2.12 Imaging Spectrometer

2.12 Imaging Spectrometer

Imaging spectrometers are characterized by a multispectral scanner with a very large number of channels (64-256 channels) with very narrow band widths, though the basic scheme is almost the same as an optical mechanical scanner or pushbroom scanner.

The optical system of imaging spectrometers are classified into three types; dioptic system, dio and catoptic system and catoptic system which are adopted depending on the scanning system. Table 2.12.1 shows a comparison of the three types. In the case of object plane scanning, the catoptic system is best chosen because the linearity of the optical axis is very good due to the narrow view angle and the observation wave range is so wide. However in the case of image plane scanning, the dioptic system or dioptic and catoptic system is best suited because the view angle should be wider.

Figure 2.12.1 shows four different types of multispectral scanner. The left upper (multispectral imaging with discrete detectors) corresponds to the optical mechanical scanner on the using the object plane scanning method used in the LANDSAT. The right upper (multispectral imaging with line arrays) corresponds to pushbroom scanner using the image plane scanning method with a linear CCD array.

The left lower (imaging spectrometry with line arrays) shows a similar scheme to the right upper system but with an additional dispersing element (grating or prism) to increase the spectral resolution. The right lower (imaging spectrometry with area arrays) shows an imaging spectrometer with area arrays.

Table 2.12.2 shows the optical scheme of the Moderate Resolution Imaging Spectrometer-Tilt (MODIS-T) which is scheduled to be carried on EOS-a (US Earth Observing Satellite). MODIS-T has an area array of 64 x 64 elements which enables 64 multispectral bands from 0.4 mm to 1.04 mm with a 64 km swath. The optical path is guided from scan mirror to Schmitt type off axis parabola of dio and catoptic system. The the light is then dispersed into 64 bands by a grating and is detected by an area CCD array of 64 x 64 elements.

As imaging spectrometer provides multiband imagery with a narrow wave length range, and is useful for rock type classification and ocean color analysis.