Catherine C. Cooksey

Reflectance measurements of human skin from the ultraviolet to the shortwave infrared (250 nm to 2500 nm)

While published literature of the optical properties of human skin is prevalent for the visible region, data are sparse in the ultraviolet and shortwave infrared. Spectral imaging has expanded from primarily an earth remote sensing tool to a range of applications including medicine and security applications, as examples. These emerging applications will likely benefit from exemplar data of human skin spectral signatures that can be used in designing and testing spectral imaging systems. This paper details an initial study of the reflectance properties over the spectral range of the ultraviolet to the shortwave infrared. A commercial spectrophotometer was used to collect the directional-hemispherical reflectance of each participant’s skin from 250 nm to 2500 nm. The measurements are directly traceable to the national scales of reflectance and include estimated measurement uncertainties. The portion of skin under test was located on the participant’s forearm and was approximately 5 mm in diameter. The results provided in this study serve as one point of reference for the optical properties of skin that in turn will aid in the development of physical and digital tissue phantoms.

Workshop on Bridging Satellite Climate Data Gaps.

Detecting the small signals of climate change for the most essential climate variables requires that satellite sensors make highly accurate and consistent measurements. Data gaps in the time series (such as gaps resulting from launch delay or failure) and inconsistencies in radiometric scales between satellites undermine the credibility of fundamental climate data records, and can lead to erroneous analysis in climate change detection. To address these issues, leading experts in Earth observations from National Aeronautics and Space Administration (NASA), National Oceanic and Atmospheric Adminstration (NOAA), United States Geological Survey (USGS), and academia assembled at the National Institute of Standards and Technology on December 10, 2009 for a workshop to prioritize strategies for bridging and mitigating data gaps in the climate record. This paper summarizes the priorities for ensuring data continuity of variables relevant to climate change in the areas of atmosphere, land, and ocean measurements and the recommendations made at the workshop for overcoming planned and unplanned gaps in the climate record.

A collection and statistical analysis of skin reflectance signatures for inherent variability over the 250 nm to 2500 nm spectral range

The spectral reflectance signature of human skin provides opportunities to advance observations ranging from medical treatment to security applications. In this study 28 volunteers participated in a skin reflectance measurement of the inside of the right forearm. The reflectance measurements were made over the 250 nm to 2500 nm spectral range. The analysis included estimates of the variability attributed to the instrument, variability of the same subject, and variability among subjects. This allowed for determining measures of similarity and differences that indicate the inherent separability within the distribution. While this sample size may not fully represent a full diverse-population, it does provide a provisional reference point for modeling and simulation.

The evaluation of hyperspectral imaging for the detection of person-borne threat objects over the 400nm to 1700nm spectral region

The detection of person-borne threat objects, such as improvised explosive devices, at a safe distance is an ongoing challenge. While much attention has been given to other parts of the electromagnetic spectrum, very little is known about what potential exists to detect clothing obscured threats over the ultraviolet through the shortwave-infrared spectral region. Hyperspectral imaging may provide a greater ability to discriminate between target and non-target by using the full spectrum. This study investigates this potential by the collection and analysis of hyperspectral images of obscured proxy threat objects. The results of this study indicate a consistent ability to detect the presence of concealed objects. The study included the use of VNIR (400 nm to 1000 nm) and SWIR (1000 nm to 1700 nm), as defined here, hyperspectral imagers. Both spectral ranges provided comparable results, however, potential advantages of the SWIR spectral region are discussed.

SWIR calibration of Spectralon reflectance factor

Satellite instruments operating in the reflective solar wavelength region require accurate and precise determination of the Bidirectional Reflectance Factor (BRF) of laboratory-based diffusers used in their pre-flight and on-orbit radiometric calibrations. BRF measurements are required throughout the reflected-solar spectrum from the ultraviolet through the shortwave infrared. Spectralon diffusers are commonly used as a reflectance standard for bidirectional and hemispherical geometries. The Diffuser Calibration Laboratory (DCaL) at NASAs Goddard Space Flight Center is a secondary calibration facility with reflectance measurements traceable to those made by the Spectral Tri-function Automated Reference Reflectometer (STARR) facility at the National Institute of Standards and Technology (NIST). For more than two decades, the DCaL has provided numerous NASA projects with BRF data in the ultraviolet (UV), visible (VIS) and the Near InfraRed (NIR) spectral regions. Presented in this paper are measurements of BRF from 1475 nm to 1625 nm obtained using an indium gallium arsenide detector and a tunable coherent light source. The sample was a 50.8 mm (2 in) diameter, 99% white Spectralon target. The BRF results are discussed and compared to empirically generated data from a model based on NIST certified values of 6°directional-hemispherical spectral reflectance factors from 900 nm to 2500 nm. Employing a new NIST capability for measuring bidirectional reflectance using a cooled, extended InGaAs detector, BRF calibration measurements of the same sample were also made using NISTs STARR from 1475 nm to 1625 nm at an incident angle of 0° and at viewing angle of 45°. The total combined uncertainty for BRF in this ShortWave Infrared (SWIR) range is less than 1%. This measurement capability will evolve into a BRF calibration service in SWIR region in support of NASA remote sensing missions.

