Polarization-difference imaging (PDI) was recently presented by us as a method of imaging through scattering media [Opt. Lett. 20, 608 (1995)]. Here, PDI is compared with conventional, polarizationblind imaging systems under a variety of conditions not previously studied. Through visual and numerical comparison of polarization-difference and polarization-sum images of metallic targets suspended in scattering media, target features initially visible in both types of images are shown to disappear in polarization-sum images as the scatterer concentration is increased, whereas these features remain visible in polarization-difference images. Target features producing an observed degree of linear polarization of less than 1% are visible in polarization-difference images. The ability of PDI to suppress partially polarized background variations selectively is demonstrated, and discrimination of target features on the basis of polarization information is discussed. Our results show that, when compared with conventional imaging, PDI yields a factor of 2-3 increase in the distance at which certain target features can be detected.
Many animals have visual systems that exploit the polarization of light, and some of these systems are thought to compute difference signals in parallel from arrays of photoreceptors optimally tuned to orthogonal polarizations. We hypothesize that such polarization-difference systems can improve the visibility of objects in scattering media by serving as common-mode rejection amplifiers that reduce the effects of background scattering and amplify the signal from targets whose polarization-difference magnitude is distinct from the background. We present experimental results obtained with a target in a highly scattering medium, demonstrating that a manmade polarization-difference system can render readily visible surface features invisible to conventional imaging.
The close apposition of the inner segments of the two cones that combine to form a double cone causes the pair of cone inner segments to guide light as a unitary structure whose transverse sections are roughly elliptical. Electron micrographs of the photoreceptors of a green sunfish (Lepomis cyanellus) retina provide evidence that the refractive index in the ellipsoid region of the inner segments of the double cones is higher in the center than at the perimeter. The hypothesis that the shape and refractive-index gradient could confer differential polarization sensitivity on double cones is examined with a two-dimensional waveguide model of a double-cone inner segment. The model has a dielectric constant that varies parabolically along the narrowest (x) dimension, leading to the index profile: n(x) = nmax[1-(x/x0)2]1/2, where nmax is the peak value of the index and x0 is a parameter specifying the rate at which the index decreases with increasing magnitude of x. A quantity, the polarization contrast, is introduced as a measure of the differential polarization sensitivity of adjacent receptors in the square mosaic of double cones in the sunfish retina. Polarization contrast is proportional to the relative difference in power absorbed by two double cones oriented with their shortest axes orthogonal to each other and stimulated by a field of uniform polarization. Polarization contrast is computed as a function of wavelength for appropriate values of nmax and x0. For normally incident light polarized parallel to one of the two axes of the double cones' cross sections, the polarization contrast is generally between 1% and 5% for wavelengths ranging from 550 to 750 nm. Over most of those wavelengths the polarization contrast of the graded-index-model double cone is approximately five times as large as that of a homogeneous-slab model of the same size and average refractive index. Additional benefits of a graded index, optical isolation of adjacent photoreceptors and antireflection at the photoreceptor entrance, are also discussed.
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