A full sky imaging spectropolarimeter that measures spectrally resolved (~2.5 nm resolution) radiance and polarization (s0, s1, s2 Stokes Elements) over approximately 2π sr between 400nm and 1000nm will be used to quantitatively characterize the spectral dependence of the polarization state of the sunlight scattered in the sky. The sensor is based on a scanning push broom hyperspectral imager configured with a continuously rotating polarizer (sequential measurement in time polarimeter). This study will help optimize sky polarimetry by offering information that can be used to select the best spectral band (or which spectra to reject) for a given application. Findings to be presented are sky maps of the angle of polarization and degree of polarization for different spectral bands, spectral dependency of degree (and angle) of polarization, and example data sets supporting each.
There is a rapidly growing need for position, navigation, and timing (PNT) capability that remains effective when GPS is degraded or denied. Naturally occurring sky polarization was used as long ago as the Vikings for navigation purposes. With current polarimetric sensors, the additional polarization information measured by these sensors can be used to increase the accuracy and the availability of this technique. The Sky Polarization Azimuth Sensing System (SkyPASS) sensor measures this naturally occurring sky polarization to give absolute heading information to less than 0.1° and offers significant performance enhancement over digital compasses and sun sensors. SkyPASS has been under development for some time for terrestrial applications, but use above the atmosphere may be possible and the performance specifications and SWAP are attractive for use as an additional pose sensor on a satellite. In this paper, we will describe the phenomenology, the sensor performance, and the latest test results of terrestrial SkyPASS; we will also discuss the potential for use above the atmosphere and the expected benefits and limitations.
Unpolarized light from the Sun incident upon the Earth’s atmosphere becomes polarized and presents a polarization pattern in the viewable sky dome that depends on the position of the Sun, the viewer’s position on the Earth, and the time of the observation. In clear and slightly overcast skies, both the degree of linear polarization and the polarization orientation can be predicted to first order using Rayleigh scattering theory. Conversely, measuring this polarization pattern provides information about the pose of a sensing platform equipped with an imaging polarimeter. We present here an investigation of the predicted polarization patterns in conjunction with a set of polarimetric measurements to show how the pointing direction of the platform hosting the polarimeter can be recovered. This direction derives solely from the measured polarization of a subsection of the hemispherical polarization pattern centered near the zenith and can be determined to high accuracy.
There is a strong need for the ability to terrestrially image resident space objects (RSOs) and other low earth orbit (LEO)
objects for Space Situational Awareness (SSA) applications. The Synthetic Aperture Imaging Polarimeter (SAIP)
investigates an alternative means for imaging an object in LEO illuminated by laser radiation. A prototype array
consisting of 36 division of amplitude polarimeters was built and tested. The design, assembly procedure, calibration
data and test results are presented. All 36 polarimeters were calibrated to a high degree of accuracy. Pupil plane
imaging tests were performed in by using cross-correlation image reconstruction algorithm to determine the prototype
Oxygen saturation measurements in the retina is an essential measurement in monitoring eye health of diabetic
patient. In this paper, preliminary result of oxygen saturation measurements for a healthy patient retina is
presented. The retinal oximeter used is based on a regular fundus camera to which was added an optimized
optical train designed to perform aperture division whereas a filter array help select the requested wavelengths.
Hence, nine equivalent wavelength-dependent sub-images are taken in a snapshot which helps minimizing the
effects of eye movements. The setup is calibrated by using a set of reflectance calibration phantoms and a lookuptable
(LUT) is computed. An inverse model based on the LUT is presented to extract the optical properties of
a patient fundus and further estimate the oxygen saturation in a retina vessel.
We describe the design, construction, calibration and testing of a confocal scanning Mueller polarimeter. A polarization
state generator and polarization state analyzer have been inserted into the optical path of a conventional confocal
scanning imager to collect the reflectance Muller matrix of samples measuring up to 6.26 mm on a side. Four sources
are available for sample interrogation using diode lasers centered at 532 nm, 635 nm, 670 nm, and 785 nm. The device
captures all required imagery to calculate the Mueller matrix of each image pixel in approximately 90 s. These matrices
are then reduced into polarization imagery such as the diattenuation, retardance and depolarization index. Oftentimes
this polarization imagery is quite different and potentially more informative than a conventional intensity image. There
are a number of fields that can benefit from alternative/enhanced imagery, most notably in the biomedical,
discrimination, and target recognition communities. The sensor has been designed for biomedical applications aimed at
improving the technique of noninvasive detection of melanoma lesions.
