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Humans view the intensity of light as the various colors of the spectrum. Characteristics of light include amplitude (intensity), frequency (color), polarization, and coherence. The human eye has cone cells, that correspond reflected light from objects into colors which range from wavelengths of approximately 400-700nm. Polarization, the direction in which light vibrates, is invisible to the human eye (but visible to some species of shrimp and insects). But it provides a great deal of information about the objects with which it interacts. Insects have special photoreceptors that distinguish the electric field orientation which characterizes the polarization effect used by bees and ants to navigate.
The most common use of polarization in everyday use are polarized sunglasses. This technology eliminates glare from vectors of polarization that are reflected from roads or water. Most glare comes from horizontal surfaces such as highways and water. A pair of sunglasses designed to eliminate glare might be vertically polarized to eliminate the horizontal glare and only allow vertically polarized light through the glasses.
Displaying 3D movies and images is possible because of polarization. 3D imaging uses two images overlaid on the same screen with the use special polarized glasses creating a 3D image. With a different polarized filter on each lens, the human eye sees two images that create the 3D image.
Satellites and radar utilize polarization in the optical and non-optical fields. Communication and radar imagery use polarization to transfer information in military and commercial products. Synthetic Aperture Radar (SAR) onboard TerraSAR-X and airborne assets such as AIRSAR utilize different polarization signatures when imaging (Lou).
The power of polarization is a property of light that offers more information than the traditional intensity sensing. By measuring one or more parameters of polarization, details can be extracted from a scene that is not readily apparent when using conventional thermal or visible imagers. The polarization signature of man-made objects and certain natural substances are different than the surrounding background, thus providing additional contrast in polarization imagery when compared to standard thermal or visible imagery. For this reason, objects which blend into the background when using standard thermal or visible cameras often stand out in polarization.
For military the Polarization-based imaging systems provide daylight detail in the dark as well as visibility in low contrast conditions. These systems have provided vast improvements in mission-critical target detection and surveillance for the military warfighter. Commercial applications for these systems include, but are not limited to, autonomous vehicle navigation, facial recognition, and oil on water detection.
Cameras that see polarized light are currently used to detect material stress, enhance contrast for object detection, and analyze surface quality for dents or scratches. However, like the early color cameras, current-generation polarization-sensitive cameras are bulky. Moreover, they often rely on moving parts and are costly, severely limiting the scope of their potential application.
Polarization Imaging
Polarized light occurs when a light waves’ electric field is on distinct plane perpendicular to the transverse waves. Naturally occurring light can be unpolarized, partially polarized, or fully polarized. Sources of light include the sun, lightbulbs, candles, or any light creating object. Once light encounters a surface, it becomes partially polarized, fully polarized, or remains unpolarized, depending on the surface and angle. The production of polarized light is caused by absorption, refraction, reflection, diffraction, and birefringence.
Polarization results from the vector nature of light. Fundamental quantity along with intensity and wavelength.
- IR polarization signals result from preferential emission and reflection of polarized light
- Depends strongly on the angle of incidence and surface properties such as roughness, emissivity, and reflectivity.
- The greater the angle of incidence, the greater the polarization
- The rougher the surface, the smaller the polarization signal
Generally, manmade objects produce strong polarization signals relative to natural materials.
- Polarized signal does not depend on temperature difference
- Polarized signal shows contrast between adjacent surfaces, which makes finding different surface normals and different materials easier than conventional IR and visible imagery
- Polarization enhances edges due to the abrupt change in surface orientations making it easier to find edges within an object and to find the edges of an object in a background than conventional IR and visible imagery
- Signal may be present even when unresolved
US Army’s thermal polarimitric cameras guide soldiers in complete darkness
Thermal radiation is always present in the environment regardless of whether it’s day or night, which is why the Army uses thermal cameras to see objects that are often hidden in the dark. However, in addition to the intensity of the infrared radiation, there is another characteristic of light that is often ignored when it comes to imaging: polarization state.
“Researchers have known for about 30 years that man-made objects emit thermal radiation that is partially polarized, for example, trucks, aircraft, buildings, vehicles, etc., and that natural objects like grass, soil, trees and bushes tend to emit thermal radiation that exhibits very little polarization,” Gurton said. “We have been developing, with the help of the private sector, a special type of thermal camera that can record imagery that is based solely on the polarization state of the light rather than the intensity. This additional polarimetric information will allow Soldiers to see hidden objects that were previously not visible when using conventional thermal cameras.” Gurton is pursuing the development of the camera hardware, while Hu is working on software designed to best exploit the additional information thermal polarimetric imaging provides.
