The implications of non-uniformity in fields-of-view of commonly used field spectroradiometers

Abstract

Accurately determining the field-of-view (FOV) is a basic requirement in photogrammetry and imaging spectroscopy but has rarely been considered in detail in field spectroscopy where the specifications for different spectroradiometers generally lack clarity or detail. The issue can be further compounded with full spectral systems (0.4 to 2.5µm) which include multiple spectrometers; in these systems the size and alignment of the viewing optics may cause significant spectral non-uniformity across the theoretical measurement area. When recording spectra from heterogeneous targets it is important to have the FOV accurately defined because distinct reflecting surfaces may be present in varying proportions. We assessed the FOVs of the Analytical Spectral Devices Field Spec Pro FR and Spectra Vista Corporation GER 3700 spectroradiometers. The resulting directional response functions are plotted, highlighting several areas of concern.

Geochemical characterization of supraglacial debris via in situ and optical remote sensing methods: a case study in Khumbu Himalaya, Nepal

“…Measured target reflectance was divided by Spectralon calibrated reference surface reflectance, and multiplied by both calibration panel offsets and the user defined reflectance scale for each wavelength (0.35-2.5 µm). Due to FieldSpec Pro instrument variability from the 57 VNIR/SWIR individual optical fiber responses (MacArthur et al, 2007), signal-to-noise ratios, three detectors (Analytical Spectral Devices, 2002), as well as for comparison of field spectra with satellite spectral data, repeat field spectra over each target were averaged to form field spectral reflectance class signatures (here after referred to as spectral signatures).…”

Exploring field-of-view non-uniformities produced by a hand-held spectroradiometer

“…However, the additional 10° lens produced a rather patchy foV with a few "blind" spots where no light sensitivity is recorded. 16 In the fixed focus lens GEr3700, manufactured by Spectra Vista Corporation (a wider wavelength range version of the GEr1500), the response function testing showed a strong shape effect due to the entry slit aperture and lens that implies that these distortions may be unit specific. While the results described by MacArthur et al 16,21 are detailed, their methodology requires precise light source positioning equipment and a substantial investment of investigator time.…”

“…16 In the fixed focus lens GEr3700, manufactured by Spectra Vista Corporation (a wider wavelength range version of the GEr1500), the response function testing showed a strong shape effect due to the entry slit aperture and lens that implies that these distortions may be unit specific. While the results described by MacArthur et al 16,21 are detailed, their methodology requires precise light source positioning equipment and a substantial investment of investigator time. The present study was conceived out of the need for a more rapidly implemented foV assessment scheme that could be applied prior to laboratory studies and field work.…”

“…how uniform is the sensor n response throughout its spectral range. MacArthur et al 16 studied two widely-used spectrometers that showed foV distortions, mostly attributed to the spectrometer's light entry mechanism. Both of the tested spectrometers exhibited foV deviations of all types listed above.…”

“…recent results demonstrate that the assumption of uniformity may not be correct and failing to fully characterise the foV may result in errors caused by attributing spectral properties to the wrong target. 16 The need to define the spectrometer's foV prior to field or laboratory measurements may, therefore, be an important step before its use.…”