Measurement of oxygen saturation has proved to give important information about the eye health and the onset
of eye pathologies such as Diabetic Retinopathy. Recently, we have presented a multi-aperture system enabling
snapshot acquisition of human fundus images at six different wavelengths. In our setup a commercial fundus
ophthalmoscope was interfaced with the multi-aperture system to acquire spectroscopic sensitive images of the
retina vessel, thus enabling assessment of the oxygen saturation in the retina. Snapshot spectroscopic acquisition
is meant to minimize the effects of eye movements. Higher measurement accuracy can be achieved by increasing
the number of wavelengths at which the fundus images are taken. In this study we present an improvement of
our setup by introducing an other multi-aperture camera that enables us to take snapshot images of the fundus
at nine different wavelengths. Careful consideration is taken to improve image transfer by measuring the optical
properties of the fundus camera used in the setup and modeling the optical train in Zemax.
Polarization data of a SM gyroscope coil may correlate to drift that provides a method to statically predict a
performance range of a coil during manufacturing or at a minimum before integration into a FOG assembly. The
Crossover Free (CF) coils described here are thermally symmetric and lack fiber crossovers. This design allows
possible expansion of depolarized FOGs beyond the research environment. To that end a series of double sided
CF gyroscope coils were manufactured and analyzed using a 4 channel fiber coupled polarimeter. In addition the
coils were tested on a single axis rate table in a FOG testbed. A polarimeter was used to measure the output
polarization state of the stand-alone coils and when integrated into an experimental FOG testbed. In addition
Shupe data of the CF coils was taken to determine the thermal sensitivity of the coils. Coil geometry and
construction, polarimetric and traditional drift data, and Shupe performance will be presented.
This paper presents the design of a visible band imaging polarimeter that can also function as a low light level intensity
imager. The polarimeter is based on the division of aperture approach, acquiring four subimages simultaneously on a
single CCD array. The system is currently designed to measure the first three normalized components of the Stokes
vector through polarization filtering on three of the four available channels. The fourth channel remains unfiltered for
radiometric sensing in low power situations. The opto-mechanical design allows for ease of assembly without requiring
active alignment techniques, while maintaining a modular system. The modular nature provides a robust, flexible sensor
that can be tailored to multiple applications.
We describe experiments of fiber coils using different lengths, coil diameters, and configurations wound on a innovative
winder. Geometric and polarimetric analyses of coils and effects on Sagnac area and bending induced birefringence are
High accuracy polarized fiber optic gyroscopes require sensor coils comprised of relatively expensive polarization maintaining fiber. While this fiber insures minimal polarization cross coupling and thus increased sensitivity, the long required fiber lengths result in sensor coils that are quite expensive. Reduced cost single mode fiber based coils are attractive, but exhibit poor polarization selectivity due to cross coupling that manifests as signal fading and reduced detected signal amplitude. Moreover, random birefringences induced at fiber crossover points impart a nonreciprocity that tends to exacerbate this problem. A crossover-free winding scheme employing a single mode fiber wound in an Archimedean spiral can potentially improve the performance of single mode fiber coils by eliminating these random birefringences, thereby improving coil sensitivity. As well, the bending induced birefringence of these coils can serve to improve polarization maintenance. Geometric and polarimetric analyses of spirally wound coils describing the effects on overall Sagnac area, bending induced birefringence, and polarization mode coupling are examined here. The spatially varying induced birefringence, beat length, and associated mode coupling are modeled and it is found that elimination of the cross coupling due to random crossovers renders the spiral geometry potentially useful for high accuracy inertial guidance systems.
Development of reliable imaging polarimeters and the models that predict their performance is dependent on the ability to assess their accuracy. Field tests frequently result in contradictory data and laboratory measurements are often not representative of materials in the field. To address these concerns, we have built a device with which the calibration of imaging polarimeters (both stationary and moving) can be verified and the polarimetric properties of materials in the field can be measured with accuracy. The device is a handheld, non-imaging polarimeter that is capable of highly calibrated phenomenology measurements in both the lab and field. Multiple optical heads enable monitoring of samples from a variety of angles in order to characterize polarimetric signatures as a function of source, sample, and sensor geometry. The device may also be used in unattended diurnal monitoring of polarimetric signatures of the sky,
backgrounds, and targets of interest, providing a correlation between observed polarization phenomenology and weather conditions. The handheld device and the associated data acquisition system is small and portable enough that it can be taken to the field readily and is simple enough that calibration and system performance is predictable and verifiable. In this paper, we describe the design and performance of the non-imaging handheld polarimeter, performance specifications, and measurement results to date.
