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

Abstract

The shape of a spectroradiometer's field of view (FOV) affects the way spectral measurements are acquired. Knowing this property is a prerequisite for the correct use of the spectrometer. If the substrate is heterogeneous, the ability to accurately know what is being measured depends on knowing the FOV location, shape, spectral and spatial sensitivity. The GER1500 is a hand-held spectrometer with a fixed lens light entry slit and has a laser guide that allows control over the target by positioning the entire unit. In the current study, the FOV of the GER1500 was mapped and analysed. The spectral and spatial non-uniformities of the FOV were examined and were found to be spectrally independent. The relationship between the FOV and the built-in laser guide was tested and found to have a linear displacement dependent on the distance to the target. This allows an accurate prediction of the actual FOV position. A correction method to improve the agreement between the expected and measured reflectance over heterogeneous targets was developed and validated. The methods described are applicable and may be of use with other hand-held spectroradiometers.

In-Field Absolute Calibration of Ground and Airborne VIS-NIR-SWIR Hyperspectral Point Spectrometers

“…When a point source is measured, the signal has some (spectral and amplitude) dependence on the position of the source inside the FOV of the bare fiber [6,7]. When this point source is measured with collecting optics, this non-uniformity becomes extremely large due to the focusing of the source on different fibers in the bundle that contribute to different parts of the detector.…”

EUROSPEC: at the interface between remote-sensing and ecosystem CO<sub>2</sub> flux measurements in Europe

“…When measurements of heterogeneous surfaces were simulated using the measured DRFs, significant differences were found between simulated reflectance factors and those expected from the manufacturers' specifications (Mac Arthur et al, 2012). Even when less optically complex spectrometers, measuring only across the VNIR region are considered, the Earth surface sampled is not necessarily that inferred from the manufacturers' specified FOV-included solid angle (Caras et al, 2011). The manufacturers of some spectrometers now offer optical elements within their foreoptic mounts to defocus the foreoptics and thereby homogenize the light received (e.g.…”

“…Limited consideration had been given to the comparability of spectral measurement protocols and systems before EU-ROSPEC (but see Castro-Esau et al, 2006;Pfitzner et al, 2011). Because the use of the same type of sensor in all sites is neither realistic, and perhaps nor desirable as it would undermine the development of new sensors, we need to examine the factors that influence the variability in the data collected with different sensors.…”

“…Field spectroscopists normally assume that the Earth surface sampled by a non-imaging spectrometer with a limited FOV foreoptic is spatially delimited by the solid angle specified by the manufacturer, and that the response across the surface delimited by the FOV is the same for all points inside the given FOV (Castro-Esau et al, 2006;Ferrier et al, 2009;Murphy et al, 2005;Nichol and Grace, 2010). In practice, the spectral response within FOV of a field spectrometer is not constant (i.e.…”

“…Measurement uncertainty is instrument-and environment-specific. Accordingly, characterization of sensor performance and quantification of measurement uncertainty is crucial to producing accurate data (Anderson et al, 2011;Castro-Esau et al, 2006;Jung et al, 2012). Anderson et al (2011) have demonstrated that laboratoryderived measurement uncertainties do not present a useful means of quantifying all uncertainties in field spectroscopy.…”