An important indicator of health status is hematocrit (HCT), the fractional volume of red blood cells. As such, a noninvasive,
real-time means of its measurement is highly desirable in a space flight situation. We propose to exploit
ultrasound technology to provide such a capability. We introduce a specific measurement concept including the hardware
and requisite processing algorithms, and discuss progress towards realization of such a measurement capability.
Stochastic Huygens describes a method of propagating a partially coherent source by sampling the Huygens
wavelets that evolve from each point of the wavefront. The amplitude and phase of each wavelet is tracked as the
light passes through the optical system. We have previously described how a partially coherent wavefront may
be simulated by propagating an ensemble of wavefronts with specified first and second-order statistics through
simple optical systems. In this work we extend the modeling effort to include an ensemble of phase (or scattering)
screens that characterize thin microscope tissue samples in the optical path.
The propagation of light through complex structures, such as biological tissue, is a poorly understood phenomenon.
Typically the tissue is modeled as an effective medium, and Monte Carlo techniques are used to solve the radiative
transport equation. In such an approach the medium is characterized in terms of a limited number of physical scatter and
absorption parameters, but is otherwise considered homogeneous. For exploration of propagation phenomena such as
spatial coherence, however, a physical model of the tissue medium that allows multiscale structure is required. We
present a particularly simple means of establishing such a multiscale tissue characterization based on measurements
using a differential interference contrast (DIC) microscope. This characterization is in terms of spatially resolved maps
of the (polar and azimuthal) angular ray deviations. With such data, tissues can be characterized in terms of their first
and second order scatter properties. We discuss a simple means of calibrating a DIC microscope, the measurement
procedure and quantitative interpretation of the ensuing data. These characterizations are in terms of the scatter phase
function and the spatial power spectral density
KEYWORDS: Atmospheric propagation, Tissues, Scattering, Monte Carlo methods, Statistical analysis, Tissue optics, Microscopy, Digital image correlation, Atmospheric optics, Correlation function
Analysis of the first and second order statistical properties of light is a powerful means of establishing the properties of a
medium with which the light has interacted. In turn, the first and second order statistical properties of the medium dictate
the manner in which light interacts with the medium. The former is the inverse problem and the latter is the forward
problem. Towards an understanding of the propagation of light through complex structures, such as biological tissue, one
might choose to explore either the inverse or the forward problem. Fundamental to the problem, however, is a physical
parametric model that relates the two halves; a model that allows prediction of the measured effect or prediction of the
parameters based on measurements. This is the objective of our study. As a means of characterizing the first and second
order properties of tissue, we discuss measurements with differential interference contrast microscopy using a phasestepping
approach. First and second order properties are characterized respectively in terms of scatter phase functions
and spatial power spectral densities. Results are shown for representative tissue.
The propagation of light through complex structures, such as biological tissue, is a poorly understood phenomenon.
Current practice typically ignores the coherence of the optical field. Propagation is treated by Monte
Carlo implementation of the radiative transport equation, in which the field is taken to be incoherent and is
described solely by the first-order statistical moment of the intensity. Although recent Monte Carlo studies have
explored the evolution of polarization using a Stokes vector description, these efforts, too are single-point statistical
characterizations and thus ignore the wave nature of light. As a result, the manner in which propagation
affects coherence and polarization cannot be predicted.
In this paper, we demonstrate a Monte Carlo approach for propagating partially coherent fields through complicated
deterministic optical systems. Random sources with arbitrary spatial coherence properties are generated
using a Gaussian copula. Physical optics and Monte Carlo predictions of the first and second order statistics of
the field are shown for coherent and partially coherent sources for a variety of imaging and non-imaging configurations.
Excellent agreement between the physical optics and Monte Carlo predictions is demonstrated in all
cases. Finally, we discuss convergence criteria for judging the quality of the Monte Carlo predictions.
Ultimately, this formalism will be utilized to determine certain properties of a given optical system from
measurements of the spatial coherence of the field at an output plane. Although our specific interests lie in
biomedical imaging applications, it is expected that this work will find application to important radiometric
problems as well.
We demonstrate the use of phase-shifting interferometry in OCT to determine the optical phase and fringe visibility within the coherence envelope. Phase-shifting algorithms provide both the optical phase and visibility from a series of intensity measurements corresponding to controlled phase shifts. In addition to providing phase information which supplements the visibility or envelope data which is traditionally obtained in OCT, this technique will provide an independent, highly sensitive measurement of the coherence envelope which may be used for a precise determination of the source power spectrum.
KEYWORDS: Dielectrics, Scattering, Spatial frequencies, Near field, Optical spheres, 3D image processing, Fourier transforms, Radon transform, 3D modeling, Near field scanning optical microscopy
The structural information carried by the evanescent components of the scattered field is discussed for the case of a single homogeneous plane wave incident on a weakly scattering three-dimensional medium. It is shown that, unlike the homogeneous components of the scattered field, the evanescent components are related to the three-dimensional Fourier transform of the dielectric susceptibility through a generalized Radon transform. The region of the three- dimensional Fourier space that is accessible from evanescent wave measurements is discussed, as well as the spatial resolution attainable in a typical multiple-view scattering experiment.
We have developed a model that predicts the effective optical path length (OPL) through a thick, refractive specimen on a reflective substrate as measured with a high numerical aperture, confocal interference microscope. Assuming an infinitesimal pinhole, only one 'magic ray' contributes to the measured OPL. It is possible to correct for the refractive errors and to unambiguously interpret the data. We present a comparison of our model predictions with experimental measurements of a fluid drop on a silicon substrate, obtained with a phase shifting laser feedback microscope.
Calibration of the axial response of interference microscopes has received considerable attention in the past two decades. In addition to systematic errors which could be caused by components in the microscope or measurement technique, a numerical correction factor associated with imaging at high apertures must be determined. Unfortunately, the cost of reference height standards increases sharply with their spatial homogeneity and calibration accuracy and these standards may be easily contaminated and therefore require sophisticated cleaning and re-calibration. To address these problems, we have investigated the interferometric measurement of the equilibrium shape of static fluid drops on coated substrates. For drops with small Bond number (the Bond number is a ratio of gravitational to capillary forces), the surface of the drop forms a spherical cap. It appears that nature forms a highly smooth, curved surface. By varying the surface energy, it is possible to obtain a wide range of static contact angles. For example, silicone oil [polydimethylsiloxane (PDMS)] on glass forms a contact angle of about 5 degrees, while it forms an angle of 38 degrees on Teflon and 68 degrees on a fluorinated silicon surface. We have measured contact angles as large as 68 degrees for PDMS on a single crystal silicon wafer with a 50 X/0.8 NA objective using a custom-made phase-shifted, laser feedback microscope. The method for preparing these static drops is simple and we envision that microscopists will be able to prepare easily disposable calibration standards in their laboratories.
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