Spectral reflectance variability of skin and attributing factors

Knowledge of the spectral reflectance signature of human skin over a wide spectral range will help advance the development of sensing systems for many applications, ranging from medical treatment to security technology. A critical component of the signature of human skin is the variability across the population. We describe a simple measurement method to measure human skin reflectance of the inside of the forearm. The variability of the reflectance spectra for a number of subjects measured at NIST is determined using statistical methods. The degree of variability is explored and discussed. We also propose a method for collaborating with other scientists, outside of NIST, to expand the data set of signatures to include a more diverse population and perform a meta-analysis to further investigate the variability of human skin reflectance.

Establishment and application of the 0/45 reflectance factor scale over the shortwave infrared

This paper describes the establishment and application of the 0/45 reflectance factor scale in the shortwave infrared (SWIR) from 1100 to 2500 nm. Design, characterization, and the demonstration of a four-stage, extended indium-gallium-arsenide radiometer to perform reflectance measurements in the SWIR have been previously discussed. Here, we focus on the incorporation of the radiometer into the national reference reflectometer, its validation through comparison measurements, and the uncertainty budget. Next, this capability is applied to the measurement of three different diffuser materials. The 0/45 spectral reflectance factors for these materials are reported and compared to their respective 6/di spectral reflectance factors.

Correction of an adding-doubling inversion algorithm for the measurement of the optical parameters of turbid media

We present broadband measurements of the optical properties of tissue-mimicking solid phantoms using a single integrating sphere to measure the hemispherical reflectance and transmittance under a direct illumination at the normal incident angle. These measurements are traceable to reflectance and transmittance scales. An inversion routine using the output of the adding-doubling algorithm restricted to the reflectance and transmittance under a direct illumination was developed to produce the optical parameters of the sample along with an uncertainty budget at each wavelength. The results for two types of phantoms are compared to measurements by time-resolved approaches. The results between our method and these independent measurements agree within the estimated measurement uncertainties.

Low-NEP pyroelectric detectors for calibration of UV and IR sources and detectors

Pyroelectric radiometers with spectrally constant response have been developed at NIST with the cooperation of a few detector manufacturers. The new devices have noise-equivalent-power (NEP) values less than 1 nW/Hz1/2 sufficiently low for use at the output of regular monochromators. Their response flatness is an order of magnitude better than that of filtered Si detectors and can be used to realize simple and low-uncertainty responsivity scales for the UV and IR wavelength ranges. For the first time, the UV irradiance responsivity of a pyroelectric detector has been determined. Based on spectral reflectance measurements of the black coating of the pyroelectric detector, the relative spectral response was determined between 0.25 μm and 30 μm. The relative response was then converted into spectral power and irradiance responsivities using absolute tie points from a silicon-trap-detector in the VIS range. In addition to the UV irradiance responsivity scale realization, the flat response between 1.6 μm and 2.6 μm was utilized and a constant irradiance responsivity was realized and applied as a reference scale for the Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources (SIRCUS) facility of NIST. The spectral power responsivity of the low-NEP pyroelectric detector is the internal standard of the NIST VIS-IR detector calibration facility for the 0.6 μm to 24 μm wavelength range. The pyroelectric standard is used to calibrate other types of detectors for spectral responsivity using detector substitution. The flat-response interval of the pyroelectric standard, calibrated for irradiance responsivity, was also used to measure the integrated irradiance from UV LED sources without using any source standard. The broadband radiometric measurements can be applied to IR LEDs emitting low fluxes between 750 nm and 4300 nm. All pyroelectric detector based calibrations were performed with expanded uncertainties of about 2 % (k=2).

Algorithm for rapid determination of optical scattering parameters

Preliminary experiments at the NIST Spectral Tri-function Automated Reference Reflectometer (STARR) facility have been conducted with the goal of providing the diffuse optical properties of a solid reference standard with optical properties similar to human skin. Here, we describe an algorithm for determining the best-fit parameters and the statistical uncertainty associated with the measurement. The objective function is determined from the profile log likelihood, including both experimental and Monte Carlo uncertainties. Initially, the log likelihood is determined over a large parameter search box using a relatively small number of Monte Carlo samples such as 2·104. The search area is iteratively reduced to include the 99.9999% confidence region, while doubling the number of samples at each iteration until the experimental uncertainty dominates over the Monte Carlo uncertainty. Typically this occurs by 1.28·106 samples. The log likelihood is then fit to determine a 95% confidence ellipse. The inverse problem requires the values of the log likelihood on many points. Our implementation uses importance sampling to calculate these points on a grid in an efficient manner. Ultimately, the time-to-solution is approximately six times the cost of a Monte Carlo simulation of the radiation transport problem for a single set of parameters with the largest number of photons required. The results are found to be 64 times faster than our implementation of Particle Swarm Optimization.