The Fields of View and Directional Response Functions of Two Field Spectroradiometers

Accurately determining field-of-view has rarely been considered in field spectroscopy where specifications for fore optics used are generally limited and the influence of the spectroradiometer rarely considered. The issue can be compounded with full wavelength spectroradiometric systems which include multiple spectrometers. In these systems, the size and alignment of the viewing optics and technology adopted to transfer light from the fore optic to individual spectrometers may cause significant nonuniformity of spectral response across the area of measurement support, and this area may not align with that assumed from the specification that is supplied for the fore optic. When recording spectra from heterogeneous earth surface targets, it is important to have the area of measurement support accurately defined as individual reflecting surfaces may be present in varying proportions within this area, and these proportions need to be determined to relate spectral reflectance or spectral radiance to state variables or target classifications being considered. The area of measurement support and the spatial and spectral responsivity of an ASD Field Spec Pro FR spectroradiometer and a SVC GER 3700 spectroradiometer have been determined by measuring the directional response function (DRF) of each instrument. This research highlights several areas of concern and makes recommendations for the improvement of field spectroradiometers and field spectroscopy methodologies. These results are specific to the spectroradiometer/fore optic combinations investigated and at the measurement distances specified. Although similar characteristics can be expected for other instruments/fore optics of the same design, and at other measurement distances, the DRFs will vary from those reported here.