Thermal Infrared Reflectance Research Articles

Thermal infrared reflectance characteristics of natural leaves in 8–14 μm region: Mechanistic modeling and relationships with leaf water content

The use of thermal infrared (TIR) remote sensing, between 8 and 14 μm, is an important method to monitor vegetation water stress status. To comprehend the spectral characteristics of natural leaves, understanding the relationship between TIR reflectance and leaf water content is helpful. In this study, we selected Magnolia cylindrica E.H.Wilson, Canna indica Rosc. and Gardenia jasminoides J. Ellis leaves to isolate cuticles with varying thicknesses. Subsequently, we constructed a leaf thermal infrared model (Leaf-TIR model) to examine the impact of cuticle thickness and inside wet epidermal cell wall on the TIR reflectance of natural leaves. Our findings indicate that the cuticle thicknesses of the three selected leaves are 12.2, 5.5, and 1.1 μm, respectively. Moreover, as cuticle thickness decreases, the similarity between the isolated cuticle and the leaf reflectance behaviors also decreases because a thinner cuticle has lower absorptance and a weaker effect on the reflection characteristics of natural leaves. Moreover, we observed that as the water content in the cell wall decreases, the reflectance of the leaves significantly increases when the cuticle thickness becomes 5.5 or 1.1 μm. This outcome is due to the increasing difference in the refractive indices of the cuticle and cell wall with declining water content. Our findings suggest that the Leaf-TIR model can quantitatively determine the relationship between TIR reflectance and water content in the cell wall. This information is crucial in interpreting TIR spectral behaviors of natural leaves and provides an important theoretical basis for future research.

Mineral Physicochemistry Underlying Feature-Based Extraction of Mineral Abundance and Composition from Shortwave, Mid and Thermal Infrared Reflectance Spectra

Reflectance spectroscopy allows cost-effective and rapid mineral characterisation, addressing mineral exploration and mining challenges. Shortwave (SWIR), mid (MIR) and thermal (TIR) infrared reflectance spectra are collected in a wide range of environments and scales, with instrumentation ranging from spaceborne, airborne, field and drill core sensors to IR microscopy. However, interpretation of reflectance spectra is, due to the abundance of potential vibrational modes in mineral assemblages, non-trivial and requires a thorough understanding of the potential factors contributing to the reflectance spectra. In order to close the gap between understanding mineral-diagnostic absorption features and efficient interpretation of reflectance spectra, an up-to-date overview of major vibrational modes of rock-forming minerals in the SWIR, MIR and TIR is provided. A series of scripts are proposed that allow the extraction of the relative intensity or wavelength position of single absorption and other mineral-diagnostic features. Binary discrimination diagrams can assist in rapidly evaluating mineral assemblages, and relative abundance and chemical composition of key vector minerals, in hydrothermal ore deposits. The aim of this contribution is to make geologically relevant information more easily extractable from reflectance spectra, enabling the mineral resources and geoscience communities to realise the full potential of hyperspectral sensing technologies.

INDO-FRENCH HIGH-RESOLUTION THERMAL INFRARED SPACE MISSION FOR EARTH NATURAL RESOURCES ASSESSMENT AND MONITORING – CONCEPT AND DEFINITION OF TRISHNA

Abstract. The Indian and French Space Agencies, ISRO and CNES, have conceptualized a space-borne Thermal Infrared Reflectance (TIR) mission, TRISHNA (Thermal infRared Imaging Satellite for High-resolution Natural Resource Assessment). The primary design drivers of TRISHNA are the monitoring of (i) terrestrial water stress and use, and of (ii) coastal and continental water. A suit of four TIR bands and six optical bands is planned. The TIR bands will be centred at 8.6 μm, 9.1 μm, 10.3 μm and 11.5 μm to provide noon-night global observations at 57m nadir resolution over land and coastal regions. The field of view (FOV) is ±34° and the orbit of 761 km altitude was designed to allow 3 sub-cycle acquisitions during the 8-day cycle. The optical bands correspond to blue, green, red, and NIR plus two SWIR bands at 1.38 μm and 1.61 μm. The green, red, NIR and the 1.61 μm SWIR bands will have better radiometry quality than those of AWiFS. ISRO and CNES will develop optical and TIR payloads, respectively. Assessing evapotranspiration and furthermore Gross and Net Primary Productivity (GPP and NPP) will in turn assist in quantifying water use in rainfed and irrigated agriculture, water stress and water use efficiency, with expected applications to agricultural drought and early warning, crop yield prediction, water allocation, implementation of water rights, crop insurance business and agro-advisories to farmers. The other scientific objectives of TRISHNA are also briefly described. TRISHNA instrument will fly aboard a ISRO spacecraft scheduled to be launched from 2024 for a minimum period of 5 years’ mission lifetime.

