Getting students interested in science, specifically in optics and photonics, is a worthwhile challenge. We developed and implemented an outreach campaign that sought to engage high school students in the science of polarized light. We traveled to Montana high schools and presented on the physics of light, the ways that it becomes polarized, how polarization is useful, and how to take pictures with linear polarizers to see polarization. Students took pictures that showed polarization in either a natural setting or a contrived scene. We visited 13 high schools, and presented live to approximately 450 students.
Randal Larimer, Angela DesJardins, Joseph Shaw, Charles Kankelborg, Christopher Palmer, Larry Springer, Joey Key, W. Berk Knighton, Kevin Repasky, Nathan Pust, Mitchell Hobish, Edmond Wilson, Carrie Fitzgerald, Ryan Fitzgerald, Thomas Trickel, Clyde Jensen, Skye Dorsett, Matt Anderson, Jim Boger, Nate McCrady, Jaylene Naylor, Laurie Battle
The yearly National Student Solar Spectrograph Competition (NSSSC) is Montana Space Grant Consortium's Education
and Public Outreach (EP/O) Program for NASA's Interface Region Imaging Spectrograph (IRIS) mission. The NSSSC is
designed to give schools with less aerospace activity such as Minority Serving Institutions and Community Colleges an
opportunity for hands on real world research experience. The NSSSC provides students from across the country the
opportunity to work as part of an undergraduate interdisciplinary team to design, build and test a ground based solar
spectrograph. Over the course of nine months, teams come up with their own science goals and then build an instrument
to collect data in support of their goals. Teams then travel to Bozeman, MT to demonstrate their instruments and present
their results in a competitive science fair environment. This paper and poster will discuss the 2011-2012 competition
along with results as well as provide information on the 2012 -2013 competition opportunities.
Recent developments for long-wave infrared (LWIR) imaging polarimeters include incorporating a microgrid polarizer array onto the focal plane array. Inherent advantages over other classes of polarimeters include rugged packaging, inherent alignment of the optomechanical system, and temporal synchronization that facilitates instantaneous acquisition of both thermal and polarimetric information. On the other hand, the pixel-to-pixel instantaneous field-of-view error that is inherent in the microgrid strategy leads to false polarization signatures. Because of this error, residual pixel-to-pixel variations in the gain-corrected responsivity, the noise-equivalent input, and variations in the pixel-to-pixel micropolarizer performance are extremely important. The degree of linear polarization is highly sensitive to these parameters and is consequently used as a metric to explore instrument sensitivities. We explore the unpolarized calibration issues associated with this class of LWIR polarimeters and discuss the resulting false polarization signature for thermally flat test scenes.
Division of focal plane (DoFP) polarimeters are a particular class of imaging device that consists of an array
of micropolarizers integrated upon a focal plane array sensor (FPA). Such devices are also called microgrid
polarimeters and have been studied over the past decade with systems being designed and built in all regions
of the optical spectrum. These systems are advantageous due to their rugged, compact design and ability to
obtain a complete set of polarimetric measurements during a single frame capture. One inherent disadvantage
of DoFP systems is that each pixel of the FPA sensor makes a polarized intensity measurement of a different
scene point. These spatial measurements are then used to estimate the Stokes vectors across the scene. Since
each polarized intensity measurement has a different instantaneous field-of-view (IFOV), artifacts are introduced
that can degrade the quality of estimated polarization imagery. Here we develop and demonstrate a visual
enhancement technique that is able to reduce false polarization caused by IFOV error while preserving true
polarization content within the Stokes parameter images. The technique is straight-forward conceptually and is
computationally efficient. All results are presented using data acquired from an actual LWIR microgrid sensor.