“Soldier-specific applications that we have been investigating include the detection of hidden trip-wires and booby-traps, enhanced ability to see camouflaged targets, identification of buried land-mines and improvised explosive devices, and enhanced targeting and tracking of missiles, mortars, unmanned aerial vehicles and other airborne threats,” Gurton said.
The team’s most recent and exciting discovery involves the ability to detect and identify specific human subjects during complete darkness. “Prior to our research at ARL, the only way to view humans at night was to use conventional thermal imaging,” Gurton said. “Unfortunately, such imagery is plagued by a “ghosting” effect in which detailed facial features required for human identification are lost. However, when polarization information is included in the thermal image, i.e., a thermal polarimetric image, fine facial details emerge, which allows facial recognition algorithms to be applied.”
Due to the technical difficulty in building thermal polarimetric camera systems, very little research had been conducted prior to Gurton and Hu’s involvement studying this novel phenomena starting in 2005. “Our primary goal was to develop a new type of camera system that could detect objects that were difficult, or impossible, to see using current state-of-the-art thermal cameras,” Gurton said.
“We are working with the private sector on a two-prong approach in which both research grade and ruggedized commercial grade polarimetric cameras are being developed,” Gurton said. “It’s our hope that in the future, all deployed Department of Defense thermal imaging systems will have a polarimetric ability that can be implemented with a simple press of a button.”
Portable polarization-sensitive camera could be used in machine vision, autonomous vehicles, security and more
Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a highly compact, portable camera that can image polarization in a single shot. The miniature camera—about the size of a thumb—could find a place in the vision systems of autonomous vehicles, onboard planes or satellites to study atmospheric chemistry, or be used to detect camouflaged objects. The research is published in Science.
“This research is game-changing for imaging,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and senior author of the paper. “Most cameras can typically only detect the intensity and color of light but can’t see polarization. This camera is a new eye on reality, allowing us to reveal how light is reflected and transmitted by the world around us.”
“Polarization is a feature of light that is changed upon reflection off a surface,” said Paul Chevalier, a postdoctoral fellow at SEAS and co-author of the study. “Based on that change, polarization can help us in the 3-D reconstruction of an object, to estimate its depth, texture and shape, and to distinguish man-made objects from natural ones, even if they’re the same shape and color.”
To unlock that powerful world of polarization, Capasso and his team harnessed the potential of metasurfaces, nanoscale structures that interact with light at wavelength size-scales.
“If we want to measure the light’s full polarization state, we need to take several pictures along different polarization directions,” said Noah Rubin, first author of the paper and graduate student in the Capasso Lab. “Previous devices either used moving parts or sent light along multiple paths to acquire the multiple images, resulting in bulky optics. A newer strategy uses specially patterned camera pixels, but this approach does not measure the full polarization state and requires a non-standard imaging sensor. In this work, we were able to take all of the optics needed and integrate them in a single, simple device with a metasurface.”
The portable polarization camera is about two centimeters in diameter and uses a metasurface with an array of subwavelength spaced nanopillars to direct light based on its polarization.
Using a new understanding how polarized light interacts with objects, the researchers designed a metasurface that uses an array of subwavelength spaced nanopillars to direct light based on its polarization. The light then forms four images, each one showing a different aspect of the polarization. Taken together, these give a full snapshot of polarization at every pixel.
The device is about two centimeters in length and no more complicated than a camera on a smartphone. With an attached lens and protective case, the device is about the size of a small lunch box. The researchers tested the camera to show defects in injection-molded plastic objects, took it outside to film the polarization off car windshields and even took selfies to demonstrate how a polarization camera can visualize the 3-D contours of a face.
“This technology could be integrated into existing imaging systems, such as the one in your cell phone or car, enabling the widespread adoption of polarization imaging and new applications previously unforeseen,” said Rubin.
“This research opens an exciting new direction for camera technology with unprecedented compactness, allowing us to envision applications in atmospheric science, remote sensing, facial recognition, machine vision and more,” said Capasso.