Effect of halite coatings on thermal infrared spectra

AbstractCharacterizing the occurrence and distribution of soluble salts on planetary surfaces allows us to model or monitor aqueous and geochemical conditions. Thermal infrared (TIR) remote sensing is useful for identifying some types of salt deposits; however, mineral abundance determinations are based on the interaction of infrared with only the top few hundred micrometers of the surface. Thus, distinguishing massive deposits from coatings presents a challenge to TIR remote sensing. To better understand the TIR properties of spectrally transmissive halite coatings, we investigated the effects of coating thickness and texture on the TIR reflectance spectra of halite‐coated glasses. We analyzed two coating textures with variable thickness: (1) continuous and (2) discontinuous particulate coatings. As halite coating thickness increases, the intensity of substrate absorption bands decreases nonlinearly. The substrate is not detected with halite coatings >150 µm thick. Therefore, when coatings are present, it is not possible to apply TIR models of mineral abundances using linear deconvolution algorithms. We show that continuous and coarse particulate halite coatings increase the reflectance minimum (emissivity maximum), making them potentially detectable on planetary surfaces by the methods of Osterloo et al. (2008). However, the reflectance minimum does not change if coatings have low areal coverage (<50%) or are composed of fine particles (<5 µm). Thus, halite that forms as efflorescent coatings or has been redistributed as fine dust is not detected using TIR spectroscopy. Our results explain why Cl salts are not detected with TIR spectroscopy at many locations on Mars despite high Cl contents.

Mineralogy and chemistry of altered Icelandic basalts: Application to clay mineral detection and understanding aqueous environments on Mars

We used a suite of techniques, including those emulating compositional data sets obtained from Mars orbit and obtainable at the Mars surface, to examine aqueous alteration of basaltic rocks from Iceland as a mineralogic and geochemical analog for Noachian environments on Mars. A sample suite was collected for laboratory measurement of (1) whole‐rock visible/near‐infrared (VNIR) reflectance and thermal infrared (TIR) emission spectra; (2) VNIR and TIR reflectance spectra of particle‐size separates derived from the bulk rock and from materials extracted from fractures/vesicles; (3) X‐ray diffraction (XRD) patterns for determination of quantitative modal mineralogy; (4) major element chemistry using flux fusion of whole‐rock powders; and (5) electron microprobe analyses of minerals in thin sections. Conclusions about aqueous alteration can be influenced by technique. For these basalts, whole‐rock chemical data showed scant evidence for chemical fractionation, but TIR, VNIR, and XRD measurements identified distinctive assemblages of hydrous silicate minerals, differing by sample. XRD provided the most complete and accurate quantitative determination of sample mineralogy. However, VNIR spectroscopy was the technique most useful for determining composition of low‐abundance smectite clays, and TIR spectroscopy was the most useful for recognizing hydrated silicates in thin surface coatings. High spatial resolution mineralogical and chemical data sets were useful for understanding the texture and distribution of alteration products and variations in fluid chemistry. No single approach provides a complete assessment of the environment of alteration, demonstrating the importance of employing multiple, synergistic mineralogical and geochemical techniques and instruments in exploration of rock strata from aqueous paleoenvironments on Mars.

Rock type classification of drill core using continuous wavelet analysis applied to thermal infrared reflectance spectra

This study investigates a core logging methodology to map rock type using thermal infrared reflectance (TIR) spectra (500–4000 cm–1 or 2.5–20.0 μm) for 74 samples encompassing 11 rock types exposed in various mines of the Sudbury Basin, Canada. A continuous wavelet transform (CWT) was used to represent the original reflectance spectra as a suite of wavelets, each capturing spectral features of different scales with the low-scale components containing mineral spectral features and the high-scale components capturing the overall continuum. Classification was driven by the use of endmember spectra and the spectral angle mapper (SAM). Modelling and validation suites were developed and the mapping accuracy evaluated iteratively for random data splits. The results were compared for reflectance and wavelets of low components of power and significance. We found that the variability amongst measurements observed for varying orientation of a sample or due to variable surface roughness can be greatly minimized with the use of low-scale components, thus improving rock type classification. The average accuracy computed for the 11 rock types is highest for the low-scale component of power (72%) data as opposed to the reflectance data (55%). The highest average accuracy per rock type is obtained using the low-scale components (average value of 82%) for seven rock units that are relatively texturally homogeneous and of uniform modal mineralogy. Lower accuracy values are observed for rock units that display pronounced textural heterogeneity at the scale of observation, or variability in modal mineralogy, or are spectrally similar to other rock types.

