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Unformatted text preview: orillonite are clay minerals that are common in soils. The strong absorption band near 1.4 µm in both spectra, along with the weak 1.9 µm band in kaolinite, are due to hydroxide ions (OH-1), while the stronger 1.9 µm band in montmorillonite is caused by bound water molecules in this hydrous clay. In contrast to these examples, orthoclase feldspar, a dominant mineral in granite, shows almost no significant absorption features in the visible to middle infrared spectral range.
Visible Near Infrared Calcite Middle Infrared 100
Orthoclase Feldspar 80
Hematite 20 0 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Wavelength (micrometers) 2.0 2.2 2.4 Reflectance spectra of some representative minerals (naturally occurring chemical compounds that are the major components of rocks and soils). page 6 Introduction to Hyperspectral Imaging Plant Spectra
The spectral reflectance curves of healthy green plants also have a characteristic shape that is dictated by various plant attributes. In the visible portion of the spectrum, the curve shape is governed by absorption effects from chlorophyll and other leaf pigments. Chlorophyll absorbs visible light very effectively but absorbs blue and red wavelengths more strongly than green, producing a characteristic small reflectance peak within the green wavelength range. As a consequence, healthy plants appear to us as green in color. Reflectance rises sharply across the boundary between red and near infrared wavelengths (sometimes referred to as the red edge) to values of around 40 to 50% for most plants. This high near-infrared reflectance is primarily due to interactions with the internal cellular structure of leaves. Most of the remaining energy is transmitted, and can interact with other leaves lower in the canopy. Leaf structure varies significantly between plant species, and can also change as a result of plant stress. Thus species type, plant stress, and canopy state all can affect near infrared reflectance measurements. Beyond 1.3 µm reflectance decreases with increasing wavelength, except for two pronounced water absorption bands near 1.4 and 1.9 µm. At the end of the growing season leaves lose water and chlorophyll. Near infrared reflectance decreases and red reflectance increases, creating the familiar yellow, brown, and red leaf colors of autumn.
Visible Chlorophyll Near Infrared Cell Structure Grass Water Middle Infrared Water 60
Reflectance (%) Walnut tree canopy Fir tree 40
Dry, yellowed grass 20 0 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Wavelength (micrometers) 2.2 2.4 Reflectance spectra of different types of green vegetation compared to a spectral curve for senescent (dry, yellowed) leaves. Different portions of the spectral curves for green vegetation are shaped by different plant components, as shown at the top. page 7 Introduction to Hyperspectral Imaging Spectral Libraries
Several libraries of reflectance spectra of natural and man-made materials are available for public use. These libraries provide a source of reference spectra that can aid the interpretation of hyperspectral and multispectral images. ASTER Spectral Library This library has been made available by NASA as part of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imaging instrument program. It includes spectral compilations from NASA’s Jet Propulsion Laboratory, Johns Hopkins University, and the United States Geological Survey (Reston). The ASTER spectral library currently contains nearly 2000 spectra, including minerals, rocks, soils, man-made materials, water, and snow. Many of the spectra cover the entire wavelength region from 0.4 to 14 µm. The library is accessible interactively via the Worldwide Web at http:// speclib.jpl.nasa.gov. You can search for spectra by category, view a spectral plot for any of the retrieved spectra, and download the data for individual spectra as a text file. These spectra can be imported into a TNTmips spectral library. You can also order the ASTER spectral library on CD-ROM at no charge from the above web address.
80 Visible Near Infrared
Middle Infrared Sample spectra from the ASTER Spectral Library. ASTER will be one of the instruments on the planned EOS AM-1 satellite, and will record image data in 14 channels from the visible through thermal infrared wavelength regions as part of NASA’s Earth Science Enterprise program. 60 40 20 Granite Concrete Asphalt roof shingles Basalt 0 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 Wavelength (micrometers) USGS Spectral Library The United States Geological Survey Spectroscopy Lab in Denver, Colorado has compiled a library of about 500 reflectance spectra of minerals and a few plants over the wavelength range from 0.2 to 3.0 µm. This library is accessible online at http://speclab.cr.usgs.gov/spectral.lib04/spectral-lib04.html. You can browse individual spectra online, or download the entire library. The USGS Spectral library is also included as a standard reference library in the TNTmips Hyperspectral Analysis process.
page 8 Introduction to Hyperspect...
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This note was uploaded on 12/16/2010 for the course ENV 148 taught by Professor Chang during the Spring '10 term at APU Japan.
- Spring '10