Hyperspectral imagers provide spectral reflectance values representative of the material being observed. Methods to evaluate uncertainties of hyperspectral imagers remains elusive. All spectrometers are subject to uncertainties attributed to various factors such as wavelength, spectral bandwidth, and linearity. Measuring a set of color tile standards with a variety of known spectral reflectance curves provides a means to quantify the differences between the measured and the known reference values. In effect, this takes a bulk of the uncertainty factors into account. Here we report on the development and application of this method to reduce the measurement errors.
Absolute flux calibration of standard stars, traceable to the International System of Units (SI) standards, is essential for 21st century astrophysics. Dark energy investigations that rely on observations of Type Ia supernovae and precise photometric redshifts of weakly lensed galaxies require a minimum uncertainty of 0.5% (k=1) in the absolute color calibration. Other areas of astronomy and astrophysics, e.g. fundamental stellar astrophysics, will also benefit. In the era of large telescopes and all sky surveys, well-calibrated standard stars that do not saturate, are available over the whole sky, and extend to fainter magnitudes are needed. Our collaboration, NIST Stars, has developed a novel, fully SI-traceable laboratory calibration strategy that will enable achieving the demanding 0.5% requirement which we shall describe here. We discuss our results from a pilot study to determine the top-of-the-atmosphere absolute spectral irradiance of bright stars and the next steps.
Sets of chromatic and neutral ceramic tiles are widely used as measurement standards for reflectance factors in color applications. The usual instrument for color measurements is a spectrophotometer that measures the tiles using either a 0:45 or 45:0 illumination and viewing geometry, or with an integrating sphere in order to measure the reflectance factor in either specular excluded or specular included conditions. Having the corresponding measurements of the tile set from a calibrated instrument, systematic errors in the instrument under study can be diagnosed and corrected using a model of the errors and fitting it to the difference in measurements. One such is the Berns-Petersen model, which models photometric scale errors as well as wavelength registration and bandwidth errors using the spectra and its derivatives from the studied instrument. This allows for a simple multilinear regression to be used for recovering the model coefficients either collectively or separately for each wavelength band.
In this study we examine the potential of a ceramic tile set for transferring the calibration from a calibrated spectrophotometer to a hyperspectral imager using the Berns-Petersen model. In particular we compare different approaches that are needed for matching the measurements from the two instruments with respect to the differences in the spectral and spatial resolution that need to be considered for the application of the regression procedure to be possible. We also compare the effectiveness of the procedure when using single model parameters for the whole imaging sensor to that obtained by doing the procedure separately for different spatial regions of the imager. The accuracy of the imager before and after the procedure is evaluated both in absolute radiometric terms as well as colorimetric quantities. We also examine the effect of using different subsets of the available full set of ceramic tiles for the resulting model fit.
We describe the robotic optical scatter instrument (ROSI), a new robotic arm-based goniometer for in-plane and outof- plane reflectance and bidirectional reflectance distribution function (BRDF) measurements of surfaces. The goniometer enables BRDF measurements to be made at nearly any combination of incident and scattering angles, without obstruction from frames or cradles that occur in traditional goniometers made of nested rotation stages. We present exploratory measurements of in-plane and hemispherically-scanned out-of-plane BRDF on a sintered white polytetrafluoroethylene (PTFE) sample using a supercontinuum fiber laser-based tunable light source operated at a wavelength of 550 nm, in order to demonstrate the capabilities of the system. An initial assessment of uncertainties is presented.
STARR II is a planned NIST facility for spectral measurements of specular reflectance and diffuse bidirectional
reflectance distribution function (BRDF) that is the follow-on to the current NIST STARR (Spectral Tri-function Automated Reference Reflectometer) facility. STARR II will improve and expand on the measurement capabilities of STARR, increasing spectral coverage in the short-wave infrared spectral region and adding out-of-plane BRDF measurement capability. We present characteristics of a tunable light source constructed for STARR II, which is based upon a supercontinuum fiber laser and has a tuning range from 500 nm to 2450 nm, alongside those of the current lampbased system in STARR. We then discuss considerations for measuring BRDF using this source. The STARR II goniometer will employ an articulated robotic arm to center and orient the sample, and we calculate the transformations from orientation of the sample and receiver to incident and scattering angles for measurements of in-plane and out-ofplane BRDF. We also present, for the first time, an analytic expression for the correction of measured BRDF to true BRDF due to the finite illumination area and collection aperture, and to the effects of uniformity of illumination. Finally, we present exploratory BRDF measurements on a diffuse sample made using the tunable supercontinnum source.
The Optical Technology Division of the National Institute of Standards and Technology (NIST) provides reference
measurements of specular and diffuse reflectance of materials, including measurements that provide traceability for
diffuser plaques that are used as onboard calibration standards in remote sensing. We are developing new
instrumentation that will enable angle-resolved Bidirectional Reflectance Distribution Function (BRDF) measurements
using a supercontinuum fiber laser-based source and a tunable monochromator. A significant improvement in optical
power density at the specimen over that of lamp-based sources is expected. We present an overview of the source design
and evaluation, including the predicted impact of supercontinuum sources on our next generation of BRDF measurement
instrumentation.
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