PortalEarth Sciences Portal Relevant DisciplinesGeologyRemote Sensing Related Information Spectroradiometer Spectroscopy Radiometry
Spectroradiometry is a technique in Earth and planetary remote sensing, which makes use of light behaviour, specifically how light energy is reflected, emitted, and scattered by substances, to explore their properties in the electromagnetic (light) spectrum and identify or differentiate between them. The interaction between light radiation and the surface of a given material determines the manner in which the radiation reflects back to a detector, i.e., a spectroradiometer. Combining the elements of spectroscopy and radiometry, spectroradiometry carries out precise measurements of electromagnetic radiation and associated parameters within different wavelength ranges. This technique forms the basis of multi- and hyperspectral imaging and reflectance spectroscopy, commonly applied across numerous geoscience disciplines, which evaluates the spectral properties exhibited by various materials found on Earth and planetary bodies.
Spectral properties such as brightness and reflectance patterns vary depending on the mineralogical compositions and crystalline structures of the given material. This variation is contributed by the presence of spectrally active components within the material, such as metallic oxides and clay minerals, which give rise to unique absorption features. Upon measurements with a spectroradiometer, these absorption features can be quantified as characteristic absorption bands in a reflectance spectra. The specific shapes associated with the bands that occur at distinctive wavelength positions enable the identification of minerals and facilitate lithological interpretations.
Conventionally, spectroradiometry is applied to the following portions of wavelengths in the electromagnetic (light) spectrum:
Ultraviolet (UV): 1 nm – 400 nm Visible-near Infrared (VNIR): 400 nm – 750 nm Short-wave Infrared (SWIR): 750 nm – 2500 nm Mid Infrared (MIR): 2500 nm – 5000 nm Thermal Infrared (TIR): 7500 nm – 15000 nm
Today, most geological applications with spectroradiometry are focused within the visible-near infrared and short-wave infrared wavelength ranges. Spectroradiometry offers a simple, non-destructive, rapid, and efficient approach that complements traditional and heavy-duty geochemical methods, to characterize mineral assemblages and rock textures. It thereby facilitates the study of geological processes, exploration for natural resources, and reconstruction of past environments and climates. Its application extends not only to Earth but also to extraterrestrial planets, broadening our understanding of geological processes beyond our own planet.
How spectroradiometry works In spectroradiometry, spectral features can be recognized and quantified by making use of the spectra containing different parameters measured by spectroradiometers. The most widely used spectral parameter in spectroradiometry for applications in geosciences is reflectance.
Spectroradiometry can be imaging and non-imaging in practice. Imaging spectroradiometry captures spectral data from a specific region or a scene, creating a two-dimensional with recorded spectral information dedicated to each pixel. Non-imaging spectroradiometry, on the contrary, measures spectral data from a single point or a small focused area, offering more detailed information about the spectral properties of a specific material.
Experimental set-up The experimental set-up for imaging spectroradiometry is simple because image processing can be conducted through computer software to show the spectral parameters for analysis, such as reflectance and brightness. Most images are freely available worldwide, widely used by different institutions, and have an extensive spatial coverage. Values of spectral parameters like reflectance can then be directly extracted from all pixels in the imagery, aggregated and averaged to produce a reflectance curve for spectral analysis.
In terms of non-imaging spectroradiometry, data collection and sampling are usually conducted through direct scanning with spectroradiometers in the laboratory or in the field. To ensure data accuracy, it is important to carry out the experiment under a stable and controlled environment. For instance, most laboratory scanning practices are performed in the dark to minimize ambient light and scattering, while field scanning is typically conducted with a contact scanning probe, such that measurements are taken in direct contact with the sample surface, free from external light sources, and in a localized setting. In both scenarios, the spectroradiometers are frequently calibrated with a white diffusion reflectance panel, which provides a reference reflectance value (99%) to maintain experimental accuracy. During spectral measurements, they exert homogeneous illumination straight towards the sample surface. Spectral data acquired will then be presented through digital software associated with the spectroradiometers.
Analysis of spectral features Reflectance appears as individual absorption bands spanning the electromagnetic spectra, which vary with mineralogy and chemical compositions. Reflectance spectra obtained directly from the spectroradiometers without data processing are known as raw reflectance spectra. Although some prominent patterns of absorption may be identified, they are prone to influence from the overall spectra trends, and features like amplitudes and magnitudes could mislead the interpretations under such circumstances.
In order to facilitate data analysis, the raw reflectance spectra are commonly normalized to provide better visualization and quantification of trends and patterns of spectral parameters. This is commonly done by statistical techniques including detrending and continuum removal.
1. Detrending:
Removing the trend components present in the raw reflectance spectra to produce detrended spectra (usually flattened) revealing the true shapes, patterns, and distributions of absorption features.
