Polarization Imaging Reveals New Views of Hunt Library, Insight Into Exciting Field of Research

Editor’s Note: This is a guest post by Brett Pantalone, a graduate student in NC State’s Department of Electrical and Computer Engineering, and Michael Kudenov, an assistant professor in the department.

These two photos of the James B. Hunt, Jr. Library (one above, one below) use a false-color technique to reveal a characteristic of light called polarization.

Like color or brightness, polarization is an intrinsic property of light. The polarization angle of a light wave describes the orientation of its electric field. Although human eyes are not sensitive to polarization, many animals – including insects, bats, and fish – can see it and use it to locate food, navigate, and possibly even communicate. Cuttlefish, for example, can change the polarization patterns of their skin to hide themselves or signal other animals.

Even though people can’t directly see polarization, it turns up in a surprising number of human activities. The measurement of polarization angles, or polarimetry, is useful in many industrial and scientific settings. For example, many drugs contain natural compounds that exist in more than one chemical form, called isomers. Sometimes the wrong form is ineffective or even harmful if it finds its way into a drug. However, different isomers can often be identified by the way they polarize light passing through them. Polarimetry can be used to measure the relative amounts of different isomers, providing quality control in many industrial and pharmaceutical processes.

The polarization of reflected light can also reveal clues about the reflecting surface. For instance, the polarization state of weather radar echoes can be used to estimate the size of raindrops. And light that is polarized by shiny metallic or glass surfaces can help military forces detect vehicles and equipment hidden under camouflage.

As part of the polarimetry class taught by Michael Escuti and Michael Kudenov at NC State, students were instructed to build a simple polarimeter using an off-the-shelf digital camera and a polarizing filter. The images here were taken with a Canon EOS Rebel XSi digital SLR. The filter only passes light having a polarization that matches the orientation of the filter. To make each picture, three snapshots were taken with the filter oriented at 0, 45, and 90 degrees. Each snapshot represents an image formed by light waves having only a single polarization angle.

Although a tripod was used to take the snapshots, a little jostling of the camera was unavoidable. To reduce errors caused by movement of the camera between shots, a software program called Iris was used to precisely align the three images. The aligned images were then loaded into MATLAB, a numerical tool often used for image processing. The variation in brightness between each image was used to estimate the actual polarization angle of each pixel. Finally, a composite image was created, using different colors to represent various polarization angles.

The results demonstrate several aspects of polarization. The first image (at the top of this post) shows the northeast corner of the library, with the sun coming in from the left. The sky appears as a deep burgundy color, which indicates that the light from that direction has a dominant polarization of about 30 degrees. However, light reflecting off the windows appears as dark green, indicating a nearly 90 degree, or vertical, polarization. The colorful “ghosts” in the foreground are pedestrians who appeared in different places in each of the three filtered snapshots; since two-thirds of their filter data is missing at that particular location in the image, they appear as false polarization signatures. Ultimately, these signatures are “false” because they are caused by movement in the scene between the three snapshots, and not by actual polarization. It should be mentioned that currently, research is underway at NC State, by Kudenov and Brendan O’Connor to develop new compact polarization sensors that lack this limitation.

Meanwhile, the second image (inset) was taken from the northwest, with the sun high overhead, behind the library. Because of how the atmosphere scatters sunlight, there is less polarization in the direction of the sun; thus, this area of the picture appears as a neutral gray (no color means no dominant polarization). To the right of the sun, the polarization angle approaches 45 degrees (red) and to the left, the angle tends toward 135 degrees (blue). In fact, the polarization varies continuously, from 0 to 360 degrees, as one revolves around it in this fashion. This polarization “pattern” progresses with the sun throughout the day, and is what enables many insects to navigate even when they cannot see the sun directly (for instance, when it is obscured by clouds).

Polarimetry is an active area of research, with applications ranging from agriculture to astronomy. Someday soon, farmers may get information about the health of their crops by analyzing the polarization of light reflected from plant foliage.

But some of the most thought-provoking uses for polarimetry come from the field of cosmology, where theories about events following the Big Bang are being tested using polarimetry measurements of the universe’s first light, and extra-solar planet detection is being better enabled via analysis of polarized light. Meanwhile, other researchers and applications include quantifying aerosol size distribution for improving estimates and projections of global warming, improving polarimetric instrumentation and applications, and investigating new methods of measuring polarization in high speed imaging scenarios.