We present adaptations of the channelled spectropolarimetry technique, a method which allows both spectral and polarization information to be captured in a single integration period. The first adaptation uses a mathematical decomposition of the system matrix, which is then modified for imaging spectropolarimetry; the second adaptation is applied first to a single-point and then to an imaging system, for which we also show applications and measurements from experimental work.
We present the implementation of snapshot imaging spectropolarimetry in a short-wave infrared (SWIR) system. It is the first of its kind to provide imaging spectropolarimetry with no moving parts and <b>snapshot</b> capability. This has applications in many fields, such as mining, biomedical imaging, and astronomy. The SWIR Computed Tomographic Imaging Channeled Spectropolarimeter (CTICS) is a snapshot imaging spectropolarimeter with 54X46 pixel spatial resolution and 10-band spectral resolution from 1.25-1.99 μm for the purpose of object identification. First, we present the design of the two main parts: the Computed Tomography Imaging Spectrometer (CTIS) and the channeled spectropolarimetry components. A discussion follows on the reconstruction technique. We then present the final assembled system and testing results.
Spectrometry and polarimetry measurements are important to modern science and engineering in an extremely wide variety of fields such as atomic and chemical processes, materials identification and characterization, astronomy, remote sensing, and stress analysis. The basic principle is that when light is emitted or absorbed by, scattered or reflected from, or transmitted through a physical material, its spectral content and polarization state are often affected. Analysis of the changes imposed by these processes then has the potential to reveal useful information about the sources. Example applications are: (1) stress-induced birefringence (photoelasticity); (2) remote sensing, object discrimination, shape measurement; (3) communications (polarization shift keying, deterimental effects on fiber networks); (4) astronomy (solar magnetic fields); (5) scattering, materials identification (retinal nerve fiber layer thickness measurement); (6) ellipsometry (materials characterization, complex index of refraction, layer thicknesses); (7) atomic physics; (8) displays (color LCDs merge colorimetry and polarization).
Channeled spectropolarimetry is a technique for measuring the spectral dependence of the polarization state of light. Passive polarization optics are used to encode the spectral dependence of the four Stokes components s<SUB>k</SUB> into a single irradiance spectrum. We treat the technique as a linear operator and compute its singular value decomposition numerically. The resulting singular functions divide into three distinct groups representing s<SUB>0</SUB>, s<SUB>1</SUB> and mixtures of s<SUB>2</SUB> and s<SUB>3</SUB>. The corresponding singular values indicate that measurements of the latter two groups will have signal-to-noise ratios reduced form that of s<SUB>0</SUB> by factors of 0.6 and 0.4 respectively. The structure of the singular vectors is in agreement with a separate estimate of the system's resolution.
We present and analyze a technique for snapshot imaging spectropolarimetry. The technique involves the combination of channeled spectropolarimetry with computed tomography imaging spectrometry (CTIS). Channeled spectropolarimetry uses modulation to encode the spatial dependence of all four Stokes parameters in a single spectrum. CTIS is a snapshot imaging spectrometry method in which a computer-generated holographic disperser is employed to acquire dispersed images of the target scene, and both spatial and spectral information is reconstructed using the mathematics of computed tomography. The combination of these techniques provides the basis for a snapshot imaging complete Stokes spectropolarimeter which can be implemented with no moving parts. We present results of a simulation that we did using four input Stokes vectors that varied with wavelength. The reconstruction took into account dispersion from the retarders and that low frequency components will be missing in CTIS.
Figures of merit for optimization of a complete Stokes polarimeter based on its measurement matrix are described which are not limited in their application to cases in which four measurements are used in the determination of a single Stokes vector. Singular value decomposition and probability theory are used to investigate the behavior and significance of these figures of merit. Their use to optimize a system consisting of a rotatable retarder and fixed polarizer indicates that a retardance of 132° (approximately three-eighths wave) and retarder orientation angles of ±51.7° and ±15.1° are favorable when four measurements are used. The performance of this system is demonstrated with experimental data.