Thermal infrared reflectance and emission spectroscopy of quartzofeldspathic glasses

This investigation seeks to better understand the thermal infrared (TIR) spectral characteristics of naturally‐occurring amorphous materials through laboratory synthesis and analysis of glasses. Because spectra of glass phases differ markedly from their mineral counterparts, examination of glasses is important to accurately determine the composition of amorphous surface materials using remote sensing datasets. Quantitatively characterizing TIR (5–25 μm) spectral changes that accompany structural changes between glasses and mineral crystals provides the means to understand natural glasses on Earth and Mars. A suite of glasses with compositions analogous to common terrestrial volcanic glasses was created and analyzed using TIR reflectance and emission techniques. Documented spectral characteristics provide a basis for comparison with TIR spectra of other amorphous materials (glasses, clays, etc.). Our results provide the means to better detect and characterize glasses associated with terrestrial volcanoes, as well as contribute toward understanding the nature of amorphous silicates detected on Mars.

Ore detection and grade estimation in the Sudbury mines using thermal infrared reflectance spectroscopy

This pilot study investigated the usefulness of thermal infrared reflectance (TIR) spectroscopy to estimate ore grade in an underground environment and to separate ore‐bearing samples from their host rocks. Work was carried out under laboratory conditions to test the initial concept; all samples had naturally broken faces to mimic the situation in a freshly blasted underground opening. A total of 26 samples, including massive and disseminated ores, were collected from eight mines around the Sudbury basin in Ontario. Rock surfaces were measured wet and dry to address environmental conditions encountered underground. To separate barren rocks from ores and for ore‐grade estimation, an important finding of this research is that, in the region of [Formula: see text], most known silicate minerals converge to a common reflectance minima (<1.5%), but massive and disseminated sulfides have distinctly higher reflectance. Individual sulfide minerals (chalcopyrite, pyrrhotite, pentlandite), however, do not reveal diagnostic features in this spectral region. When sulfides are disseminated in the host rock, the average reflectance of the rock increases but the correlation with abundance is not systematic. However, sulfide concentration as a function of continuum‐removed reflectance (CRR) is systematic. The empirical correlation between CRR at [Formula: see text] versus the total sulfide concentration, estimated via thin‐section point counts, gives a coefficient of determination value [Formula: see text] of 0.93 for measurement of dry and wet surfaces when averaged. Similar results are observed when dry and wet locations are analyzed separately. The relationship demonstrates the feasibility to estimate total sulfide concentration from TIR reflectance data even when samples are wet.

Evaluation of visible and near-infrared and thermal infrared reflectance spectra for studying thermal alteration of Pierre Shale, Wolcott, Colorado : J Geophys Res V96, NB11, Oct 1991, P18047–18057

Analyses of visible and near-infrared (0.4–2.5 μm) and thermal-infrared (2.5–25.0 μm) spectral reflectance of whole rock and kerogen concentrates of dike-intruded Pierre Shale indicate that the degree of thermal alteration of contained organic matter (OM) can be estimated by use of spectral reflectance. With increasing thermal alteration of the OM, whole rock, and kerogen spectra show the following trends: (1) visible and near-infrared (VNIR) and thermal-infrared (TIR) reflectance decreases progressively, with the largest change in the VNIR occurring between 0.90 and 1.50 μm; and (2) the intensities of absorption features in the 3- to 5-μm region, which are related to C-H stretching vibrations, decrease. The VNIR and TIR spectral reflectance trends are highly correlated with H/C ratios and vitrinite reflectance data. The largest changes in all these parameters occur at a distance that is equivalent to about one-half the dike thickness, where the temperature range is estimated to have been 240°–350° C. The VNIR and TIR spectral reflectance variations are related to thermally induced composition and structural changes in the OM. Landsat Thematic Mapper channels 3, 4, and 5 should have excellent potential for remote detection of OM maturity. Analysis of in situ VNIR and TIR measurements may provide a rapid method for estimating thermal maturity in the field. In addition, laboratory measurements of TIR spectral reflectance are a more sensitive and convenient means of measuring the maturity of kerogen than the traditional transmittance method. Further work is necessary to examine the effects of variable OM abundance and type, mineralogic composition, grain size, and weathering on spectral reflectance characteristics.

Thermal infrared spectra and images of altered volcanic rocks in the Virginia Range, Nevada

Abstract Airborne Thermal Infrared Multispectral Scanner (TIMS) data and laboratory thermal infrared reflectance spectra were collected from sites of hydrothermally altered andesitic volcanic rocks near Virginia City, Nevada. Alunitic, kaolinitic, illitic, and propylitic alteration types have distinct laboratory spectral curves, although the individual spectra are difficult to interpret mineralogically. TIMS emittance spectra show a general shift in the wavelength of the silicate emittance minima to shorter wavelengths with increasing intensity of alteration, owing to the increasing abundance of secondary framework and sheet silicate minerals. Strongly altered volcanic rocksare identifiable on TIMS radiance and emittance imagery.