Microgrid polarimeters, also known as division of focal plane (DoFP) polarimeters, are composed of an integrated
array of micropolarizing elements that immediately precedes the FPA. The result of the DoFP device is that
neighboring pixels sense different polarization states. The measurements made at each pixel can be combined to
estimate the Stokes vector at every reconstruction point in a scene. DoFP devices have the advantage that they
are mechanically rugged and inherently optically aligned. However, they suffer from the severe disadvantage
that the neighboring pixels that make up the Stokes vector estimates have different instantaneous fields of view
(IFOV). This IFOV error leads to spatial differencing that causes false polarization signatures, especially in
regions of the image where the scene changes rapidly in space. Furthermore, when the polarimeter is operating
in the LWIR, the FPA has inherent response problems such as nonuniformity and dead pixels that make the
false polarization problem that much worse. In this paper, we present methods that use spatial information from
the scene to mitigate two of the biggest problems that confront DoFP devices. The first is a polarimetric dead
pixel replacement (DPR) scheme, and the second is a reconstruction method that chooses the most appropriate
polarimetric interpolation scheme for each particular pixel in the image based on the scene properties. We have
found that these two methods can greatly improve both the visual appearance of polarization products as well
as the accuracy of the polarization estimates, and can be implemented with minimal computational cost.
Remote sensing applications make use of the optical polarization characteristics of a scene to enhance
target detection and discrimination. Imaging polarimeters typically utilize polarizing arrays located in
front of a focal plane array as a means of extracting polarization information from the optical scene. Over
the last few years, technology development efforts have resulted in FPAs that integrate the polarizer with
the infrared focal plane array (FPA). This paper will report on the radiometric and polarization
characterization of a micro-grid polarizer FPA from DRS Infrared Technologies, L.P. (DRS). These
measurements were performed to evaluate the radiometric performance and the polarization
characteristics of the FPA.
Remote sensing of the polarized light emitted or reflected from a scene has the potential to improve object detection, object identification, or to enhance images. The need to accurately measure individual Stokes components depends on the application. In all applications, it is desirable to estimate the uncertainty associated with Stokes images. A process is described that uses an instrument computer simulation to compute expected uncertainties in imaging polarimeter data. The simulation takes measured and/or estimated uncertainties in individual instrument components and propagates them to evaluate their effect on the Stokes measurement. Individual uncertainties are then combined to obtain an overall uncertainty estimate for the given Stokes input. Methods of visualizing the effect of instrument uncertainty on various Stokes input states are also described.
Recent developments for Long Wave InfraRed (LWIR) imaging polarimeters include incorporating a microgrid polarizer array onto the focal plane array (FPA). Inherent advantages over typical polarimeters include packaging and instantaneous acquisition of thermal and polarimetric information. This allows for real time video of thermal and polarimetric products. The microgrid approach has inherent polarization measurement error due to the spatial sampling of a non-uniform scene, residual pixel to pixel variations in the gain corrected responsivity and in the noise equivalent input (NEI), and variations in the pixel to pixel micro-polarizer performance. The Degree of Linear Polarization (DoLP) is highly sensitive to these parameters and is consequently used as a metric to explore instrument sensitivities. Image processing and fusion techniques are used to take advantage of the inherent thermal and polarimetric sensing capability of this FPA, providing additional scene information in real time. Optimal operating conditions are employed to improve FPA uniformity and sensitivity. Data from two DRS Infrared Technologies, L.P. (DRS) microgrid polarizer HgCdTe FPAs are presented. One FPA resides in a liquid nitrogen (LN2) pour filled dewar with a 80°K nominal operating temperature. The other FPA resides in a cryogenic (cryo) dewar with a 60° K nominal operating temperature.
Long-wave infrared imaging Stokes vector polarimeters are used in many remote sensing applications. Imaging polarimeters require that several measurements be made under optically different conditions in order to estimate the polarization signature at a given scene point. This multiple-measurement requirement introduces error in the signature estimates, and the errors differ depending upon the type of measurement scheme used. Here, we investigate a LWIR linear microgrid polarimeter. This type of instrument consists of a mosaic of micropolarizers at different orientations that are masked directly onto a focal plane array sensor. In this scheme, each polarization measurement is acquired spatially and hence each is made at a different point in the scene. This is a significant source of error, as it violates the requirement that each polarization measurement have the same instantaneous field-of-view (IFOV). In this paper, we first study the amount of error introduced by the IFOV handicap in microgrid instruments. We then proceed to investigate means for mitigating the effects of these errors to improve the quality of polarimetric imagery. In particular, we examine different interpolation schemes and gauge their performance. These studies are completed through the use of both real instrumental and modeled data.
