Modelling and instrumentation for polarized light imaging and spectroscopy of scattering media (original) (raw)
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Degree of polarization as an objective method of estimating scattering
Journal of the Optical Society of America A, 2004
A new method of determining objectively the amount of scattered light in an optical system has been developed. It is based on measuring the degree of polarization of the light in images formed after a double pass through the system. A dual apparatus composed of a modified double-pass imaging polarimeter and a wavefront sensor was used to measure polarization properties and aberrations of the system under test. We studied the accuracy of the procedure in a system that included a lanthanum-modified lead zirconate titanate (PLZT) ceramic plate able to generate variable amounts of scattered light as a function of the applied voltage. Changes in the voltage applied to the ceramics plate modified significantly the scattering contribution while hardly altering the wave-front aberration. The degree of polarization was well correlated with the level of scattering in the system as determined by direct-intensity measurements at the tails of the double-pass images. This indicates that this polarimetric parameter provides accurate relative estimates of the amount of scattering generated in a system. The technique can be used in a number of applications, for example, to determine objectively the amount of scattered light in the human eye.
Polarization imaging: principles and integrated polarimeters
IEEE Sensors Journal, 2002
Polarization is a general descriptor of light and contains information about reflecting objects that traditional intensity-based sensors ignore. Difficult computer vision tasks such as image segmentation and object orientation are made tractable with polarization vision techniques. Specularities, occluding contours, and material properties can be readily extracted if the Stokes polarization parameters are available. Astrophysicists employ polarization information to measure the spatial distribution of magnetic fields on the surface of the sun. In the medical field, analysis of the polarization allows the diagnose of disease in the eyes. The retinae of most insect and certain vertebrate species are sensitive to polarization in their environment, but humans are blind to this property of light. Biologists use polarimeters to investigate behaviors of animals-vis-à-vis polarization-in their natural habitats. In this paper, we first present the basics of polarization sensing and then discuss integrated polarization imaging sensors developed in our laboratory.
Comparison of Methods for Reducing the Effects of Scattering in Spectrophotometry
Light scattering provides a problem in optical spectroscopy as the relationship between attenuation and absorption in the presence of scattering is nonlinear. Three simple methods of reducing the effects of scattering and hence returning to an approximately linear relationship are considered in this paper, namely, extracting light that has maintained its original polarization state through subtraction of orthogonal polarization states, use of an added absorber, and spatial filtering. These can all be applied relatively easily to conventional spectrophotometers. However, there is an inevitable trade-off between the accuracy of the measurement and the signal-to-noise ratio as scattered light is rejected from the detector. It is demonstrated that polarization subtraction is the most efficient technique at selecting weakly scattered photons from a scattered light background as it enables the relationship between attenuation and absorption coefficient to become more linear while maintaining a higher number of detected photons. In practical use, the drawback of polarization subtraction over added absorber and spatial filtering methods is that a large dc background light level is maintained, which contributes to a higher shot noise. This means that when the scattering coefficient is high (ls ! 7 mm À1 ) the added absorber method offers better performance for shot noise limited detection.
Active polarization compensation and goniometer for angularly resolved light scattering measurements
Applied Optics, 1988
A method for active polarization compensation for a scanning goniometer is presented. This is part of instrumentation constructed for measurements of light scattering by particulates in a high voltage spark. Individual optical element characterization is made using ellipsometry. Mueller matrix calculations are used to model the optical system. The inverse of the optical system is used to calculate the necessary input polarization state. A polarized source with angularly controlled halfwave and quarterwave retarders is used to introduce the necessary polarization state into the goniometer.
Polarization is a fundamental property of light and an important concept in the field of optics [2]. Three different polarization states can exist e.g, linear circular and elliptical. In this experiment, we studied polarization of light through a laser light source, polarizers, and retarders (called wave plates). Our calculated Brewster angle was 56.31̊ , while polarizer angle was set to 55 ̊ and the laser source was rotated at an angle of 56 ̊. In later parts, we calculated the refractive index of the glass using Fresnel Rhomb. Our calculated refractive index of the glass be 1.2, 1.43 (using Brewster angle) and 1.48 (using minimum deviation technique). Finally, we studied the depolarization of light upon reflection through linear polarization.
Effect of the structure of polarimeter characteristic matrix on light polarization measurements
Semiconductor physics, quantum electronics and optoelectronics, 2009
In the paper, we carried out the comparative analysis of three polarimeters among the most usable their variants: (i) Stokes polarimeter based on phenomenological definition of Stokes parameters; (ii) Stokes polarimeter based on the method of four intensities; (iii) Stokes dynamic polarimeter. We show that, since the accuracy in determination of individual Stokes parameter is different for different types of polarimeters, and, therewith, it depends on polarization of input light. All that strongly motivates the choice of type of polarimeter to provide minimum errors in determination of polarization parameters (ellipticity angle ε, azimuth β, and degree of polarization P).