We present an extension to existing Monte Carlo photon transport methods to simulate integrating sphere experiments. This method uses a Monte Carlo approach to simulate photon paths in tissue and an analytical expression for the probability of a photon in an integrating sphere being re-incident on the tissue. Analytical models, previous works on Monte Carlo photon transport, and measurements of a synthetic tissue phantom validate this method. We present two approaches to back-calculate the optical properties of samples. Experimental and simulation uncertainties are propagated through both methods. Both back-calculation methods find the optical properties of a sample accurately and precisely.
Computational models are useful tools for simulating the thermal response of biological tissue to laser exposure. These models typically include a solution to a bio-heat equation and require a set of physical properties for each distinct tissue as inputs. In applications such as cauterization and surgery, the irradiated tissues may undergo severe heating, resulting in substantial denaturation and a probable change in one or more of their associated physical properties. While the wellestablished temperature-dependent behavior of water can approximate the changes in tissue thermal properties during heating, there is minimal available research on the dynamics of optical properties of tissue. This study characterized optical absorption and scattering of porcine skin tissues that had undergone temperature increases up to 90°C. We excised thin samples of porcine dermis and subcutaneous fat, placed them in a custom sealed tissue mount with a built-in temperature sensor, and raised them to various temperature intervals through submergence in a controlled hot water bath. Following heating, a series of goniometric spectrophotometry measurements of transmittance and reflectance, focusing on the near-infrared band, allowed for calculation of optical absorption and scattering coefficients as a function of tissue temperature and rate process model damage. The results allow for more accurate representation of tissue characteristics in computational models.
The reflections of high energy laser off surfaces can present hazards to persons and instruments at significant
distances. The heating from these lasers cause changes in the reflection characteristics of surfaces they impact. As
such, the reflections from these surfaces cannot be properly modeled with static bidirectional reflectance distribution
functions (BRDFs), but require time-dynamic BRDFs. Moreover, the time-evolution of the surface reflections is not
deterministic, but can vary even when the materials and irradiance conditions are nearly identical, such that only
probabilistic characterization is realistic. Due to the swiftly changing nature of the reflections, traditional BRDF
measurements with goniometric instruments is impossible, and BRDFs must be deduced from images of the
reflected light incident on a screen which intercepts a portion of the reflection solid angle. A model has been
constructed to describe these complex probabilistic dynamic BRDFs with only a moderate number of intuitive
parameters, where these parameters have central values and statistical variances. These simple parametric
representations are appropriate for use in predictive modeling codes and are also easily adjustable to allow facile
exploration of the sensitivity of hazards to laser, material, and model uncertainties. An automated procedure has
been created for determining appropriate parameter values and variances from captured screen images, without the
need for case-by-case human judgment. Examples of the parameter determination procedure are presented.
We describe the development of a mechanically simple, radiometrically stable transfer radiometer designed for both radiance and irradiance measurements. The filter radiometer consists of a six-element Si trap detector, a temperature stabilized filter wheel with up to 5 filters, and two precision apertures in a Gershun tube arrangement. With the Gershun tube installed, the instrument operates in radiance mode; with the front aperture removed, in irradiance mode. Two trap detector filter radiometers have been designed and built by the National Institute of Standards and Technology (NIST) for use in remote sensing applications. The filter radiometers have been characterized for optical and electrical performance, and have been calibrated for responsivity using both narrow-band, tunable-laser-illuminated and broad-band, lamp-illuminated integrating sphere sources. This paper describes the filter radiometer design, characterization, and deployments for two remote sensing projects.
In the field of spectrophotometry, the value of the quantities depends upon the geometry under which they are measured. Therefore, it is imperative to completely describe the measurement geometry. Many documentary standards specify the geometry for a particular application. However, to accurately specify the geometry, a general, basic understanding of the relevant parameters for describing the geometry is required. A systematic approach for describing the measurement geometry is presented, which will hopefully have a positive impact on documentary standards. The key to describing the geometry is to consider the illuminator and receiver of the instrument as optical systems with pupils and windows. It is these optical systems, together with the reference plane, that determine the sampling aperture of the instrument. The geometry is then completely described by the relations between the sampling aperture and the optical systems of the illuminator and receiver. These concepts are illustrated by considering three configurations of pupils and windows relative to the focal point of an optical system.