Long-wave infrared (LWIR) imaging is a prominent and useful technique for remote sensing applications. Moreover, polarization imaging has been shown to provide additional information about the imaged scene. However, polarization estimation requires that multiple measurements be made of each observed scene point under optically different conditions. This challenging measurement strategy makes the polarization estimates prone to error. The sources of this error differ depending upon the type of measurement scheme used. In this paper, we examine one particular measurement scheme, namely, a simultaneous multiple-measurement imaging polarimeter (SIP) using a microgrid polarizer array. The imager is composed of a microgrid polarizer masking a LWIR HgCdTe focal plane array (operating at 8.3-9.3 μm), and is able to make simultaneous modulated scene measurements. In this paper we present an analytical model that is used to predict the performance of the system in order to help interpret real results. This model is radiometrically accurate and accounts for the temperature of the camera system optics, spatial nonuniformity and drift, optical resolution and other sources of noise. This model is then used in simulation to validate it against laboratory measurements. The precision and accuracy of the SIP instrument is then studied.
One of the most significant challenges in performing infrared (IR) polarimetery is the focal plane array (FPA) nonuniformity (NU) noise that is inherent in virtually all IR photodetector technologies that operate in the midwave IR (MWIR) or long-wave IR (LWIR). NU noise results from pixel-to-pixel variations in the repsonsivity of the photodetectors. This problem is especially severy in the microengineered IR FPA materials like HgCdTe and InSb, as well as in uncooled IR microbolometer sensors. Such problems are largely absent from Si based visible spectrum FPAs. The pixel response is usually a variable nonlinear response function, and even when the response is linearized over some range of temperatures, the gain and offset of the resulting response is usually highly variable. NU noise is normally corrected by applying a linear calibration to the data, but the resulting imagery still retains residual nonuniformity due to the nonlinearity of the photodetector responses. This residual nonuniformity is particularly troublesome for polarimeters because of the addition and subtraction operations that must be performed on the images in order to construct the Stokes parameters or other polarization products. In this paper we explore the impact of NU noise on full stokes and linear-polarization-only IR polarimeters. We
compare the performance of division of time, division of amplitude, and division of array polarimeters in the presence of both NU and temporal noise, and assess the ability of calibration-based NU correction schemes to clean up the data.
Accurately identifying and bounding error sources in imaging spectro-polarimeters is a challenging task. Here we present an error evaluation methodology intended as an organizational tool for both itemizing and quantifying sources of error in polarimetric instruments. Associated with each source of error are both a metric and test by which these errors may be quantified. Using this procedure, we examine the accuracy and precision of a particular imaging Stokes vector hyper-spectral polarimeter. A subset of the identified error sources are selected and propagated through the system. These measured error quantities are then used to put absolute error bounds on the data acquired by our instrument. These measured error quantities are further documented and presented in the form of an error evaluation sheet.
The polarizing characteristics of materials such as paints, metals and dielectrics, are distinct. Measurements of the Stokes vector or the Mueller matrix provide quantitative information about the material characteristics. This paper describes a laboratory experiment, performed under the support of the US Air Force Research Laboratory, the results of which establish the ability to numerically distinguish materials using a non-imaging active laser system. Such an approach is described in the literature as sub-pixel de- mixing.
In this paper we outline and address some of the challenges encountered in calibrating a visible 4-channel imaging reduced Stokes polarimeter. Specifically we expose the errors associated with the retardance of the optical elements in the system and show how the presence of circular polarization can affect the measurement of the linear states of polarization. These errors are analyzed in some detail and several possible solutions are proposed.
