Arthur Lompado

Mueller matrix imaging polarimetry in dermatology

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.

Effect of multiple light paths on retinal vessel oximetry

Techniques for noninvasively measuring the oxygen saturation of blood in retinal arteries and veins are reported in the literature, but none have been sufficiently accurate and reliable for clinical use. Addressing the need for increased accuracy, we present a series of oximetric equations that explicitly consider the effects of backscattering by red blood cells and lateral diffusion of light in the ocular fundus. The equations are derived for the specific geometry of a scanning-beam retinal vessel oximeter; however, the results should also be applicable to photographic oximeters. We present in vitro and in vivo data that suggest the validity of these equations.

Properties of chemical-vapor-deposited silicon carbide for optics applications in severe environments

Important data on chemical-vapor-deposited (CVD) SiC concerning the elastic modulus, polishability, scattering measurement, thermal and cryogenic stability, and degradation owing to the effects of atomic oxygen and electron beams have been obtained with the aim of assessing the suitability of SiC as an optical substrate for severe environments. These measurements show that CVD SiC substrates exhibit excellent polishability (<0.1 nm rms) with low scatter, good retention of mechanical properties up to 1500 degrees C, superior thermal and cryogenic stability (-190 degrees to 1350 degrees C) and high resistance to atomic-oxygen and electron-beam degradation. These results suggest that CVD SiC optical substrates will perform extremely well in severe environments such as outer space, and when used in lasers, combustion, and synchrotron x rays.

Minimizing the influence of fundus pigmentation on retinal vessel oximetry measurements

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.

Multispectral confocal scanning laser ophthalmoscope for retinal vessel oximetry

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.

Retinal vessel oximetry: toward absolute calibration

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.

Measurement of the transmission and reflection Mueller matrices of a thin blood column

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 vessels 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 samples 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.

In-plane scatterometry of small caliber blood column

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.

HN22 sheet polarizer, an inexpensive infrared retarder

The popular sheet polarizer, Polaroid HN22, has been measured to be a nearly half-wave retarder in the 3.6-5.4-microm spectral band with a transmittance of approximately 20%. Tuning of the retardance value between 60 degrees and 260 degrees has been demonstrated by tilting of the HN22 sheet with respect to the incident beam. The materials availability, relatively large aperture, large field of view, and low cost make it an excellent candidate for use as an infrared retarder for systems operating in this wave band. Thus HN22 may be employed as an inexpensive half-wave linear retarder and used for rotating the plane of polarization as well as for conversion between circular polarization states.

Near-achromatic retardance behavior of HN22 Polaroid 3.6 to 5.4 μm

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 materials 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.