An optimization of a dual-rotating-retarder Mueller matrix polarimeter is performed by minimizing the condition number of the system data reduction matrix. The optimum retardance for the rotating retarders is found to be 127 degrees. If exactly 16 intensity measurements are used for the calculation, we observe a complex relationship between the condition number and the size of the angular increments of the two retarders. If many intensity measurements are made, thus over-specifying the calculation, we find broad optimal ranges of angular increments of the two retarders that yield essentially equal performance.
A system for characterizing polarization controllers and other fiber components with Mueller matrices is presented. Most polarization controllers, such as lithium niobate modulators or PLZT electro-optical modulators, exhibit a wide range of polarization behaviors at constant drive voltage including elliptical retardance, polarization dependant loss (PDL), and depolarization. Specifying the half wave voltage for such devices describes their desired characteristic, an electrically addressable retardance, but not the undesired characteristics. Devices with complex polarization behaviors require a similarly comprehensive description of their polarization effects. We present example measurements that demonstrate how the Mueller matrix as a function of voltage provides a complete description of the desired retardance and the undesired PDL. Such polarization controller Mueller matrices can be multiplied with Mueller matrices for other photonic components to quantify how component polarizations interact.
In recent years, several investigators have begun to explore polarized light imagery as a potential diagnostic tool. For example, polarimetric images have shown promise in identifying a variety of dermatological conditions. Because tissues tend to depolarize a large fraction (~85%) of incident light, the Mueller calculus lends itself well to these applications. A particular property of the Mueller matrix, the Depolarization Index, has demonstrated promise in discriminating between cancerous and benign moles. In this paper, we discuss the depolarizing aspects of tissues, however we primarily attempt to analyze the small fraction of light that has retained a polarization state. Analyzing the residual polarizing properties of a sample is challenging, and it requires a polar decomposition of the measured Mueller matrix into the basic properties of diattenuation, retardance, and depolarization. The diattenuation and retardance images contain information about the complex refractive index of the tissue, including any spatial variations in the index. We present measurements of the diattenuation and retardance of laser light reflected from skin as a function if incident angle and scattered angle.
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.
Previous infrared polarization imaging research has shown manmade objects to be sources of emitted and reflected polarized radiation while natural backgrounds are predominantly unpolarized. The prior work underscored the dramatic improvements in signal to clutter ratio that could be achieved in a typical target acquisition scenario using polarization sensing techniques. Initial investigations into the polarization signatures of surface scattered mines have also shown polarization techniques are able to provide strong cues to mine presence. The US Army Night Vision and Electronics Sensors Directorate has developed complete Stokes imaging polarimeters in the midwave infrared (MWIR, 3-5 µm)1,2, the short wave infrared (SWIR, 1-2 µm) and the visible wavebands in order to investigate polarization phenomenology and to quantify expected improvements to target acquisition and mine detection. This paper will review the designs of the polarization cameras and will summarize efforts to calibrate them. An example phenomenology study of MWIR polarization from surface scattered landmines will be presented.
Accurately calibrating an infrared polarimeter presents several challenges. Some of these challenges include characterization of the polarization elements within the polarimeter, overcoming the effects of thermal emissions within the polarimeter, and developing a test setup and procedure for conclusively verifying the instrument calibration. We describe our efforts to absolutely calibrate our imaging Stokes polarimeter that operates in the mid-wave infrared band (3 to 5 microns). We have developed a generator that provides well-controlled polarization states for calibrating the polarimeter. This generator overcomes problems associated with thermal emissions, stray reflections, and narcissus effects. While the polarization state emitted by the generator is not known with extreme accuracy, we are able to rotate the generated state without affecting its degree of polarization or ellipticity. We show that we can create a complete set of input states to allow a full calibration of the polarimeter, and we describe a technique for optimizing the calibration based on a variance-minimization. Results of our calibration are presented, indicating that our polarimeter precision is better than 0.1%. This technique is not limited to infrared polarimeters and should therefore have broad applicability.