The specification of the color of an object is important, sometimes crucially, to its acceptability for an intended application. When quantitative measurements are used to determine the color of an object, an uncertainty is required to satisfy the requirements of traceability. Determining the uncertainty of a color measurement has been a topic of interest for many years, and has received additional impetus with the publication of the ISO Guide to the Expression of Uncertainty in Measurement, which describes methods for estimating uncertainties in a consistent manner. These methods are applied to the case of reflectance colorimetry, where many of the sources of uncertainty are caused by systematic effects and are therefore correlated. Analytical expressions are derived for the combined uncertainties in the quantitative expression of color due to both correlated and uncorrelated effects. The measurement equation is used to derive the uncertainties in reflectance factor from different sources, and to take correlations between signals at the same wavelength into account. The correlations between reflectance factors at different wavelengths are also included in the analysis. These concepts are illustrated by considering uncertainties from common sources for selected colored specimens.
The measurement and long-term monitoring of global total ozone by ultraviolet albedo measuring satellite instruments require accurate and precise determination of the Bi-directional Reflectance Distribution Function (BRDF) of laboratory-based diffusers used in the pre-launch calibration of those instruments. To assess the ability of laboratories to provide accurate UltraViolet (UV) diffuse BRDF measurements, a BRDF measurement comparison was initiated by the NASA Total Ozone Mapping Spectrometer (TOMS) Project. From December 1998 to September 1999, NASA's Goddard Space Flight Center (GSFC), TPD TNO (formerly the TNO Institute of Applied Physics), and the National Institute of Standards and Technology (NIST) made BRDF measurements on four Spectralon diffusers used in the pre-launch calibration of three TOMS instruments. The diffusers were measured at the six TOMS wavelengths and at the incident and scatter angles used in the TOMS pre-launch calibration. The participation of GSFC, TPD TNO, and NIST in the comparison establishes a link between the diffuser calibrations of the on-orbit TOMS instruments, the Ozone Monitoring Instrument (OMI), and a national standards laboratory. The results of the comparison show that all of the BRDF measurements on the four diffusers agreed within +0.85 % to -1.10 % of the average BRDF and were well within the combined measurement uncertainties of the participating laboratories.
As part of the Triana mission, the Scripps Earth Polychromatic Imaging Camera (Scripps-EPIC) will view the full sunlit side of Earth from the Lagrange-1 point. The National Institute of Standards and Technology and the Scripps Institution of Oceanography, in collaboration with the contractor, Lockheed-Martin, planned the radiometric calibration of Scripps-EPIC. The measurements for this radiometric calibration were selected based upon the optical characteristics of Scripps-EPIC, the measurement equation relating signal to spectral radiance, and the available optical sources and calibrated radiometers. The guiding principle for the calibration was to perform separate, controlled measurements for each parameter in the measurement equation, namely dark signal, linearity, exposure time, and spectral radiance responsivity.
We have performed regular spectral transmittance and reflectance measurements over the 1 (mu) m to 2.5 (mu) m wavelength region on several different types of materials using three different spectrophotometer systems. Two of the systems employ grating-based monochromators and InGaAs photovoltaic detectors. The beam at the sample position is nearly collimated. The other system uses an FTIR (Fourier Transform Infrared) spectrophometer as a source and a diffuse Au-coated integrating sphere with a photoconductive HgCdTe detector. In this system, the sample is placed at the focus of an f/6 converging beam. Measurements are performed on transmissive materials as well as either highly reflective or absorptive mirrors and the results are compared, taking into account any differences in beam geometry and polarization among the different systems.
