Co-polarized Intensity Research Articles

Integrating intensity and context for improved supervised river ice classification from dual-pol Sentinel-1 SAR data

River ice is a major contributor to flood risk in cold regions due to the physical impediment of flow caused by ice jamming. Although a variety of classifiers have been developed to distinguish ice types using HH or VV intensity of SAR data, mostly based on data from RADARSAT-1 and -2, these classifiers still experience problems with breakup classification, because meltwater development causes overlap in co-polarization backscatter intensities of open water and sheet ice pixels. In this study, we develop a Random Forest classifier based on multiple features of Sentinel-1 data for three main classes generally present during breakup: rubble ice, sheet ice and open water, in a case study over the Athabasca River in Canada. For each ice stage, intensity of the VV and VH backscatter, pseudo-polarimetric decomposition parameters and Grey Level Co-occurrence Matrix texture features were computed for 70 verified sample areas. Several classifiers were developed, based on i) solely intensity features or on ii) a combination of intensity, pseudo-polarimetric and texture features and each classifier was evaluated based on Recursive Feature Elimination with Cross-Validation and pair-wise correlation of the studied features. Results show improved classifier performance when including GLCM mean of VV intensity, and VH intensity features instead of the conventional classifier based solely on intensity. This highlights the complementary nature of texture and intensity for the classification of breaking river ice. GLCM mean incorporates spatial patterns of the co-polarized intensity and sensitivity to context, while VH intensity introduces cross-polarized surface and volume scattering signals and is less sensitive to wind than the commonly used co-polarized intensity. We conclude that the proposed method based on the combination of texture and intensity features is suitable for and performs well in physically complex situations such as breakup, which are hard to classify otherwise. This method has a high potential for classifying river ice operationally, also for data from other SAR missions. Since it is a generic approach, it also has potential to classify river ice along other rivers globally.

Determination of target depth in a turbid medium with polarization-dependent transmitted signals.

We demonstrate a novel method for target depth determination in a turbid medium with experiments and Monte Carlo simulations. This method relies on the strong dependence of transmitted co-polarized intensity on target depth. Such a dependence originates from the inclusion of certain diffuse photons in the co-polarized intensity. A target of stronger scattering located closer to the transmitter results in stronger photon divergence and hence weaker co-polarized intensity at the receiver of a finite aperture. On the other hand, the degree of polarization (DOP) carries only information about ballistic and snake photons. It is weakly dependent on the target depth. The DOP data can be used as a reference of absolute scattering strength in the turbid system. Our experimental and Monte Carlo simulation results show the feasibility of the proposed target depth determination method. Meanwhile, it is shown that an appropriate time-gating process could help in improving the accuracy of target depth. In addition, the results show that the proposed method has quite large applicable ranges of scattering coefficient and absorption coefficient.

Optical Imaging and Tissue Characterization with Polarization Discrimination of Time-Gated Signals

In this paper we demonstrated the effectiveness of imaging in a tissue phantom with isotropic scattering using polarization discrimination combined with the time gating method. This simple polarization discrimination technique was shown ineffective when it was applied to filamentous tissues. In this situation, we utilized the time-gated degree of polarization (DOP) imaging technique to show that the DOP measurement was quite effective for high-quality imaging of objects in filamentous tissues. We also applied this technique to the characterization of myocardial tissues and showed the difference between normal and abnormal tissues. In addition, we demonstrated a novel method for target depth determination in a turbid medium based on co-polarized light measurements. This method relied on the strong dependence of transmitted co-polarized intensity on target depth.

Determination of the depth of a scattering target in a turbid medium with polarization discrimination of transmitted signals.

We demonstrate the feasibility of a novel method of determining target depth in a turbid medium through Monte Carlo simulations and experiments. The method is based on the strong and weak dependencies of the copolarized component and the degree of polarization (DOP), respectively, of the transmitted intensity on the target depth. The two-way measurements of the copolarized intensity can be used for determination of target depth, whereas the transversely scanned DOP results are used for estimating the two-dimensional image in a turbid system. The combination of these two sets of data could provide useful results for estimating three-dimensional images.