Several techniques for measuring the oxygen saturation of blood in retinal vessels have been reported. One interesting application of retinal vessel oximetry is the identification of occult blood loss in trauma victims. However, all the devices described to date are too bulky and cumbersome to be used in a trauma bay or in the field. We present a design for a handheld instrument that performs four-wavelength retinal vessel oximetry. This device is comparable in size and weight to a commercially available camcorder, and is suitable for use in the trauma bay. The compact size of this device could also extend its applications beyond traditional clinical settings, as it could be used by primary care physicians and home health care workers for the screening and monitoring of ophthalmic diseases. Principles of operation and preliminary data from the device will be described.
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.
We describe the design and development of an imaging polarimeter that will simultaneously measure Stokes vector images in the mid-wave IR and long-wave IR wavebands. We present an analysis of the expected errors that arise due to spectral variations in the polarization elements of the instrument across each waveband. FInally, instrument calibration and polarization images acquired in the MWIR waveband are presented.
Most organic materials are quite opaque to IR radiation, making it nearly impossible to measure the polarization properties at these wavelengths. By placing the highly absorbing organic material on a high index of refraction prism and making use of the evanescent field present at the interface for angles exceeding the critical angle, it is possible to measure the organic material's polarization characteristics. Attenuated Total Reflection (ATR) devices have been constructed in-house for use in our IR polarimeter operating over the spectral range of 3-14 microns. Single bounce internal reflection ZnSe prisms are used in our devices to measure the polarization characteristics of highly absorbing materials. Using attenuated total reflection along with a dual rotating retarder polarimeter, it is possible to determine the full Mueller Matrix, and thus determine the linear and circular diattenuation and retardance. With this information we can determine the complex index of refraction, dichroic ratio, and isotropic ratio of materials which are opaque in the IR region. With this knowledge of the material polarization properties, it may be possible to determine the orientation of the molecules in films made of long helical organic molecules and determine the enantiomeric purity of stereo-isomers. The theoretical performance of these devices with a focus on the organic chemical applications has been determined and compared with experiments. In this paper we present a summary of the theoretical basis for the measurements and make comparisons with our measurements.
To make accurate imagin polarimetry measurements, it is imperative that the individual polarization images comprising a data set are spatially well registered. Both the Stokes Imaging Polarimeter at the University of Alabama in Huntsville and the Multispectral IR Stokes Imagining Polarimeter at Night Vision Laboratories use a rotating quarter-wave linear retarder followed by a fixed linear polarizer to measure Stokes vector images. The rotating retarder cause small amounts of image wander at the detector array. In this paper, we quantify the effects of this image wander on the calculated polarization parameters, and present data indicating the maximum acceptable image wander for typical scenes.
Retinal vessel oxygen saturation has been suggested as a parameter for monitoring a wide range of conditions including occult blood los and a variety of ophthalmic diseases. We have developed an Eye Oximeter (EOX), that noninvasively measures the oxygen saturation of the blood in individual large retinal vessels using scanning lasers. 1D vessel extinction profiles are obtained at four wavelengths (629, 678, 821 and 899 nm), and the vessel transmittances computed. The oxygen saturation of blood within the vessel is then calculated from the transmittance data. We have performed an in vitro experiment on human blood which demonstrates the calibration of the EOX measurements and validates our oximetry equations. Retinal vessel oxygen saturation was measured in a human subject and found to be 65%O2Sat and 101 - 102%O2Sat in the veins and arteries on the optic disk. Irregularities in the background measured away from the optic disk resulted in a large variance in the calculated saturation when compared to measurements made on the disk.
Imaging polarimetry is a novel method of characterizing the polarization effects of optoelectronic devices. From the Mueller matrix image, any polarization property of a device can be determined. High resolution polarization images of the outcoupling faces of several self-imaging GaAs/AlGaAs waveguide beamsplitters were made in the Mueller matrix imaging polarimeter at the University of Alabama in Huntsville. Interesting polarization states of the device modes (TE and TM), the magnitude of linear retardance varied significantly across a device. Polarization losses were also observed to vary across the faces of the devices. These effects could not have been observed by simply measuring the crosstalk between the TE and TM modes. The results of this study could lead to the detection of defect mechanisms in optoelectronic devices through Mueller matrix measurements.
