A dual-spectrometer approach to reflectance measurements under sub-optimal sky conditions

Abstract

This paper presents a practical method for the development of spectral reflectance libraries under sub-optimal sky conditions. Although there are commercially available spectrometers which simultaneously measure both downwelling and upwelling radiance to mitigate the impact of sub-optimal sky conditions, these spectrometers only record in the visible and near infra-red. There are presently no commercially available spectrometers with this capability that can record the visible through shortwave infra-red. This paper presents a practical method of recording and processing data using coordinated measurements from two full-range spectrometers and discusses potential pitfalls and solutions required to achieve accurate reflectance spectra. Results demonstrate that high-quality spectral reflectance libraries can be developed with this approach. A. White, C. A. Jones, and K. Lee, "Coastal characterization from hyperspectral imagery: an intercomparison of retrieval properties from three coast types," in ©2012 Optical Society of America

Surface Reflectance and Sun-Induced Fluorescence Spectroscopy Measurements Using a Small Hyperspectral UAS

“…In the laboratory, we cross-calibrated the spectrometers radiometric responses considering a range of light intensities typical for clear-sky days during summer. In the field, we used the empirical correction method proposed in Bachmann et al, 2012 [58]. It consists of determining the actual spectral transfer function between two spectrometers.…”

Fully automated laboratory and field-portable goniometer used for performing accurate and precise multiangular reflectance measurements

“…It is for these reasons that many researchers in the remote sensing community prefer to coincidently measure the downwelling illumination with the target reflectance and incorporate these additional measurements into their postprocessing routines to better compensate for temporal changes in illumination that occur throughout the course of a scan. 6,10,16 While the downwelling illumination measurements are useful for removing the temporal changes, these measurements do not compensate for the spatially dependent incoming illumination from nearby objects (i.e., adjacency effects). Specifically, when measuring an HCRF, it is necessary to understand the angular distribution of the incoming radiance.…”

“…A third spectrometer is then used to ensure that any temporal changes in illumination are measured for compensation in the postprocessing routines; typically, this third spectrometer will monitor the reflected radiance from a Spectralon TM calibration panel or monitor the downwelling radiance onto the surface by means of a cosine collector fore-optic attachment. 16 This third spectrometer is located at a stationary location near the GRIT-T. Due to its lightweight frame design, the GRIT-T is able to add this dual-view capability without dramatically reducing the mobility of the system, and the software is able to integrate the third spectrometer into the overall control scheme since the control of all on-board devices operates on a separate internal network. Figure 3 shows the placement of the spectrometers along with the fiber optic cables and the design of the fore-optic pointing head.…”

A hyperspectral vehicle BRDF sampling experiment

“…During this period of time the scene illumination can vary considerably due to changing atmospheric conditions such as clouds or haze. 6 To capture this temporal illuminant variation, GRIT uses a second spectrometer of the same make and model as the first that stares downward normal to a white Spectralon TM panel. 6 The second radiometer is referred to as the base station and is usually placed closely enough to the goniometer that both share the same incident irradiance but far enough that its presence does not contaminate the goniometer measurements.…”