Polarized Light in Animal Vision

Since the structuring and variability of the Sun and other stars are governed by magnetic fields, much of present-day stellar physics centers around the measurement and understanding of the magnetic fields and their interactions. The Sun, being a prototypical star, plays a unique role in astrophysics, since its proximity allows the fundamental processes to be explored in detail. The PRL anniversary gives us an opportunity to look back at past milestones and try to identify the main unsolved issues that will be addressed in the future.

Solar magnetic fields

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-a-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.

Polarization imaging: principles and integrated polarimeters

As we resolve ever smaller structures in the solar atmosphere, it has become clear that magnetism is an important component of those small structures. Small-scale magnetism holds the key to many poorly understood facets of solar magnetism on all scales, such as the existence of a local dynamo, chromospheric heating, and flux emergence, to name a few. Here, we review our knowledge of small-scale photospheric fields, with particular emphasis on quiet-sun field, and discuss the implications of several results obtained recently using new instruments, as well as future prospects in this field of research.

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Small-Scale Solar Magnetic Fields

Using numerical simulations of flux and polarization spectra of visible to near-infrared starlight reflected by Jupiter-like extrasolar planets, we show that polarimetry can be used both for the detection and for the characterization of extrasolar planets. Polarimetry is valuable for detection because direct, unscattered starlight is generally unpolarized, while starlight that has been reflected by a planet will generally be polarized. Polarimetry is valuable for planet characterization because the degree of polarization of starlight that has been reflected by a planet depends strongly on the composition and structure of the planetary atmosphere.

/pdf/using-polarimetry-to-detect-and-characterize-jupiter-like-1v0m9yimav.pdf

Using polarimetry to detect and characterize Jupiter-like extrasolar planets

A new type of 2-D polarimeter is developed for use in high-resolution observations of solar magnetic fields. Two piezoelastic polarization modulators are used in combination with charge-coupled-device (CCD) image sensors to simultaneously record all four Stokes parameters. Demodulation of the fast 50 and 100 kHz intensity modulations produced by the piezoelastic modulators is achieved by CCD sensors used as synchronous integrators sensitive to a single frequency. The temporary buffer storage needed to separate the charges generated during the two modulation half periods is obtained by covering every second row of the CCD sensors with an opaque mask. The charges are shifted back and forth between the photosensitive uncovered and the adjacent storage rows in phase with the modulation. The polarization signal is calculated from the difference between the charges accumulated in two adjacent rows. A separate CCD sensor is needed for each normalized Stokes parameter Q/I, U/I, or V/I. Because the high modulation frequency lies well above the seeing frequencies occurring in solar observations, precision polarimetry becomes possible. We have demonstrated the capability of this new type of instrument to achieve a polarimetric sensitivity below 10-3 in a single frame. By frame averaging the noise level in the fractional polarization can be reduced to the order of 10-5.

Imaging Stokes polarimetry with piezoelastic modulators and charge-coupled-device image sensors

Hanspeter Povel Achim Gandorfer Peter Steiner Pietro N. Bernasconi ETH Zentrum Institute of Astronomy CH-8092 Zürich, Switzerland Abstract. A hybrid microlens/aperture mask assembly is fabricated, aligned and mounted on a CCD imager for an application in an imaging polarimeter. The microlens array is produced by direct laser writing in photoresist followed by electroforming and replication onto a glass substrate in registration with a slit aperture array on the reverse side. An alignment of the combined arrays to the CCD imager of better than 0.7 mm in lateral position and 0.005 deg rotation is achieved. Stray light of less than 1% was measured for the hybrid module. The optical microsystem satisfied the challenging requirements of the imaging polarimeter and the technique has applications in other areas of optical sensing, metrology and processing. © 1997 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(97)02905-X]

Hans-Peter Povel

Papers

Imaging Stokes polarimetry with piezoelastic modulators and charge-coupled-device image sensors

Active alignment of replicated microlens arrays on a charge-coupled device imager