2. Continuum removal (Convex hull transformation):
Removing overall shape, level changes, and slopes induced by other materials in the raw reflectance spectra (indicated as continuum lines) to produce continuum-removed spectra which allows effective comparison of the individual absorption features under a common baseline.
From the normalized spectra, spectral features can be accurately identified, analyzed, and compared to that among different materials. Spectrograms can also be generated using the normalized spectra to further enhance visualization. The features can be characterized using geometrical parameters describing the shapes and appearance of a particular reflectance spectrum:
Position (P): The position of absorption band at a certain wavelength interval. Depth (D): The depth of absorption band (absorption value) at a certain wavelength interval. Width (W): The width of absorption band at a certain wavelength interval. Full width at half maximum (F): The width of an imaginary horizontal line positioned at half of the maximum absorption strength of a particular absorption band. Asymmetry (AS): The asymmetry of shape with respect to an absorption band. It can be quantified by the ratio between left and right widths with respect to the half maximum. Hence, an AS value of 1.0 will represent a symmetrical absorption band, while AS < 1.0 and AS > 1.0 will indicate leftward and rightward asymmetry.
The investigation of spectral features is often followed by building spectral indices to characterize specific minerals, i.e., parameterizing. The indices are based on the unique spectral properties exhibited by the materials, such as the positions and depths of absorption bands. A similar example of such indices is the Normalized difference vegetation index (NDVI).
Mineral identification Minerals are identified with spectroradiometry by examining their spectral response to incoming radiation, such as brightness and reflectance, across different wavelengths. In particular, absorption bands observed in the reflectance spectra can be unique towards different minerals which allow the differentiation between one another. These absorption features arise from the distinct electronic and vibrational processes associated with the energy, molecular components, and internal structures of minerals. Electronic processes of minerals comprise charge transfers, crystal field effects (electrons achieving higher energy states), conduction bands, and colour centres, whereas vibrational processes in minerals involve stretching, bending, and rotation, which are influenced by the functional groups present in the minerals.
For example, water-bearing minerals commonly share distinctive absorption features indicating the presence of hydroxyl groups (-OH) and molecular water (H2O), which include the asymmetrical absorption features due to overtones near 1400 nm (AS1400), as well as absorption peaks near 1900 (D1900) and 2200 nm (D2200). With higher molecular water contents, the AS1400 feature becomes more asymmetrical, the absorption near 2200 nm strengthens, but the one near 1900 nm weakens. Hence, the asymmetrical absorption feature (AS1400), together with the ratio between absorption depths near 2200 and 1900 nm (D2200/D1900) are used a parameter to quantify water contents.
In practice, however, certain minerals may exhibit absorption features that coincide with those of water in similar wavelength intervals. This can potentially lead to the overlapping or masking of absorption features associated with the original minerals. Such situations may arise during field scanning or when dealing with wet samples, introducing confusion in mineral identification. Therefore, to minimize such interference, drying of samples prior to spectral scanning for mineral identification is essential. The drying process should be conducted at a temperature of 105 °C or below, which can ensure the removal of adsorbed water without causing any disruption to the internal structures of minerals.
Considering the identification capabilities of spectroradiometry for different minerals and rocks, the comprehensive databases that encompass spectral signatures are crucial. Such databases serve as valuable resources which contributes to advancing our understanding and characterization of Earth materials. Notably, the United States Geological Survey (USGS) spectral library and the ECOSTRESS spectral library represent present examples of such databases. The USGS spectral library provides a collection of reflectance spectra for rock-forming minerals and other Earth materials, spanning from ultraviolet (350 nm) to shortwave infrared (SWIR) regions (2500 nm). Likewise, the ECOSTRESS spectral library integrates spectral data from multiple spectral libraries, consolidating information on minerals and rocks into a standardized data format.
Geomorphology and surface mapping Geomorphology is the study of geological processes occurring at the surface of Earth that subsequently shape different kinds of landforms. Surface mapping is a common approach to understand these processes and their effects, which can be done by using spectroradiometry. Chemical weathering is one of the dominant processes controlling the morphology of Earth's surface, which produces iron oxides as coatings on particle surfaces, as well as clay minerals that evolve from hydrothermal alteration and decomposition of feldspars in surface soils and granitic rock bodies. These minerals are sensitive to spectral parameters like brightness and reflectance, and they exhibit distinctive absorption features on reflectance spectra, which facilitates easy diagnosis and determination of weathering states.
(1) Clay minerals (Phyllosilicates) Illite: Narrow absorption peaks at 1400, 1900, 2200, and 2300 nm. Chlorite: Triple absorption features near 2300 nm.