We present a method of calculating analytic formulas for the second-order statistics - the signal, variance, and SNR - of a variety of linear Stokes polarization measurement techniques. The advantage of the method is that it is easy to perform and can be used to produce analytic rather than numeric results. Using the derived formulae, we compare a number of different polarimetric designs.
Imaging the thermal changes in a scene at the millikelvin level reveals a fascinating world that we normally cannot see. Wind passing over the ground produces dynamic thermal striations that indicate the wind direction and speed. Trace quantities of infrared gases passing across the field of view create subtle thermal dynamics patterns that can be used to detect gas leaks. Combining these two effects, we show that the thermal signatures induced by air turbulence create a fundamental lower limit on the ability to detect trace gases with infrared imaging, independent of measurement noise.
We introduce a tomographic approach for three-dimensional imaging of evoked hemodynamic activity, using broadband illumination and diffuse optical tomography (DOT) image reconstruction. Changes in diffuse reflectance in the rat somatosensory cortex due to stimulation of a single whisker were imaged at a frame rate of 5 Hz using a hyperspectral image mapping spectrometer. In each frame, images in 38 wavelength bands from 484 to 652 nm were acquired simultaneously. For data analysis, we developed a hyperspectral DOT algorithm that used the Rytov approximation to quantify changes in tissue concentration of oxyhemoglobin (ctHbO2) and deoxyhemoglobin (ctHb) in three dimensions. Using this algorithm, the maximum changes in ctHbO2 and ctHb were found to occur at 0.29±0.02 and 0.66±0.04 mm beneath the surface of the cortex, respectively. Rytov tomographic reconstructions revealed maximal spatially localized increases and decreases in ctHbO2 and ctHb of 321±53 and 555±96 nM, respectively, with these maximum changes occurring at 4±0.2 s poststimulus. The localized optical signals from the Rytov approximation were greater than those from modified Beer–Lambert, likely due in part to the inability of planar reflectance to account for partial volume effects.
Flat field calibration of broadband imaging systems is widely used, and it has been said that users should try to make the spectrum of the flatfield calibration light source as close as possible to that of the measurement object. However, a quantitative analysis of the error induced by a mismatch of calibration and object spectra has been lacking. In order to develop this quantitative analysis, we provide a theoretical radiometric model for flatfield calibration and show how this spectral mismatching error arises. Simulations covering a variety of measurement scenarios indicate that spectral mismatching can create quantitative errors of up to a factor of 5 in situations that are regularly encountered by researchers performing quantitative work.
A novel way to measure the Mueller matrix image enables a sample's diattenuation, retardance, and depolarization to be measured within a single camera integration period. Since the Mueller matrix components are modulated onto coincident carrier frequencies, the described technique provides unique solutions to image registration problems for moving objects. In this paper, a snapshot imaging Mueller matrix polarimeter is theoretically described, and preliminary results shows it to be a viable approach for use in surface characterization of moving objects.
Within the field of spectral imaging, the vast majority of instruments used are scanning devices. Recently, several snapshot spectral imaging systems have become commercially available, providing new functionality for users and opening up the field to a wide array of new applications. A comprehensive survey of the available snapshot technologies is provided, and an attempt has been made to show how the new capabilities of snapshot approaches can be fully utilized.
We have recently constructed and tested a gas cloud imager which demonstrates the rst-ever video-rate detection (15 frames/sec) of gas leaks using an uncooled LWIR detector array. Laboratory and outdoor measurements, taken in collaboration with BP Products North America Inc. and IES Inc., show detection sensitivities comparable to existing cooled systems for detecting hydrocarbon gases. Gases imaged for these experiments include methane, propane, propylene, ethane, ethylene, butane, and iso-butylene, but any gases with absorption features in the LWIR band could potentially be detected, such as sarin and other toxic gases. These results show that practical continuous monitoring of gas leaks with uncooled imaging sensors is now possible.
