Polarimetry

Polarimetry is the measurement and interpretation of the polarization of transverse waves, most notably electromagnetic waves, such as radio or light waves. Typically polarimetry is done on electromagnetic waves that have traveled through or have been reflected, refracted or diffracted by some material in order to characterize that object. Polarimetry is the measurement and interpretation of the polarization of transverse waves, most notably electromagnetic waves, such as radio or light waves. Typically polarimetry is done on electromagnetic waves that have traveled through or have been reflected, refracted or diffracted by some material in order to characterize that object. Polarimetry of thin films and surfaces is commonly known as ellipsometry. Polarimetry is used in remote sensing applications, such as planetary science, astronomy, and weather radar. Polarimetry can also be included in computational analysis of waves. For example, radars often consider wave polarization in post-processing to improve the characterization of the targets. In this case, polarimetry can be used to estimate the fine texture of a material, help resolve the orientation of small structures in the target, and, when circularly-polarized antennas are used, resolve the number of bounces of the received signal (the chirality of circularly polarized waves alternates with each reflection). In 2003, a visible-near IR (VNIR) Spectropolarimetric Imager with an acousto-optic tunable filter (AOTF) was reported. These hyperspectral and spectropolarimetric imager functioned in radiation regions spanning from ultraviolet (UV) to long-wave infrared (LWIR). In AOTFs a piezoelectric transducer converts a radio frequency (RF) signal into an ultrasonic wave. This wave then travels through a crystal attached to the transducer and upon entering an acoustic absorber is diffracted. The wavelength of the resulting light beams can be modified by altering the initial RF signal. VNIR and LWIR hyperspectral imaging consistently perform better as hyperspectral imagers. This technology was developed at the U.S. Army Research Laboratory. The researchers reported visible near infrared system (VISNIR) data (.4-.9 micrometers) which required an RF signal below 1 W power. The reported experimental data indicates that polarimetric signatures are unique to manmade items and are not found in natural objects. The researchers state that a dual system, collecting both hyperspectral and spectropolarimetric information, is an advantage in image production for target tracking. A polarimeter is the basic scientific instrument used to make these measurements, although this term is rarely used to describe a polarimetry process performed by a computer, such as is done in polarimetric synthetic aperture radar. Polarimetry can be used to measure various optical properties of a material, including linear birefringence, circular birefringence (also known as optical rotation or optical rotary dispersion), linear dichroism, circular dichroism and scattering. To measure these various properties, there have been many designs of polarimeters, some archaic and some in current use. The most sensitive are based on interferometers, while more conventional polarimeters are based on arrangements of polarising filters, wave plates or other devices. Polarimetry is used in many areas of astronomy to study physical characteristics of sources including active galactic nuclei and blazars, exoplanets, gas and dust in the interstellar medium, supernovae, gamma-ray bursts, stellar rotation, stellar magnetic fields, debris disks, reflection in binary stars and the cosmic microwave background radiation. Astronomical polarimetry observations are carried out either as imaging polarimetry, where polarization is measured as a function of position in imaging data, or spectropolarimetry, where polarization is measured as a function of wavelength of light, or broad-band aperture polarimetry.