Imaging spectrometer
The principle of operation is the same as that of the simple spectrometer, but special care is taken to avoid optical aberrations for better image quality.
In 1704, Sir Isaac Newton demonstrated that white light could be split up into component colours.
The subsequent history of spectroscopy led to precise measurements and provided the empirical foundations for atomic and molecular physics (Born & Wolf, 1999).
Significant achievements in imaging spectroscopy are attributed to airborne instruments, particularly arising in the early 1980s and 1990s (Goetz et al., 1985; Vane et al., 1984).
The source image is reimaged, every point, as a line spectrum on what is called a detector-array column.
Simultaneously, the system provides spectral information about the source area and its line of spatially resolved points.
More precisely, it is the simultaneous acquisition of spatially coregistered images in many spectrally contiguous bands.
) and be used to measure local motion (via the Doppler shift) and even the magnetic field (via the Zeeman splitting or Hanle effect) at each location in the image plane.
Non-linear mixing results from multiple scattering often due to non-flat surface such as buildings and vegetation.
The practical application of imaging spectrometers is they are used to observe the planet Earth from orbiting satellites.
The advantages of spectral content data include vegetation identification, physical condition analysis, mineral identification for the purpose of potential mining, and the assessment of polluted waters in oceans, coastal zones and inland waterways.
Prism spectrometers are ideal for Earth observation because they measure wide spectral ranges competently.
One application is spectral geophysical imaging, which allows quantitative and qualitative characterization of the surface and of the atmosphere, using radiometric measurements.
These measurements can then be used for unambiguous direct and indirect identification of surface materials and atmospheric trace gases, the measurement of their relative concentrations, subsequently the assignment of the proportional contribution of mixed pixel signals (e.g., the spectral unmixing problem), the derivation of their spatial distribution (mapping problem), and finally their study over time (multi-temporal analysis).
Moreover, distortion of the slit image at each wavelength can complicate the interpretation of the spectral data.
The refracting lenses used in the imaging spectrometer limit performance by the axial chromatic aberrations of the lens.
It is harder to correct chromatic aberrations over wider spectral ranges without further optical complexity.
Spectrometers using area-array detectors need more complex mirror systems to provide good resolution.
It is conceivable that a collimator could be made that would prevent all aberrations; however, this design is expensive because it requires the use of aspherical mirrors.
Scattered radiation can interfere with the detector by entering into it and causing errors in recorded spectra.
