The Role of Field Spectroscopy

A Definition

Field spectroscopy is defined as the quantitative measurement of radiance, irradiance, reflectance or transmission in the field. In literature, it can sometimes be called other names, such as ground based radiometry, field spectroradiometry, or more narrowly, reflectance spectrometry. Fundamentally, the technique is a method for inferring properties (biological, chemical or physical) about the world based on quantitive measurements of the interaction with light with the Earth’s surface, whether that be vegetation, soils, animals etc.

Uses of Field Spectroscopy

With this definition in mind, field spectroscopy can be used as a tool for a number of different applications, such as:

to make direct measurements of, radiance (and exitance), irradiance or transmission in the field
to convert image-based measurements of radiance to calibrated radiance or reflectance (calibration of images acquired by space or airborne sensors)
for validation of measurements made from satellite or airborne platforms
to better understand the nature of the interaction of electromagnetic radiation with earth surface objects
to improve the quantitative determination of Earth surface or media
to provide data for input into radiative transfer models or validation of modelled results
to build spectral databases, perhaps for spectral unmixture modelling

Features of Field Spectrometers

Field spectroscopy is notable for it’s fine scale and ground validated measurements, both in the spectral and spatial domain. With a modern field spectrometer, and with expertise in knowing what you are taking measurements of with such an instrument, you are able to take hyperspectral measurements with excellent spectral resolution at extremely high spatial (i.e. covering very small areas, much less than what a satellite or airborne sensor sees) resolution. In certain circumstances, this allows for a unique “spectral fingerprint” of an object to be determined, which can then allow for identification from in-orbit or airbrone sensors.

As mentioned, most modern field spectrometers are hyperspectral. In this context, hyper- denotes that the instrument takes measurements at many different spectral intervals, as opposed to multi-, which take measurements over relatively few bands. As an example, the multispectral sensor on ESA’s Sentinel-2, covers 13 bands over a spectral range from 400 to 2500 nm. By comparison, a modern field spectrometer usually takes measurements at over 1000 bands. Measurements from field spectrometers are also contiguous (immediately successive) and continuous (having no breaks, unbroken, and uninterrupted in sequence).

This leads to very rich datasets in field spectroscopy, which can pose problems when analysed. Data collection in the field can also prove complex.

However, as the field continues to grow, including the growth of in-orbit hyperspectral imagers, field spectroscopy – with it’s ability to provide ground based, high spectral and spatial resolution measurements – will continue to be a powerful tool for remote sensing.