Imaging Spectroscopy

The terms imaging spectroscopy, imaging spectrometry, hyperspectral imaging, and, occasionally, ultraspectral imaging are often used interchangeably. Even though semantic differences might exist, a common definition for such terms is as follows: the simultaneous acquisition of spatially coregistered images, in many narrow, spectrally contiguous bands, measured in calibrated radiance units, from a remotely operated platform. A variety of imaging spectrometers exist for imaging spectroscopy applications. Imaging-spectrometer-collected data facilitate quantitative and qualitative characterization of both the surface and the atmosphere, using geometrically coherent spectral measurements. This result can then be used for the unambiguous direct and indirect identification of surface materials and atmospheric trace gases, the measurement of their relative concentrations, and subsequently the assignment of the proportional contribution of mixed pixel signals (e.g., spectral unmixing), the [Page 1542]derivation of their spatial distribution (e.g., mapping), and finally their evolution over time (multitemporal analysis).

More than 300 yrs. (years) ago, in 1704, Sir Isaac Newton published his Treatise of Light, which presented the concept of dispersion of light. He demonstrated that white light could be split up into its component colors by means of a prism and found that each pure color is characterized by a specific refrangibility. Newton's corpuscular theory was gradually succeeded over time by the wave theory. Consequently, the substantial summary of past experiences performed by James Maxwell in 1873 resulted in his equations of electromagnetic waves. But it was not until the 19th century that the quantitative measurement of dispersed light was recognized and standardized.

A major contribution was Joseph von Fraun-hofer's discovery of the dark lines in the solar spectrum in the early 1800s and their interpretation as absorption lines on the basis of experiments by Robert Bunsen and Gustav Kirchhoff in the mid 1800s. The term spectroscopy was first used in the late 19th century and provides the empirical foundations for atomic and molecular physics. Significant achievements in imaging spectroscopy are attributed to airborne instruments, particularly arising in the early 1980s and 1990s. However, it was not until 1999 that the first imaging spectrometer was launched in space (the National Aeronautics and Space Administration's Moderate-Resolution Imaging Spectroradiometer, or MODIS).