Image mapping spectrometry (IMS) is a hyperspectral imaging technique that simultaneously captures spatial and spectral information about an object in real-time. We present a new calibration procedure for the IMS as well as the first detailed evaluation of system performance. We correlate optical components and device calibration to performance metrics such as light throughput, scattered light, distortion, spectral image coregistration, and spatial/spectral resolution. Spectral sensitivity and motion artifacts are also evaluated with a dynamic biological experiment. The presented methodology of evaluation is useful in assessment of a variety of hyperspectral and multi-spectral modalities. Results are important to any potential users/developers of an IMS instrument and to anyone who may wish to compare the IMS to other imaging spectrometers.
The snapshot advantage is a large increase in light collection efficiency available to high-dimensional measurement systems that avoid filtering and scanning. After discussing this advantage in the context of imaging spectrometry, where the greatest effort towards developing snapshot systems has been made, we describe the types of measurements where it is applicable. We then generalize it to the larger context of high-dimensional measurements, where the advantage increases geometrically with measurement dimensionality.
The gas cloud imager (GCI) is a passive uncooled multispectral camera capable of unprecedented sensitivity for
analyzing hydrocarbon gas mixtures in a scene. The GCI is currently finishing its final stages of development,
and promises to obtain a 220×220 image of gas concentrations at 30 frames/sec, allowing for real-time display in
a compact instrument without moving parts. We summarize measurement approach and discuss the advantages
of the GCI instrument design against conventional instruments.
We present a foveated miniature endoscopic lens implemented by amplifying the optical distortion of the lens. The resulting system provides a high-resolution region in the central field of view and low resolution in the outer fields, such that a standard imaging fiber bundle can provide both the high resolution needed to determine tissue health and the wide field of view needed to determine the location within the inspected organ. Our proof of concept device achieves 7 ∼ 8 μm resolution in the fovea and an overall field of view of 4.6 mm. Example images and videos show the foveated lens' capabilities.
We present a rapid, noncontact imaging technique which can obtain the spectrally- and spatially-resolved scattering
and absorption coefficients of a turbid medium. The measurement involves combining a spatially modulated
illumination pattern with a snapshot imaging spectrometer for measurement. After capture of an (x, y, λ)
datacube, an image demodulation scheme is applied in post-processing to obtain the spatial maps of diffuse
reflectance, absorption coefficient, and reduced scattering coefficient. The resulting system is used to dynamic
maps (in 1 s intervals) of the brain's intrinsic optical signal.
We have designed and constructed a coded aperture spectrometer for use in the deep UV range. The Czerny-Turner design provides sufficient spectral resolution to observe Raman scattering features, while the use of a coded aperture provides a greatly improved light collection efficiency for scattering sources. The resulting instrument
is capable of analyzing Raman spectra from samples at a 1 meter viewing distance. We provide an overview of the system, its optical design, and some preliminary measurements.
We demonstrate a static multiplex spectrometer based on a Fabry-Perot interferometric filter for measuring the
mean spectral content of diffuse sources. By replacing the slit of a low-dispersion grating spectrometer with
a Fabry-Perot interferometric filter, we improve the resolving power of the instrument while simultaneously
overcoming the free spectral range limitation of the Fabry Perot. The resulting instrument is smaller than conventional
spectrometers having the same resolving power. We present experimental results from the spectrometer
using neon lamp, He-Ne laser, and diode laser sources over a wavelength range from 620 nm to 660 nm.
This paper will describe methods of measuring all of the components of the Stokes polarization vector for each pixel in a
scene using only one frame of passive optical sensor data, one radar pulse, or one radiometer integration interval. Both
active and passive sensors operating in any waveband from microwave to visible will be considered. For systems
operating in the millimeter wave and terahertz bands, the techniques developed by Dereniak and his students at the
University of Arizona will be discussed. For other wavebands, a technique developed by the author that requires the
coherent reception of two orthogonally-polarized signal components will be presented. This latter method works for both
for both broad-band and narrow-band active or passive signals, but requires focal planes and hardware in the visible and
infrared bands that may be too complicated for many applications. Results of calculations made for the millimeter and
terahertz bands will be presented.
