Effects of Spectralon absorption on reflectance spectra of typical planetary surface analog materials.

Hao Zhang,Weidong Jin,Yazhou Yang,Chujian Liu,Weibiao Hsu

doi:10.1364/oe.22.021280

Abstract

Acquiring accurate visible and near-infrared (VisNIR) reflectance values of atmosphereless celestial bodies is very important in inferring the physical and geological properties of their surficial materials. When a calibration target with inherent non-trivial absorption features is used, the calibrated reflectance would essentially always contain spurious spectral features and the spectroscopic data may easily be misinterpreted if the artifact is not properly taken care of. We demonstrate with laboratory reflectance measurements that the VisNIR spectra of three typical planetary surface analog materials, lunar simulant JSC-1A, olivine and pyroxene grains, have an artificial peak at 2.1 µm when Spectralon-type plaque made of polytetrafluoroethylene is used as the calibration target in the NIR region. The degree of severity of this artifact is dependent on the strength of the 2.0 µm absorption feature of the mineral. Empirical methods are proposed to remove this artifact to bring the spectra close to that calibrated by a gold mirror which does not have any conspicuous absorption features in the NIR region. The correction methods may be applied to reflectance data acquired by the VisNIR imaging spectrometer onboard the Yutu Rover of the Chinese Chang'E 3 lunar mission which employed an onboard Spectralon-type calibration target.

Highlights

To accurately extract the surficial mineralogical, compositional and physical information of atmosphereless celestial bodies using visible and near-infrared (VisNIR) reflectance spectroscopy in space missions, space-borne and in situ spectrometers should be rigorously calibrated to obtain the reliable reflectance of the surface materials (e.g., [1,2,3,4])
We demonstrate that the absorption feature at 2.14 μm of the Spectralon plaque incurs spurious spectral features with varying degrees of severity in sample spectra calibrated by Spectralon plaque in the NIR region
Clark and colleagues ([8,23]) and others as early as more than 20 years ago, it seems that reflectance spectra containing this artifact floating around in the literature have not diminished substantially

Summary

Introduction

To accurately extract the surficial mineralogical, compositional and physical information of atmosphereless celestial bodies using visible and near-infrared (VisNIR) reflectance spectroscopy in space missions, space-borne and in situ spectrometers should be rigorously calibrated to obtain the reliable reflectance of the surface materials (e.g., [1,2,3,4]). InfraGold plate was chosen as the calibration target over Spectralon in the Near-Infrared Spectrometer (NIS) onboard the Near Earth Asteroid Rendezvous (NEAR) spacecraft for its resistance to solar irradiation damages [17] to ensure the long-term stability of measured spectra (0.8-2.7 μm) [2,3]. Longer than 1.8 μm or so, the reflectance starts to drop below 98% and reaches a “local minimum” value of 94% at 2.14 μm This absorption feature (a dip of ~5%) must have caused the spurious peak located at 2.1 μm of the sample Rs spectra when radiance of the sample is ratioed by radiance from the plaque during the calibration process. In the NIR region above 1 μm the gold reflectance is smoother than that of Spectralon

Laboratory measurements

Discussions

Findings

Conclusions