The extinction of light passing through a blood vessel comprises both absorbed and scattered components, the latter of which includes relatively strong forwardly transmitted and directly reflected components. The effect of such vessels on incident light beams of arbitrary polarization is most thoroughly described by the vessel's transmission and reflection Mueller matrices. The Mueller matrices of illuminated mock blood vessels (diameter 102-278 micrometers ) in these two important directions have been measured at a wavelength of 633 nm using a Mueller matrix imaging polarimeter. The measured Mueller matrices are presented, decomposed, and analyzed to determine the sample's retardance and depolarization as a function of vessel diameter. It is expected that characterization of these matrices should broaden light-vessel modeling techniques by permitting calculation of the transmitted and reflected properties of arbitrary input polarization states.
The goal of making calibrated oxygen saturation measurements of blood in retinal arteries and veins via a noninvasive spectroscopic technique has nearly been realized. Semi-continuous advancement in the field of retinal vessel oximetry over the last three decades has resulted in several technologies that seem poised for commercialization. In this paper, we present our instrumentation and technique for making well-calibrated saturation measurements of the blood in retinal vessels. The Eye Oximeter (EOX) is a confocal scanning laser ophthalmoscope capable of acquiring multi-spectral images. Analysis of these spectral vessel images allows spectroscopic determination of the oxygen saturation of blood within each vessel. The primary emphasis of this paper is to illustrate the effect of fundus pigmentation on these oximetric measurements. We show that decreasing fundus reflectivity is mathematically similar to decreasing the vessel thickness. The apparent decreased vessel thickness is a direct consequence of scattering by red blood cells. We present in vitro and in vivo measurements that demonstrate an instrument calibration that is nearly independent of vessel diameter and fundus reflectivity.
Accurately measuring the oxygen saturation of blood within retinal arteries and veins has proven to be a deceptively difficult task. Despite the excellent optical accessibility of the vessels and a wide range of reported instrumentation, we are unaware of any measurement technique that has proven to be calibrated across wide ranges of vessel diameter and fundus pigmentation. We present an overview of our retinal oximetry technique, present the results of an in vitro calibration experiment, and present preliminary human data.
The scattering of He-Ne laser light incident on a flowing column of whole human blood has been measured and analyzed. An automated scatterometer whose sample chamber simulates a small caliber blood vessel was used to perform the measurements and is described. Angular scattered light distributions due to flowing blood columns for two independently varied parameters, blood oxygenation and hemoglobin concentration, are presented. It is found that the dependence of the scattering distribution on blood oxygenation is minimal while the dependence on hemoglobin concentration is strong. A nominally transparent sample of human plasma has also been investigated to quantify its scattering characteristics. The whole blood scattering results are compared to theoretical predictions obtained using a Monte Carlo simulation employing the Mie single particle phase function and macroscopic transport coefficients obtained from published literature. The best correlation was found when the largest published scattering coefficient was employed in the simulation. However, a strong correlation between the measured and predicted scattering distributions was only obtained when unphysically high values of the scattering coefficient were used in the simulation.
Recent studies have indicated that polarized light may be useful in the discrimination between benign and malignant moles. In fact, imaging polarimetry could provide noninvasive diagnosis of a range of dermatological disease states. However, in order to design an efficacious sensor for clinical use, the complete polarization-altering properties of a particular disease must be well understood. We present Mueller matrix imaging polarimetry as a technique for characterizing various dermatological diseases. Preliminary Mueller matrix imagery at 633 nm suggests that both malignant moles and lupus lesions may be identified through polarimetric measurements. Malignant moles are found to be less depolarizing than the surrounding tissue, and lupus lesions are found to have rapidly varying retardance orientation.
Scanning laser microscopy is a widely used technique in ophthalmoscopy for providing high-resolution real time images of the retina. We describe a scanning laser ophthalmoscope that acquires retinal images at four wavelengths for the purpose of measuring the oxygen saturation of blood in retinal arteries and veins. Images at all four wavelengths are obtained across a single video frame using a temporal interlacing technique. An extraction procedure then permits analysis of four monochromatic images. A technique for calculating oxygen saturation from a multi-spectral image set is presented, along with preliminary measurements. The choice of wavelengths dramatically affects the oxygen saturation calculation accuracy and we present an optimized wavelength set and the calculated oxygen saturation results. The potential applications for this technology range from the diagnosis of various ophthalmic diseases to the detection of blood loss in trauma victims.
Polaroid HN22, a popular sheet polarizer, has been measured to be a nearly half wave retarder in the 3.6 to 5.4 micrometers spectral band with a transmittance of approximately 20 percent. The exact retardance value may be tuned to the range of 60 degrees - 260 degrees by tilting the HN22 with respect to the incident beam. The material's polarizing effects have been shown to be minimal in this waveband. Its availability, relatively large available aperture, large filed of view, and low cost make HN22 an excellent candidate for use as an IR retarder for systems operating from 3.6 to 5.4 micrometers . As such, HN22 may be used for rotating the plane of polarization of an incident linearly polarized beam as well as to convert between circular polarization states.