Field spectroscopy generally refers to the use of non-imaging spectrometers near the ground surface and it is usually aimed at evaluating spectral reflectance of the investigated target. For this purpose, consecutive measurements of total incident solar irradiance and of radiance or irradiance upwelling from the target are collected by an operator, or more recently by new instruments for long-term and unattended field spectroscopy measurements. The incident irradiance is usually computed by measuring the radiance upwelling from a white calibrated panel which represents the ideal Lambertian surface. Upwelling fluxes are instead usually collected holding the sensor vertically over the surface (nadir view), although spectral libraries collected observing the target from different viewing angles are also available.
Field spectrometry is also referred to as ‘proximal sensing’ to underline that spectra are collected with portable spectroradiometers in the vicinity of the target, in contrast to ‘remote sensing’, which is instead usually performed with satellite or airborne sensors.
Field spectroscopy is therefore an in-situ method for characterising the reflectance of natural or artificial surfaces and thereby provides reference data for the calibration or validation (cal/val) of airborne and satellite sensors. This method provides a means of scaling-up measurements from small areas (e.g. leaves, rocks) to composite scenes (e.g. vegetation canopies), and ultimately to pixels.
Field spectroscopy is used in different applications, for example, soils, rocks, vegetation and chlorophyll fluorescence, water, snow surfaces and atmosphere. Long-lasting field spectroscopy campaigns based on manual measurements are extremely resource-demanding and do not ensure repeatability of the acquisition conditions as the instrument setup is initialized each day. To overcome such limitations a few research groups have initiated automatic tower-based spectral reflectance measurements using different devices. With such setups, non-imaging spectrometers are installed in the field and are operated automatically for long periods (i.e. months to years) and different networks of hyperspectral instruments are now becoming operational (e.g. RadCal Net).
Field spectroscopy can be also used to predict optimum spectral bands, viewing configuration, spectral calibration and time to perform a particular remote sensing task but also to develop, refine and test models relating biophysical attributes to remotely-sensed data. In this context, ground reflectance measurements are therefore mainly used as input in simulation study for sensor design, calibration/validation data for remote sensing sensors, for spectral mixture analysis and for the development of relationships between field data and radiometric variables.
Since spectroscopy is the study of matter using electromagnetic radiation, point or imaging field spectrometers are instruments which allow the measurements of reflected or emitted electromagnetic radiation. In particular, portable or hand-held spectroradiometers are small instruments that spectrally measure the radiation reflected or emitted by a target and they are useful in obtaining accurate spectral data over different surfaces. In remote sensing, they generally cover the 400-2500 nm spectral range and operate with a full width at half-maximum of about 1.5/3 nm, so that they can collect radiation in a continuous way across the spectrum. The final output is therefore the hyperspectral signature of reflectance of the surfaces versus the considered wavelength.