The polarization and depolarization properties of two types of targets have been derived from the experimental determination of their Mueller matrix image. Each polarization signature is deduced from the polar decomposition of the Mueller matrix into images of retardance, diattenuation, polarizance, and depolarization. Different correlations between the polarization parameters and synthesized angle of incidence and angle of scatter images have been developed to determine an approximation of the angle of incidence from the polarization signatures.
Mueller matrix imaging polarimetry represents a novel means of characterizing the polarization effects of optoelectronic devices. The Mueller matrix contains the complete polarization properties of a sample, and can therefore be used to calculate properties such as phase retardance, polarization dependant losses and polarization crosstalk. The complete polarization properties of a series of GaAs/AlGaAs self-imaging waveguide beamsplitters were measured with an imaging Mueller matrix polarimeter. Polarization properties were mapped across high resolution images of the devices' outcoupling faces, and the uniformity of the polarization properties was measured. Properties investigated include magnitude and orientation of linear retardance, polarization dependant losses, and crosstalk between TE and TM modes.
We describe a noninvasive technique and instrumentation for measuring the oxygen saturation of blood in retinal arteries and veins. A white-light image of a subject's retina is presented to an operator. The operator targets a particular retinal artery or vein and initiates the measurement procedure. The measurements are made by scanning low-power lasers into the eye and across the retinal vessel. The light reflected and scattered back out of the eye is collected and measured. The oxygen saturation of blood within the vessel is spectroscopically determined by analyzing the vessel absorption profiles at two or more wavelengths. A complete saturation measurement can be made in less than one second, allowing real-time measurement during physiological changes. The sensitivity of this measurement technique to changes in retinal saturation has been demonstrated through a series of pilot studies in anesthetized swine. We present data indicating that retinal venous oxygen saturation decreases predictably during ongoing blood loss, indicating a potential application of an eye oximeter to nonivasively monitor blood loss. Current invasive techniques for monitoring bleeding, such as fiber optic pulmonary catheters, are not suited for use early in trauma situations. A portable eye oximeter may therefore provide a new technique for reducing mortality in the emergency department setting.
Fractals have been used successfully in generating missing information in scene reconstruction. If this information is processed using coherent optical techniques, then it is important to understand the properties of the Fourier transforms of fractals. Since fractals with square or rectangular bases are representative of pixel structures found on most electrically adressed spatial light modulators, a square-based fractal has been generated and its far field diffraction pattern calculated. The properties of the resulting Fourier spectrum are discussed.
Although zero-order quartz waveplates are widely used in instrumentation that needs good temperature and field-of-view characteristics, the residual errors associated with these devices can be very important in high-resolution polarimetry measurements. How the field-of-view characteristics are affected by retardation errors and the misalignment of optic axes in a double-crystal waveplate is discussed. The retardation measurements made on zero-order quartz and single-order "achromatic" waveplates and how the misalignment errors affect those measurements are discussed.
Although zero-order quartz waveplates are widely used in instrumentation that needs good temperature and field-of-view characteristics, the residual errors associated with these devices can be very important in high resolution polarimetry measurements. This paper will discuss how the field-of-view characteristics are affected by retardation errors and the misalignment of optic axes in a double crystal waveplate. The paper will then describe the retardation measurements that were made on zero-order quartz and single-order `achromatic' waveplates and how the misalignment errors affect those measurements.
A new vector magnetograph is being developed at NASA's Marshall Space Flight Center (MSFC) to measure polarization from active regions on the Sun. This paper will describe the polarization characteristics of the elements that make up the MSFC EXperimental Vector Magnetograph (EXVM) polarimeter. Based on those characteristics, the systematic errors associated with this polarimeter will be modeled and the polarization resolution that could be achieved in a space-based (or balloon-based) instrument will be discussed.
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