We present a snapshot technique for performing spectrally-resolved Mueller matrix polarimetry, based on channeled
spectropolarimetry. After discussing the measurement theory in detail, we present a simulated measurement
of a polymer achromatic retarder. Finally, we review some methods for modifying the technique to achieve
A computed tomography imaging channeled spectropolarimeter (CTICS) is a combination of a computed tomography
imaging spectrometer (CTIS) and a channeled spectropolarimeter (CHSP). The CTICS instrument can simultaneously
obtain image spatial and spectral information as well as polarization Stokes vectors at each resolution element in a single
focal plane array (FPA) integration time with no moving parts. An instrument has been designed and built for the
visible wavelength region at the University of Arizona. Performance testing is underway. In this work, we present
initial results from data acquired during testing of the CTICS instrument.
A persistent barrier to the wider use of the Computed Tomographic Imaging Spectrometer (CTIS) has been the
extraordinary demands it places on computational resources. Raw images can be obtained at snapshot speeds,
but reconstructed datacubes typically require minutes of reconstruction time each. We present a new approach
to the CTIS reconstruction problem which makes use of the spatial shift-invariance in a CTIS system to greatly
reduce the dimensionality of the matrix inversion process performed during reconstruction. Preliminary results
indicate that a speedup by a factor of 4000 is possible.
A Computed Tomography Imaging Spectrometer (CTIS) is an imaging spectrometer which can acquire a multi-spectral
data set in a single snapshot (one focal plane array integration time) with no moving parts. Currently, CTIS instruments
use a specially designed computer generated hologram (CGH) to disperse the light from a given spectral band into a
grid of diffraction orders. The capabilities of the CTIS instrument can be greatly improved by replacing the static CGH
dispersing element with a reconfigurable liquid crystal spatial light modulator. The liquid crystal spatial light modulator
is tuned electronically, enabling the CTIS to remain a non-scanning imaging spectrometer with no moving parts. The
ability to rapidly reconfigure the dispersing element of the CTIS allows the spectral and spatial resolution to change by
varying the number of diffraction orders, diffraction efficiency, etc. In this work, we present the initial results of using
a fully addressable, 2-D liquid crystal spatial light modulator as the dispersing element in a CTIS instrument.
We present some new grating designs for use in a computed tomographic imaging spectrometer (CTIS) and
discuss their differences with previous gratings. One of the advantages of the new designs is that they provide
added flexibility for a tunable CTIS instrument, and we show some preliminary data illustrating this advantage.
This paper covers the design and construction of a snapshot imaging spectropolarimeter for use in the long wave
infrared, 8 to 12 micron region. This imaging device is unique in the fact that system is nonscanning, contains no
moving parts, and in a single integration period is able to record spectral data as well as the polarization state as a
function of wavelength from every spatial location in a 2D image. The system is based on the Computed Tomographic
Imaging Spectrometer, commonly referred to as CTIS, and has been modified to incorporate components of Channeled
Spectropolarimetry. The paper presents an overview of how both the CTIS and the CTICS (Computed Tomographic
Imaging Channeled Spectropolarimeter) systems work, details on the specific components used in the LWIR system, and
preliminary results from a completed LWIR CTIS system, which is the first of its kind.
A spectropolarimeter utilizing an Oriel MIR8000 Fourier Transform Spectrometer in the MWIR is demonstrated. The
use of the channeled spectral technique, originally developed by K. Oka, is created with the use of two AR coated
Yttrium Vanadate (YVO4) crystal retarders with a 2:1 thickness ratio. A basic mathematical model for the system is
presented, showing that the Stokes parameters are directly present in the interferogram. Theoretical results are then
compared with real data from the system, an improved model is provided to simulate the effects of absorption within the
crystal, and error between reconstructions with phase-corrected and raw interferograms is analyzed.
