LAND SURFACE EMISSIVITY Long-Wave Infrared (LWIR)
Total Page：16
File Type：pdf, Size：1020Kb
L LAND SURFACE EMISSIVITY Long-wave infrared (LWIR). For most terrestrial surfaces (340 K to 240 K), peak thermal emittance occurs in the LWIR (8–14 mm). Alan Gillespie Mid-infrared (MIR). Forest fires (1,000–600 K) have Department of Earth and Space Sciences, University peak thermal emittances in the MIR (3–5 mm). of Washington, Seattle, WA, USA Noise equivalent D temperature (NEDT). Random mea- surement error in radiance propagated through Planck’s law to give the equivalent uncertainty in temperature. Definitions Path radiance S↑. The power per unit area incident on Land surface emissivity (LSE). Average emissivity of a detector and emitted upward from within the atmosphere À À an element of the surface of the Earth calculated (W m 2 sr 1). from measured radiance and land surface temperature Planck’s law. A mathematical expression relating spectral (LST) (for a complete definition, see Norman and Becker, radiance emitted from an ideal surface to its temperature 1995). (Equation 1, in the entry Land Surface Temperature). Atmospheric window. A spectral wavelength region in Radiance. The power per unit area from a surface directed À À which the atmosphere is nearly transparent, separated by toward a sensor, in units of W m 2 sr 1. wavelengths at which atmospheric gases absorb radiation. Reflectivity r. The efficiency with which a surface reflects The three pertinent regions are “visible/near-infrared” energy incident on it. (0.4–2.5 mm), mid-wave infrared (3–5 mm) and Reststrahlen bands. Spectral bands in which there is long-wave infrared (8–14 mm). a broad minimum of emissivity associated in silica Blackbody. An ideal material absorbing all incident energy minerals with interatomic stretching vibrations of Si and or emitting all thermal energy possible. A cavity with O bound in the crystal lattice. a pinhole aperture approximates a blackbody. SEBASS. Spatially Enhanced Broadband Array Spectro- Brightness temperature. The temperature of a blackbody graph System, a hyperspectral TIR imager (Hackwell that would give the radiance measured for a surface. et al., 1996). Color temperature. Temperature satisfying Planck’s law Short-wave infrared (SWIR). Erupting basaltic lavas for spectral radiances measured at two different (1,400 K) have their maximum thermal emittance at wavelengths. 2.1 mm in an atmospheric window at 0.4–2.5 mm. Part Contrast stretch. Mathematical transform that adjusts the of this spectral region (1.4–2.5 mm) is called the SWIR. way in which acquired radiance data translate to the Sky irradiance I#. The irradiance on the Earth’s surface black/white dynamic range of the display monitor. originating as thermal energy radiated downward by the À À À Emissivity e. The efficiency with which a surface radiates atmosphere (W m 2) (spectral irradiance: W m 2 mm 1). its thermal energy. Spectral radiance L. Radiance per wavelength, in units of À À À Irradiance. The power incident on a unit area, integrated Wm 2 mm 1 sr 1. over all directions (W mÀ2). Thermal infrared (TIR). Thermal energy is radiated from Graybody. A material having constant but non-unity a body at frequencies or wavelengths in proportion to its emissivity. temperature. The wavelengths for which this radiant energy E.G. Njoku (ed.), Encyclopedia of Remote Sensing, DOI 10.1007/978-0-387-36699-9, © Springer Science+Business Media New York 2014 304 LAND SURFACE EMISSIVITY is significant for most terrestrial surfaces (1.4–14 mm) are de-emphasizes the temperature, shown as dark/light inten- longer than the wavelength of visible red light and hence sity. In addition to composition, the daytime image gives are known as thermal infrared. The TIR is subdivided into a good sense of topography, because sunlit slopes are three ranges (LWIR, MIR, SWIR) for which the atmo- warmer than shadowed slopes. In the nighttime image, sphere is transparent (atmospheric “windows”) so that the most temperature effects are subdued, and the image energy can be measured from space. closely resembles the Land Surface Emissivity (LSE) alone. Introduction Exceptions include standing water, which is cooler than Thermal emissivity e is the efficiency with which a surface the land during the day but warmer at night. Standing emits its stored heat as thermal infrared (TIR) radiation. water (C) in the floor of Death Valley shows dark green It is useful to know because it indicates the composition in the daytime image but light pink in the nighttime image. of the radiating surface and because it is necessary as Vegetation (A) appears dark in the daytime image, when it a control in atmospheric and energy-balance models, since is cooling its canopy by evapotranspiration. The toe of an it must be known along with brightness temperature to alluvial fan (B) appears darker at night, when soil moisture establish the