the surface materiel as a means to adapt an object to a certain optical signature. It has also been shown that the surface almost completely depolarises the emitted radiation, which makes it more difficult to observe with a polarization measurement. These properties make the surface suitable as a reference surface for polarization measurements. A more systematic development of these surfaces and investigation are reported here.
Using passive IR polarization enhances the possibility to detect man made objects. Usually, man made objects consist of flat surfaces which polarize the emitted and reflected radiation, thereby increasing the contrast relative to an almost completely depolarized background. This makes reconnaissance and surveillance sensors and warners more efficient. In order to protect an object from being detected by a polarization sensitive sensor the surface of the object should be less sensitive to polarization. This is achieved by increasing the surface roughness. In this paper describes the use of cenosphere particles to increase the surface roughness and hence reducing the polarization. At the same time it is important to decide the value of the emissivity due to the need of contrast reduction. Evaporating gold with different coverage on the surface does this.
The equipment used here consist of two IR Thermovision cameras, one for the wavelength region 3-5 μm and the other for the region 8-12μm. By applying linear polarizing filters in front of the cameras, it is possible to measure the emissivity curves.
The emissivity of the used cenosphere samples varies between about 0.90 and 0.23. For all samples the radiation is close to Lambertian and they all depolarize the radiation completely.
Exploitation of polarization increases the contrast in imaging sensor and makes reconnaissance and surveillance sensors more efficient. This paper reports about two methods of polarization measurements. The used equipments are two IR Thermo vision 900 cameras, one in the wavelength region 3-5 micrometers and the other in the region 8-12 micrometers . The cameras have polarizing filters in front of the sensors. A calibration method is applied. In that way it is possible to correct for the signal contribution from the polarizing filters. The contribution can be separated into two parts, one consists of contributions to the detector signal due to the temperature of the filters and the other consists of reflections at the filter surfaces. The measurement methods are illustrated by laboratory measurements of surfaces of different surface roughness. The objectives of these measurements are to understand the physical properties of different surfaces for camouflage work. Also, calibrated sensitive polarization measurements of backgrounds are reported. The methods make it possible for the sensors to increase their ability for detection, recognition and classification.
Polarization measurements in the IR region are useful for the detection of man-made objects in natural environment. Surface laid landmines are examples of man-made objects that are difficult to detect, especially when a certain time have elapsed. As time goes by, both IR signature and degree of polarization are reduced, due to the dust coverage and the grass that has grown around the mines. Polarization measurements are useful when detecting Trip Wires. This article reports the results of IR polarization measurements compared to the measurements without a polarizing element. To simulate the dust and grasses that can cover the mines, a series of controlled covers are constructed. Measurements on this series are performed, and the results are shown in a diagram of relative Degree of Linear Polarization, (DoLP) as a function of coverage. The same is done for the case without a polarizer but here the relative radiance is shown. By comparing the two diagrams a measure is achieved of how effective the polarization measurements are with respect to the case without a polarizer. A simple parameter model of the coverage has been made and has been compared with the measurements. Measurements indicate that detection of trip wire is improved when using polarization measurements.
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