A technique has been developed and tested in the laboratory where speckled images are reconstructed from pupil-plane speckle data. The wavefront is estimated using a technique similar to optical heterodyne. The method requires pupil- plane data to be mixed with a plane wave whose polarization is orthogonal to the speckle polarization. A polarimeter is then used as part of the detection scheme. If the polarimeter is capable of simultaneous detection of four or more channels, a key advantage to the technique becomes its speed. Fast detection overcomes some of the problems of Doppler shifts seen in the heterodyne detection scheme. This eliminates the need for high frame rate cameras. A simple theoretical basis for the technique is presented along with a description of the experiment and the results from that experiment. Practical applications for this technique are briefly explored.
Fourier telescopy is an imaging technique in which the Fourier spectrum of an object is built up by sweeping fringe patterns of varying spatial frequency and orientation over the object. The modulated scattered radiation that results is collected by an nonimaging detector. We have performed a laboratory demonstration of Fourier telescopy in order to confirm the validity of the fundamental measurement concept and the image reconstruction method. We show experimentally obtained images and compare them to simulated images. On- going experiments will characterize the consequences of less-than-ideal measurement conditions, such as fringe- spacing errors and less-than-unity fringe modulation. Our ultimate application is the imaging of geosynchronous satellites.
A Referenced Polarization Imaging (RPI) method can be used to measure surface displacements in an uncontrolled environment without the use of complicated feedback and stability controls. The system is ideal for non-contact and nondestructive testing of parts or materials. Data can be acquired in a single frame, so that the effects of vibration are minimal. High resolution wavefront sensing is performed using polarization modulation instead of frequency shifting (which is necessary in a heterodyne detection system). A system for recording complex (amplitude and phase) images of coherently illuminated objects is described. The promise for this type of system is the ability to obtain digital complex images at real-time rates with simple hardware. These images can be digitally interfered on a standard PC to measure changes in the object at real-time rates. A simple RPI system is described and experimental results demonstrating performance are presented.
It is envisioned that large, lightweight, optical mirrors for use in space will require onboard instrumentation for measuring and controlling the gross aspects of the mirror surface figure. An optical heterodyne array is well-suited for the figure sensing portion of this operation. The basic principles of heterodyne wave front sensing are reviewed and the advantages of this approach are highlighted. We describe the development of a dedicated circuitry module for integration with a sensor array for performing heterodyne array phase measurements.
A laboratory investigation into Doppler heterodyne imaging is presented. Heterodyne detection involves beating a temporally modulated laser beam with a local oscillator. The primary advantage of this technique is that it allows measurements to be taken at low SNR levels. In heterodyne imaging, a series of temporally modulated far-field speckle patterns is measured, and demodulated to form the 2- dimensional complex field. The complex field data is inverse Fourier transformed to create a speckled image of the coherently illuminated target. Rotating targets cause the beat frequency to be Doppler broadened, and target information is encoded into temporal frequencies determined by the associated Doppler shift from which velocity information can be recovered. A significant limitation of heterodyne array imaging is that, in its fundamental form, it is sensitive to phase aberrations in the propagation path. We investigate a method whereby information on the imaging system aberrations may be obtained, and good images recovered in the presence of phase aberrations. Recoveries of laboratory data using Doppler heterodyne at high and low SNR levels are presented.
The Air Force Phillips Laboratory is in the process of demonstrating an advanced space surveillance capability with a heterodyne laser radar (ladar) system to be used, among other applications, for range-resolved imaging. Recently, image domain signal-to-noise rations (SNRs) have been derived both for the intensity projections calculated from the range-resolved reflective data and for image information obtained using linear combinations of the projections. Also, other recent results have indicated that superior image quality is obtained by first converting the heterodyne returns into intensity projections before using tomographic techniques to reconstruct an image, as intensity projections is validated using a laboratory heterodyne setup. In addition, the laboratory results are used to validate the conclusion that intensity projections provide superior image reconstructions.
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