How To Guide: Field Spectroscopy
Field spectroscopy enables scientists to measure how different materials interact with electromagnetic energy and therefore underpins the technology enabling us to view the world from the air or space. Hyper- and multi-spectral sensors in the air or in orbit around Earth are designed to measure specific wavelengths of light that have been determined to be useful using field spectroscopy. Field spectroscopy is then also used to verify or ‘ground-truth’ drone, plane and satellite data.
However, field spectroscopy is not only used to support remote sensing, it is also a critical scientific tool in its own right that is widely used for non-destructively identifying and characterising materials and surfaces. This is crucial in climate studies, precision agriculture, geological prospecting, snow melt mapping, spatial ecology, pollution management and many other fields. While there are many applications for spectroscopy in the laboratory under fixed lighting conditions, this guide will focus specifically on using portable spectrometers outdoors in a naturally lit environment, since this is most relevant for environmental applications and validating satellite and airborne remote sensing instruments.
The field spectrometer enables the measurement of two particularly important properties of surfaces: the albedo and spectral reflectance factor. The albedo is the ratio of all downwelling solar energy hitting a surface to the upwelling light escaping from it. It is crucial for determining energy balance and calculating the amount of solar energy being absorbed in forest canopies, snowpacks and used for photosynthesis in crops and oceanic algal blooms. The reflectance factor is the amount of incoming solar energy reflected in a particular direction, relative to the approximately ‘perfect’ scattering of a reflectance panel, measured using a lens with a fixed viewing angle (Nicodemus et al., 1977). Directional measurements are more similar to what is measured by satellites, planes and drones that make multi- or hyper-spectral maps of areas of Earth’s surface. Measurement protocols for each of these measurements are widely available in the scientific literature, textbooks and instruction manuals (e.g. NERC FSF).
This guide will outline some specific pitfalls that may not be immediately obvious to a new user but that could completely invalidate the measurements made using the field spectroradimeter. The aim is to help the reader to avoid some of those important but common mistakes and misunderstandings associated with obtaining albedo and directional reflectance factor measurements in the field.
Measure the correct property
The type of measurement to make using a spectroradiometer depends completely upon the research question to be answered. If the aim of the research is to validate remote sensing data, or to identify spectral features for specific materials, then it is probably most appropriate to measure reflectance in some fixed viewing angle as similar as possible to the viewing angle of the airborne or satellite instrument. However, this type of measurement omits reflected radiation from the entire viewing hemisphere apart from in a narrow cone of view. For studies of energy balance, an estimate of the surface albedo is most appropriate because it takes into account energy arriving and leaving a surface in all directions.
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Use the right lens
A directional reflectance measurement is enabled using a bare fibre, or a collimating lens that limits the instantaneous field of view. Only a small fraction of the total energy reflected from the surface is measured by the sensor. For albedo, the appropriate lens is known as a cosine collector. This device expands the viewing angle of the fibre optic to the entire hemisphere (180 degrees of zenith and 360 degrees in azimuth). The sensor is named because it uses a diffuser to measure the irradiance proportional to the cosine of the angle of incoming light. It is important to realize that different collimating lenses produce different measurement areas on the ground.
Measure in the right direction
The measurement direction is critical because all-natural surfaces reflect light in some directions slightly or dramatically more than other directions. Ice and snow, for example, show a strong preference for scattering light in the forwards direction. This is a major reason why a downwards-looking measurement with a limited viewing angle cannot be converted to surface albedo without knowledge of the anisotropic reflectance factor (ARF). Essentially, the measurement may well be missing the majority of the reflected energy. Nadir-view measurements are very common, but there may be good reasons to measure in the maximum scattering direction, or backwards scattering directions. Take care to ensure the chosen measurement direction is appropriate for your research question.
