There is strong evidence to suggest that polarimetric techniques offer significant improvements in the ability of electro-optic sensors to detect difficult targets in cluttered backgrounds. Many previous attempts to quantify the potential benefits have been hampered by an inability to gather all the polarimetric data simultaneously from a scene. Sequential data gathering can lead to artefacts in the polarimetric data which in turn lead to spurious and erroneous conclusions being drawn This paper describes work undertaken to build and test a pair of four camera, real time, polarimetric sensors that measure all four Stokes parameters simultaneously. One of the sensors operates in the visible waveband and the other in the near infrared. Example images obtained with both sensors are shown, together with measured target and background signal distributions for one of them. Preliminary results from this work show that the sensor can significantly improve target discrimination.
This paper describes a simple new optical technique for enhancing underwater target detection. Small-scale laboratory experiments using an idealised turbid medium consisting of polystyrene particles in water showed that detection ranges were up to an order of magnitude greater than those achieved using conventional methods. This work was carried out as part of the Electronic Systems Domain of the MOD Research programme.
The incorporation of polarisation sensitive optics offers considerable potential for improving the utility of remote sensing imaging systems operating within the visible or infrared wavebands. Systems now exist allowing measurement of the four components of Stokes vector arising from each pixel in the image. In order to develop the interpretation of polarimetric images, knowledge is required of the polarised directional reflectance properties of the materials in the scene, which determine the radiation reaching the sensor. Natural vegetation forms a significant element of many scenes observed with remote sensing systems. Although the size of a leaf may be below the spatial resolution of a system, the reflectance properties of individual leaves will affect the polarimetric data observed. This paper will report the results of measurements of the linear polarised bidirectional reflectance distribution function (BRDF) from two examples of leaves. The polarimetric properties of the directional reflectance from an individual leaf will depend on the surface and volume scattering properties. We report data on two leaves representing extreme cases of the leaf structure. Laurel (prunus laurecatious) has a nacreous surface creating a gloss finish to the leaf. Mullein (verbascum thapsus) has a highly pubescent surface, creating a highly diffuse surface reflectance. Measurements of the linear polarisation BRDF are reported at 632.8nm, 1064nm, 3.39pm and 10.6µm as a function of the polar scatter angles. These wavelengths characterise the polarised reflectance from the leaves under different conditions of absorption and scattering. In both cases the body of the leaf acts a highly diffuse reflector through multiple scattering, but this mechanism is only important when the absorption by the leaf constituents is low. In the spectral regions of moderate and high absorption the surface reflectance dominates. In the case of laurel the surface is relatively smooth, with an associated Brewster angle, whereas the data suggests the layer of hair covering a mullein leaf acts as an array of scattering sources.
A comparison is made between the backscattering Mueller matrices of two bead-blasted aluminium samples at both 632.8nm HeNe and 10.6micrometers CO2 lasers wavelengths. The samples are bead-blasted at two blasting pressures to vary the rms surface roughness and slope. These surfaces are characterized using a mechanical profilometer. Mueller matrices are determined with the detector fixed close to direct backscatter and the target angle varied between 0 degrees and 80 degrees. Results are presented of the angular variation of the major diagonal elements and off-diagonal elements m12 and m21 with increasing roughness to wavelength ratio. The copolarized scattering intensities and copolarized ratios derived from the Mueller matrices show similarities to microwave radar sea echo.
The backscattering Mueller matrices for a range of bead- blasted aluminium samples having a range of surface roughness and slopes are investigated using a 632.8nm linearly polarized HeNe laser. The analyzing polarizing optics and detector are fixed in the backscattering direction and the angle of incidence varied from 0 degrees to 90 degrees. A computer controlled Mueller matrix scatterometer determines the matrix elements. The incident polarization states are set via combinations of a polarizer and waveplates and the backscattered light is Fourier analyzed using a rotating compensator and fixed linear polarizer. The surfaces were characterized using a surface profilometer and the variation of the Mueller matrix elements with incidence angle determined. Only four elements of the normalized Mueller matrix are non-zero. Of these, three vary with incidence angle and the fourth is normalized to unity. A depolarization term PD calculated from the matrix elements is shown to decrease with increasing angle of incidence. Its value at normal incidence reduces with increasing bead-blast pressure.
The 8 - 12 micrometer polarization signatures of diffuse and specular aluminum geometrical darts were analyzed outdoors using a polarization sensitive thermal imager. Results of the degree and plane of polarization are presented for different thermal imager gain bands and weather conditions during a two week period. The 0 degree, 45 degree, 90 degree and 135 degree polarizer orientations were thermally calibrated and the S1 and S2 Stokes parameters shown as radiometric temperature differences. The effect on the polarization signatures of range is considered for these targets at 100 m and 370 m. A comparison of the degree of polarization to changes in the emission/reflection balance and to variations in the dart's complex refractive index is made.
Plastic mines are cheap, small, and difficult to detect using current methods. IR polarization discrimination where the mines present themselves as a flashing signal may be of potential benefit in finding surface laid or scattered types in a cluttered background. Results from a laboratory study show that plastic has significant 8-14 micrometers IR polarization in emission and reflection. We have constructed a 10.6 micrometers ellipsometer which has been used to measure the complex refractive indices of mine-like plastics. This apparatus was then modified to determine the degree of emission polarization. The measured degree of emission polarization has been compared to that predicted using the complex refractive index and good agreement found.
The copolarized backscattered intensity from surfaces composed of metallic particles on conducting flat substrates is analyzed experimentally as a function of the incidence angle. The analysis is done for particle sizes smaller than, comparable to, and larger than the incident wavelength (0.633 μm) and for low particle surface densities. Numerical calculations based on the extinction theorem for a onedimensional surface model consisting of an infinitely long cylinder located on a flat substrate for the same optical constants used in the experiment are also presented for qualitative comparison with the experimental results. This serves to analyze the effect of particle aggregation. For the surfaces with particles smaller than the incident wavelength, conclusions are drawn concerning the possible relevance of this study in radar wave scattering from the sea surface.