Spectral characterization of igneous rocks in the 8‐ to 12‐μm region

Variation in the spectral response of silicates in the thermal infrared (TIR) from 8 to 12 μm is the foundation for potentially important applications in the lithologic characterization of terrestrial and extraterrestrial surfaces. The purpose of this study is to elucidate the crystallo‐chemical basis for the variation in spectral behavior in terms which may be useful in remote sensing. This has led to the definition of a new parameter, SCFM, which is defined as the ratio SiO2/(SiO2 + CaO + FeO + MgO) in minerals and rocks. This parameter reflects the degree of depolymerization of the silica tetrahedra and is therefore highly correlated with the structure of TIR reflectance spectra of major silicate minerals in both fine‐ and coarse‐grained igneous rocks. It is also a good numerical descriptor of igneous rock composition. Thus SCFM is proposed as a very useful determinant for spectral identification of igneous rocks. The SCFM parameter was used to assess the effect of variations in location, number, and width of bands in discriminating rock composition. (An unfortunate ambiguity exists in the use of the term “band” which in spectroscopy indicates a feature in a spectrum and in remote sensing signifies a specific wavelength increment or channel. The latter definition is used in this paper.) Spectra of coarse‐particulate mineral and solid rock samples were acquired in the laboratory. A regression analysis of bands varying in width from 0.2 to 1.4 μm versus SCFM results in coefficients of determination (correlation coefficients), R2, ranging from 0.88 (for two 1.2‐μm‐wide bands) to 0.97 (for seven 0.2‐μm‐wide bands). Deletion of bands in the ozone absorption region (9.4–10.2 μm) results in a decrease in R2 by only 0.01. Stimulation of system noise by addition of ±5% random counts to the bands decreases R2 by only 0.03. Because spectra structure is modified by weathering, alteration, and texture of the samples, the validity of these results is limited to fresh igneous rocks. With this constraint, it is concluded that accurate spectral identification of igneous rock composition is not greatly affected by bandwidth or system noise. It is the overall spectral envelope rather than the fine structure in the TIR which contains most of the information diagnostic of igneous rock composition. This permits design of remote sensing systems with broad bands and corresponding higher signal‐to‐noise, which can be traded off against the important requirement for higher spatial resolution.

Optical properties of sputter-deposited ZnO:Al thin films

ZnO:Al coatings were prepared by rf magnetron sputtering of ZnO together with dc magnetron sputtering of Al onto rapidly revolving unheated substrates under weakly oxidizing conditions. Optimized films had ∼1% luminous absorptance, ∼85% thermal infrared reflectance, and ∼5×10−4 Ω cm electrical resistivity at a thickness of ∼0.3 μm. The Al content was ≲2 at. %, as determined by Rutherford backscattering spectrometry. Transmission electron microscopy and electron diffraction showed ∼50-nm average crystallite size and a hexagonal wurtzite structure. Spectrophotometric transmittance and reflectance were recorded in the 0.2–50-μm wavelength interval, and the complex dielectric function was evaluated by computation. The optical data were explained from an effective mass model for n-doped semiconductors. The Al atoms are singly ionized, and the associated electrons occupy the bottom of the conduction band as free-electron gas. The Al ions act as pointlike Coulomb scatterers and are screened by the electrons according to the random phase approximation or an extension thereof. The optical properties of ZnO:Al could be understood by considering the free electrons to be damped primarily by ionized impurity scattering. ZnO:Al films can have high luminous transmittance, high solar ultraviolet absorptance, low thermal infrared emittance, and high electrical conductance; hence, they are of large interest for energy-efficient windows.

Spectrally Selective Coatings for Energy Efficiency and Solar Applications

This paper gives a brief survey over some recent work on surface coatings for energy efficiency and solar applications. Specifically, it covers coatings for- selective absorption of solar energy (for solar collectors),- selective emission of infrared radiation in the 8-13 μm range (for passive cooling devices),- visible transmission combined with near-infrared reflectance (for windows with decreased inlet of solar energy),- solar transmission combined with thermal-infrared reflectance (for windows with decreased outlet of thermal radiation),- variable transmission of solar or visible radiation (for "smart windows" with dynamic control of radiant energy).One general conclusion from the survey is that the research field embodying these types of coatings remains, with a few exceptions, in a state of rapid progress.