Illumination conditions
Even for identical surfaces measured with identical instruments, reflectance measurements vary if the illumination conditions change. This can be due to changing cloud cover, solar angle and atmospheric composition. Cloud is more effective at absorbing longer wavelength solar energy than shorter visible light. In the case of snow and ice, this means on cloudy days the incoming light is more concentrated in wavelengths that are efficiently reflected by the surface, leading to a greater albedo than on clear-sky days (Grenfell and Maykut, 1977). Similarly, when the sun is at a lower angle incoming light can ‘skim’ the surface rather than penetrating more deeply, meaning albedo increases when the sun is lower in the sky. For these reasons, measurements are often made during a short window (2 hours either side of solar noon) to minimize errors due to the changing solar angle and in conditions of constant cloud cover.
Spectrometer warm-up time
Spectrometers measuring most of the solar spectrum usually consist of several separate internal arrays, each having slightly different warm-up times. Ignoring this can lead to artefacts in the data at the crossover points between the arrays. In the case of the widely used ASD Field Spec spectrometers a warm up time of at least 30 minutes is recommended (NERC Field Spectroscopy Facility) to ensure both arrays are operating at their optimal temperature. This is especially true for applications in cold places.
The 1000 nm artefact
There is a common feature in the spectral data from some spectrometers in the form of a step in the spectra at 1000 nm. This is the result of clustering of fibres that measure specific wavelengths within the fibre-optic cable that causes certain parts of the ice surface being measured preferentially at certain wavelengths. If there is a degree of non-uniformity on the surface, this can lead to step-shaped artefacts in the data. This can be corrected during data-processing or avoided by fitting a randomizing filter to the field instrument.
Choosing a target area
It is also important to acknowledge the area covered by a sample surface and the area imaged by the spectroradiometer. If the surface you wish to measure is 10 cm2 and the imaged area is 20 cm2 then the measured spectrum will be the integrated signal of your sample and the other surfaces surrounding it. For this reason, strive to ensure the viewing footprint of the sensor is smaller than the area covered by your sample surface. The footprint can be calculated from the sensor height and instantaneous field of view using basic trigonometry.
The perfect reference panel
For reflectance factor measurements, the quality of the reference panel is paramount. This is the standard against which your sample is measured. If there are dents, scratches, discolorations, dirt or imperfections in the panel, these propagate through all the sample measurements and can obscure or exaggerate real spectral features. The point of the panel is that it is as close as possible to a perfect Lambertian scattering surface. The best available option is flat, clean SpectralonTM, but even this is not really a perfect Lambertian scatterer, meaning any imperfections impact the quality of the final data. The reflectance of the reference panel should be measured at least before and after each sample measurement to enable corrections for changing illumination conditions.
Metadata
Because the reflectance or albedo measurement made using the spectroradiometer is so sensitive to the precise instrument configuration and illumination conditions, extensive and accurate metadata is crucial for fair comparison between different research groups, different locations, or even different measurements within your dataset. As a minimum, the date, time, instrument details, lens type, measurement height and angle, cloud cover, surface slope and aspect and basic meteorological observations should be recorded for each and every measurement made (Rasaiah et al., 2014).
References
- Nicodemus, F. E., Richmond, J.C., Hsia, J.J. 1977. Geometrical considerations and nomenclature for reflectance. Washington, DC: National Bureau of Standards, US Department of Commerce. URL: http://physics.nist.gov/ Divisions/Div844/facilities/specphoto/pdf/geoConsid.pdf
- Grenfell, T.C., Maykut, G.A. 1977. The optical properties of ice and snow in the Arctic Basin. Journal of Glaciology, 18: 445 - 463
- NERC Field Spectroscopy Facility: Instrument Guides, http://fsf.nerc.ac.uk/resources/guides/
- Rasaiah, B.A., Jones, S.D., Bellman, C., Malthus, T.J. (2014) Critical metadata for spectroscopy field campaigns, Remote Sensing, 6: 3662 – 3680, doi: 10.3390/rs6053662