A portable high accuracy double spectrometer for ultraviolet (UV) spectral irradiance measurements has been under development at the National Institute of Standards and Technology (NIST) or several years. It has been used in the comparison of NIST spectral irradiance sources: an electron storage ring, 1000 W quartz-halogen lamps, deuterium arc lamps, and a windowless argon miniarc. A UV spectral irradiance intercomparison with the Physikalische Technische Bundesanstalt of Germany has also been carried out with the instrument. This paper will discuss the modular design of this instrument, preliminary uncertainty analysis, results of standard source comparisons and results of solar ultraviolet measurements using the UV spectrometer with the NIST spectral irradiance field calibrator.
Polytetrafluoroethylene (PTFE) is widely used in remote sensing applications requiring a diffuse reflectance standard for detector calibration. The bi-directional and directional-hemispherical reflectance properties of both pressed and sintered PTFE were measured at ultraviolet wavelengths to provide information for their use as standards in this spectral range. The reflectance decreases with decreasing wavelength for both geometries, and the ratio between the reflectances for these geometries remains constant for wavelengths from 300 nm to 400 nm.
Polytetrafluoroethylene (PTFE) is widely used in applications requiring a material with a diffuse reflectance factor close to unity. The bidirectional reflectance distribution function (BRDF) of both pressed and sintered samples was measured as a function of wavelength, polarization, and incident and viewing angles. The results for both samples show that the BRDF is constant at wavelengths from 400 nm to 1000 nm and depends on the polarization of the incident beam of radiant flux, that the BRDF varies with viewing angle, and that Helmholtz reciprocity holds for angles from 10 degrees to 60 degrees.
Two types of mirrors and a glass are used as Standard Reference Materials to calibrate the specular reflectance scales of optical systems. While calibrations of these materials are routinely performed using the 6 degree/6 degree geometry, there has been an increased interest in the optical radiation measurement community for additional bidirectional geometries such as 30 degree/30 degree and 45 degree/45 degree. This paper reports the results of specular reflectance measurements of first surface aluminum and gold mirrors and polished black glass at wavelengths from 250 nm to 2250 nm. Supplemental measurements of wavelength standards are included to provide a comprehensive overview of the standards available for calibrating spectrophotometers.
A comparison of spectral diffuse reflectance between different national standards laboratories is being planned under the direction of the Comite Consultatif de Photometrie et Radiometrie (CCPR). A similar comparison of bidirectional reflectance distribution factor among laboratories in the United States in support of optical remote sensing measurements is nearing completion. Since this comparison provides valuable lessons for the one organized by the CCPR, pertinent results and their implications are presented.
Two interagency solar ultraviolet spectroradiometer intercomparisons (September 1994 and June 1995) have been conducted at Table Mountain outside Boulder, Colorado. A third intercomparison is planned for June 1996 at the same location. The intercomparisons were organized by the National Institute of Standards and Technology (NIST) at a field site supported by the National Oceanic and Atmospheric Administration and included participants representing North American agencies currently monitoring solar ultraviolet irradiance. Instruments were characterized at the intercomparisons for wavelength error, bandwidth, and stray- light rejection. All instruments were calibrated for spectral irradiance responsivity by NIST personnel with specially developed quartz-halogen field irradiance standards. Thus, comparisons of synchronized solar irradiance measurements allowed true comparisons between instruments, without the added complication of differences in irradiance scales of the participants. Estimates of differences in individual network propagation of irradiance scales were made separately. In addition, there were extensive concurrent ancillary measurements in an effort to compile a standard data set. We will discuss the results of these experiments and the overall agreement between the participants.
Several different monitoring networks using spectroradiometers are being developed to detect changes in the solar ultraviolet irradiance at the surface of the earth due to stratospheric ozone depletion. To ensure the accurate, long-term measurements that are required from these networks, a strategy for instrument intercomparisons is necessary. This involves the characterization of the instrument parameters which affect measurements of solar ultraviolet irradiance, particularly the linearity, wavelength accuracy, irradiance responsivity, slit- scattering function, and cosine response of the instrument. We have developed laboratory techniques for determining each of these parameters and have used them to characterize a specific spectroradiometer. These techniques and results are being used to develop a strategy for a planned field intercomparison of instruments from several monitoring networks.
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