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Unformatted text preview: roscopy is the study of light that is emitted by or reflected from materials and its variation in energy with wavelength. As applied to the field of optical remote sensing, spectroscopy deals with the spectrum of sunlight that is diffusely reflected (scattered) by materials at the Earth’s surface. Instruments called spectrometers (or spectroradiometers) are used to make ground-based or laboratory measurements of the light reflected from a test material. An optical dispersing element such as a grating or prism in the spectrometer splits this light into many narrow, adjacent wavelength bands and the energy in each band is measured by a separate detector. By using hundreds or even thousands of detectors, spectrometers can make spectral measurements of bands as narrow as 0.01 micrometers over a wide wavelength range, typically at least 0.4 to 2.4 micrometers (visible through middle infrared wavelength ranges). Remote imagers are designed to focus and measure the light reflected from many adjacent areas on the Earth’s surface. In many digital imagers, sequential measurements of small areas are made in a consistent geometric pattern as the sensor platform moves and subsequent processing is required to assemble them into an image. Until recently, imagers were restricted to one or a few relatively broad wavelength bands by limitations of detector designs and the requirements of data storage, transmission, and processing. Recent advances in these areas have allowed the design of imagers that have spectral ranges and resolutions comparable to ground-based spectrometers.
Scan Mirror and Other Optics λ Dispersing Element Light from a single groundresolution cell. Imaging Optics Detectors Schematic diagram of the basic elements of an imaging spectrometer. Some sensors use multiple detector arrays to measure hundreds of narrow wavelength (λ ) bands. page 4 Introduction to Hyperspectral Imaging Spectral Reflectance
In reflected-light spectroscopy the fundamental property that we want to obtain is spectral reflectance: the ratio of reflected energy to incident energy as a function of wavelength. Reflectance varies with wavelength for most materials because energy at certain wavelengths is scattered or absorbed to different degrees. These reflectance variations are evident when we compare spectral reflectance curves (plots of reflectance versus wavelength) for different materials, as in the illustration below. Pronounced downward deflections of the spectral curves mark the wavelength ranges for which the material selectively absorbs the incident energy. These features are commonly called absorption bands (not to be confused with the separate image bands in a multispectral or hyperspectral image). The overall shape of a spectral curve and the position and strength of absorption bands in many cases can be used to identify and discriminate different materials. For example, vegetation has higher reflectance in the near infrared range and lower reflectance of red light than soils.
1 2 3 4 SPOT XS Multispectral Bands 5 Reflected Infrared 123
Blue Grn Red Landsat TM Bands 7 Near Infrared Middle Infrared 60 Vegetation Reflectance (%) Dry soil (5% water) Wet soil (20% water) 40 20
Clear lake water Turbid river water 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 Representative spectral reflectance curves for several common Earth surface materials over the visible light to reflected infrared spectral range. The spectral bands used in several multispectral satellite remote sensors are shown at the top for comparison. Reflectance is a unitless quantity that ranges in value from 0 to 1.0, or it can be expressed as a percentage, as in this graph. When spectral measurements of a test material are made in the field or laboratory, values of incident energy are also required to calculate the material’s reflectance. These values are either measured directly or derived from measurements of light reflected (under the same illumination conditions as the test material) from a standard reference material with known spectral reflectance. page 5 Introduction to Hyperspectral Imaging Mineral Spectra
In inorganic materials such as minerals, chemical composition and crystalline structure control the shape of the spectral curve and the presence and positions of specific absorption bands. Wavelength-specific absorption may be caused by the presence of particular chemical elements or ions, the ionic charge of certain elements, and the geometry of chemical bonds between elements, which is governed in part by the crystal structure. The illustration below shows spectra of some common minerals that provide examples of these effects. In the spectrum of hematite (an iron-oxide mineral), the strong absorption in the visible light range is caused by ferric iron (Fe+3). In calcite, the major component of limestone, the carbonate ion (CO3-2) is responsible for the series of absorption bands between 1.8 and 2.4 micrometers (µm). Kaolinite and montm...
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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