heat content of the surface. The first practical rises to the surface and evaporates. demonstration of multispectral TIR imaging for composi- The colors in Figure 1 indicate rock type. For example, the emissivity of quartzite is low (0.8) at 8.3 and 9.1 mm tional mapping was from a NASA airborne scanner flown m over Utah (Kahle and Rowan, 1980). (blue and green) but high at 10.4 m (red); therefore, it is Emissivity differs from wavelength to wavelength, just displayed as red. Other rock types and display colors can as reflectivity r does in the spectral region of reflected be understood by comparing the images and emissivity sunlight (0.4–2.5 mm). Emissivity is defined as spectra in Figure 1. The discussion below focuses on algorithms designed Lðl; TÞ to recover emissivities from remotely sensed spectral radi- eðlÞ¼ (1) Bðl; TÞ ance data. Figure 2, of a desert landscape, compares spec- tral radiance to temperature and emissivity images where L is the measured spectral radiance and B is the recovered from it. Also shown are emissivity spectra of theoretical blackbody spectral radiance for a surface vegetation and the geologic substrate. As explained in with a skin temperature T. B is given by Planck’s law the entry Land Surface Temperature, temperature and which, together with the basic physics of TIR radiative emissivity recovery is an underdetermined problem, and transfer, is discussed in the entry Land Surface Tempera- dozens of approaches have been proposed and published ture (LST). that break down the indeterminacy. These fall in four clas- Unlike T, which is a variable property of a surface ses: deterministic algorithms that solve for LST and LSE controlled by the heating history and not directly by com- exactly, algorithms that recover the shape of the LSE spec- position, e(l) is independent of T and is a function directly trum only, model approaches that make key assumptions, of composition. Furthermore, e(l) in the TIR wavelengths and algorithms that attempt also to scale or calibrate the (3–14 mm) responds to different aspects of composition normalized spectra to their actual emissivity values. than reflectivity r(l) at 0.4–2.5 mm. In general, r at wave- In evaluating the algorithms, it is useful to ask how lengths 0.4–2.5 mm is controlled by the amounts of iron accurately it is necessary to recover LSE and LST. For oxides, chlorophyll, and water on the surface; e in the example, many analytic algorithms that seek to identify TIR is controlled more by the bond length of Si and O in surface composition rely not so much on actual emissivity silicate minerals. Examples of emissivity spectra are given values, but on the central wavelengths of emissivity min- in Figure 1. ima (e.g., reststrahlen bands), which can be diagnostic TIR spectroscopy is especially important because for many rocks and minerals. If this is your goal, it may silicate minerals are the building blocks of the geologic not be necessary to scale the spectra, relying instead on surface of Earth, and their presence and amounts can be the simpler algorithms that just recover spectral shape. inferred only indirectly at shorter wavelengths. Thus TIR Errors in LST may affect some algorithms by warping spectroscopy is complementary to spectroscopy of the spectra over several mm of wavelength. This happens reflected sunlight. Good summaries of TIR spectroscopy because the shape of the Planck function changes with and its significance in terms of surface composition may temperature (Land Surface Temperature, Figure 2). be found in Lyon (1965), Hunt (1980), and Salisbury A 5 K error at 300 K, for example, will cause a slope in and D’Aria (1992). A good introduction to spectral the recovered emissivity spectrum of 0.05 from 8 to analysis may be found in Clark et al. (2003). 14 mm. However, the sharp mineralogical features Figure 1 shows daytime and nighttime false-color com- (0.2–0.5 mm wide) are readily distinguished against this posite images of spectral radiance from a sparsely vege- distorted continuum. tated part of Death Valley, California, enhanced using The TIR is commonly a difficult spectral region in a decorrelation contrast stretch (Soha and Schwartz, which to measure spectral radiance, and the images are typ- 1978; Gillespie et al., 1986). This stretch emphasizes the ified by a low signal–noise ratio. This ratio is commonly emissivity component of the signal, shown as color, and represented by the “noise equivalent D temperature” or LAND SURFACE EMISSIVITY 305 Land Surface Emissivity, Figure 1 Airborne thermal infrared multispectral scanner (TIMS: Palluconi and Meeks, 1985) false-color TIR radiance images of Death Valley, California (RGB ¼ 10.4, 9.1, 8.3 mm). Letters A, B, and C indicate sites discussed in the text. Central column shows laboratory spectra for field samples. Inset shows similar ASTER image “draped” over topography, looking north up Death Valley.