Computed Tomographic Imaging Spectrometry (CTIS) is a technique which has been around for over a decade, providing
snapshot measurements of datacubes as large as (x,y,λ) = (100,100,300). We discuss the difficulties with
maximizing the resolution of a CTIS instrument and some new grating design ideas for realizing performance
We present the first visible-spectrum snapshot imaging spectro-polarimeter based on a Computed Tomographic Imaging Spectrometer (CTIS). Improvements to our calibration methods have provided advances in the CTIS spectral resolution and in noise suppression which obstructed previous attempts to construct such an instrument. The resulting device is capable of 75×75 spatial resolution, 1 nm spectral resolution across the visible spectrum (400 nm-720 nm), at video frame rates. The instrument is also capable of complete Stokes vector polarimetry at a reduced spectral resolution (~10 nm).
We present adaptations of the channelled spectropolarimetry technique, a method which allows both spectral and polarization information to be captured in a single integration period. The first adaptation uses a mathematical decomposition of the system matrix, which is then modified for imaging spectropolarimetry; the second adaptation is applied first to a single-point and then to an imaging system, for which we also show applications and measurements from experimental work.
The recent development of channelled spectropolarimetry presents opportunities for spectropolarimetric measurements of dynamic phenomena in a very compact instrument. We present measurements of stress-induced birefringence in an ordinary plastic by both a reference rotating-compensator fixed-analyzer polarimeter and a channelled spectropolarimeter. The agreement between the two instruments shows the promise of the channelled technique and provides a proof-of-principle that the method can be used for a very simple conversion of imaging spectrometers into imaging spectropolarimeters.
Spectrometry and polarimetry measurements are important to modern science and engineering in an extremely wide variety of fields such as atomic and chemical processes, materials identification and characterization, astronomy, remote sensing, and stress analysis. The basic principle is that when light is emitted or absorbed by, scattered or reflected from, or transmitted through a physical material, its spectral content and polarization state are often affected. Analysis of the changes imposed by these processes then has the potential to reveal useful information about the sources. Example applications are: (1) stress-induced birefringence (photoelasticity); (2) remote sensing, object discrimination, shape measurement; (3) communications (polarization shift keying, deterimental effects on fiber networks); (4) astronomy (solar magnetic fields); (5) scattering, materials identification (retinal nerve fiber layer thickness measurement); (6) ellipsometry (materials characterization, complex index of refraction, layer thicknesses); (7) atomic physics; (8) displays (color LCDs merge colorimetry and polarization).
17 November 2008 | San Diego, California, United States
SC1212: Quantitative Imaging with Uncooled Infrared Cameras
Within the infrared community, it is widely held that uncooled sensors are incapable of doing accurate quantitative work. This course aims to show that quantitative imaging is actually possible with uncooled systems by demonstrating the steps to achieve radiometric calibration in detail and establishing the limits of what can be achieved. Throughout the course, the emphasis is on material that is practical for camera users, rather than for detector designers. The course material provides a thorough introduction into microbolometer pixel design and clarifies the differences between uncooled infrared sensors and photon-integrating sensors. Many of the examples provided are drawn from outdoor measurements, and the course provides a discussion of how to model atmospheric effects on infrared sensing in order to make sense of the thermal infrared world. Examples of quantitative measurements are drawn from the author’s work in infrared gas imaging, atmospheric sensing, and imaging of thermal dynamics.
This course covers the design and analysis of imaging spectrometers, from instrumentation to evaluation and data exploitation. After surveying the fundamentals of spectral imaging, the course provides a detailed survey of various implementations of imaging spectrometers and the benefits of each approach, with special attention to snapshot systems. Noise-equivalent spectral radiance (NESR) and other evaluation metrics are introduced and explained, providing a quantitative means of comparing systems. Finally, the course will review commonly used methods for data exploitation, surveys common algorithms used with spectral imaging data, and discusses their relative strengths and weaknesses.