Comparison of focusing properties of conventional and diffractive lenses

We compare focusing properties of conventional lenses and diffractive lenses (DLs). To that end, we use a converging electric-dipole wave as a model of an optimal converging wavefront that emerges from a conventional lens system. Our numerical results show that DLs have better focusing properties than conventional lenses for f-numbers less than 1. Although both types of lenses give nearly indistinguishable co-polarized intensity components in the focal region at low f-numbers, DLs provide much lower longitudinally polarized components. However, at sufficiently low Fresnel numbers and high f-numbers DLs fail to provide a well-focused energy distribution.

Numerical and asymptotic results for focusing of two-dimensional waves in uniaxial crystals

Recently exact integral representations were presented for focusing of two-dimensional (2D) waves through a plane interface into a uniaxial crystal. These exact results were given for the special case in which (i) the incident field is a 2D transverse magnetic (TM) wave and (ii) the optical axis of the crystal lies in the plane of incidence, implying that we have a 2D vectorial problem with no coupling between transverse electric (TE) plane waves and TM plane waves upon reflection and refraction at the interface of the crystal. Here we apply these integral representations to obtain numerical results for focused TM fields inside positive and negative uniaxial crystals with several different orientations of the optical axis in the plane of incidence. Our numerical results show that the co-polarized intensity is symmetric about the direction of the Poynting vector of the transmitted axial ray, while the longitudinally-polarized intensity does not have this symmetry. Further it is shown that when the optical axis in the crystal makes a large angle with the interface normal, the longitudinally-polarized component of the electric field can be significant even for paraxial focusing geometries.

Optical free-path-length distribution in a fractal aggregate and its effect on enhanced backscattering

A free-path-length distribution function (FPDF) of multiply backscattered light is theoretically derived for a fractal aggregate of particles. An effective mean-free path-length l(D) is newly introduced as a measure of randomness analogous with a homogeneously random medium. We confirm the validity of the FPDF by demonstrating agreement between the dimensions designed for a particle distribution generated by a random walk based on the derived FPDF and estimated by the radius of gyration method. The FPDF is applied to Monte Carlo simulations for copolarized multiply backscattered light from the fractal aggregate of particles. It is shown that a copolarized intensity peak of enhanced backscattering in the far field decreases in accordance with theta(2-D) and has an angular width of lambda/l(D). This spatial feature of the backscattering enhancement corresponds to that of the copolarized intensity peak produced from a homogeneously random medium with a dimension of D = 3. As a result, the validity of the model for the fractal structure of particle aggregates and the applicability of the derived FPDF are confirmed by the numerical results.

Polarization Properties of the Enhanced Backscattering of Light from the Fractal Aggregate of Particles

The polarized intensity enhancement of the light backscattered multiply from a fractal aggregate of particles is numerically investigated using Monte Carlo simulations. It is confirmed only for the co-polarized intensity component that an effective mean free pathlength lD defined for a fractal medium has the same physical meaning as a mean free pathlength l for a homogeneously random medium. Furthermore, co- and crosspolarized components of the backscattering intensity enhancement decrease in accordance with θ2.16-1.03D and θ4-1.8D in the far-field plane and with ζ0.93D-3.94 and ζ1.07D-3.98 in the boundary plane between the scattering medium and the air. Therefore, the two-dimensional Fourier transform relation is satisfied only between the co-polarized intensity distributions in the far-field and the boundary planes but not the cross-polarized ones with the accuracy of the numerical simulation. Finally, we notice the applicability of the enhanced backscattering light to estimating the dimension of the absorbent fractal medium on the basis of the result that the slope of the intensity decay of the intensity peak is directly proportional to the dimension of the medium but is independent of the absorption.