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.
Applications Polarimetry is used in biomedical imaging as well as remote sensing applications, such as planetary science, astronomy, and weather radar. Polarimetry of thin films and surfaces is commonly known as ellipsometry. 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).
Astronomy 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.
Biomedical Mueller matrix polarimetry (MMP) has been investigated for biomedical imaging applications, where it is used to characterize tissue structure based on polarization-dependent light–matter interactions. Early studies demonstrated the feasibility of ex vivo cancer characterization using Mueller polarimetric imaging, for example in human colon tissue, where polarimetric contrast was shown to correlate with pathological changes.
Machine Learning Later studies have combined Mueller matrix polarimetry with machine-learning methods for image segmentation and tumor identification in ex vivo human brain tissue. Studies report that both polarimetric features derived from Lu-Chipman decompositions and direct use of Mueller-matrix images as inputs to convolutional neural networks can discriminate between tumorous and healthy tissue, with potential relevance for intraoperative imaging. When analyzing Mueller matrix polarimetric data using data-driven methods, conventional image augmentation techniques such as spatial rotations and reflections may violate physical polarization constraints. Physics-aware augmentation strategies have therefore been proposed that apply consistent transformations to both image geometry and Mueller-matrix elements. Such approaches preserve physical admissibility of the augmented data and have been shown to improve learning performance when training deep neural networks on polarimetric datasets limited in size.
Gemology Gemologists use polariscopes to identify various properties of gems under examination. Proper examination may require the gem to be inspected in various positions and angles. A gemologist's polariscope is a vertically oriented device, usually with two polarizing lenses with one over the other with some space in between. A light source is built into the polariscope underneath the bottom polarizing lens and pointing upwards. A gemstone will be placed on top of the lower lens and may be properly examined by looking down at it through the top lens. To operate the polariscope, a gemologist may turn the polarizing lenses by hand to observe various characteristics about a gemstone. Polariscopes make use of their polarizing filters to reveal properties of a gem about how it affects light waves passing through it. A polariscope may be first used to determine the optic character of a gem and whether it is singly refracting (isotropic), anomalously doubly refracting (isotropic), doubly refracting (anisotropic), or aggregate. If the stone is doubly refracting and is not an aggregate, the polariscope may be used to further determine the optic figure of the gemstone, or whether it is uniaxial or biaxial. This step may require use of a loupe, also known as a conoscope. Finally, a polariscope can be used to detect the pleochroism of a gemstone, although a dichroscope may be preferred for this purpose as it may show pleochroic colors side by side for easier identification.
Combined Imaging Modalities 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. Polarimetric infrared imaging and detection can also highlight and distinguish different features in a scene and give unique signatures of different objects. A nano-plasmonic chirped metal structure for polarimetric detection in the mid-wave and long-wave infrared dual bands can give unique characteristics about the different detected materials, objects, and surfaces.
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