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Imaging Spectrometers

Michael E. Schaepman

Keywords: , imaging spectrometry, hyperspectral, airbone, spaceborne.

INTRODUCTION resulting in Maxwell’s equations of electromag- netic waves (Maxwell 1873). But it was only in Imaging spectrometers have significantly im- the early nineteenth century that quantitative mea- proved the understanding of interactions of photons surement of dispersed light was recognized and with the surface and atmosphere. Spectroscopy has standardized by Joseph von Fraunhofer’s discovery existed since the eighteenth century; the imaging of the dark lines in the solar spectrum (Fraunhofer part of this term became technically possible in the 1817) and their interpretation as absorption lines on early 1980s. The first part of this chapter is devoted the basis of experiments by Bunsen and Kirchhoff to a short historical background of this evolution. (1863). The term spectroscopy was first used in the In subsequent sections, imaging spectroscopy is late nineteenth century and provides the empiri- defined and the main acquisition principles are dis- cal foundations for atomic and molecular physics cussed. The main imaging spectrometers used for (Born and Wolf 1999). Following this, astronomers Earth observation are presented, as well as emerg- began to use spectroscopy for determining radial ing concepts which give an insight in a broad velocities of stars, clusters, and galaxies and stel- range of air to spaceborne associated technology. lar compositions (Hearnshaw 1986). A historical Imaging spectroscopy has expanded to many other example of an astronomical spectrometer is George disciplines, and the approach is used in medicine, E. Hale’s spectroheliograph (Figure 12.2) of the extraterrestrial research, process, and manufactur- early twentieth century. The spectroheliograph was ing industries, just to name a few areas. In addition, designed by this American astronomer to col- much development is currently seen in other wave- lect spectral images of the by simultaneously length domains such as the ultraviolet and the scanning the sun’s image across the entrance slit thermal. However, this chapter focuses on Earth and a film plate past the exit slit of a two-prism observation based imaging spectrometers in the monochromator. solar reflective wavelength range. Advances in technology and increased aware- ness of the potential of spectroscopy in the 1960s to 1980s led to the development of the first analyti- cal methods used in (Arcybashev HISTORICAL BACKGROUND and Belov 1958, Lyon 1962), the inclusion of ‘additional’spectral bands in multispectral imagers Three centuries ago Sir Isaac Newton published (e.g., the 2.09–2.35 µm band in Landsat for the the concept of dispersion of light in his ‘Treatise of detection of hydrothermal alteration minerals), as Light,’ and the concept of a spectrometer was born well as first airborne and later spaceborne imaging (Figure 12.1). spectrometer concepts and instruments (Collins The corpuscular theory by Newton was grad- et al. 1982, Goetz et al. 1982, Vaneet al. 1984, Vane ually succeeded over time by the wave theory, 1986). Significant recent progress was achieved

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when in particular airborne imaging spectrometers understanding of the modeled interaction of pho- became available on a wider basis (Goetz et al. tons with matter (Schläpfer and Schaepman 2002) 1985, Gower et al. 1987, Kruse et al. 1990, Rickard will allow for more quantitative, direct and indirect et al. 1993, Birk and McCord 1994, Rowlands et al. identification of surface materials, and atmospheric 1994, Green et al. 1998) helping to prepare for transmittance based on spectral properties from spaceborne activities (Goetz ground, air, and space. and Herring 1989). This initial phase of develop- ment lasted until the late 1990s, when the first imag- ing spectrometers were launched in space (e.g., MODIS (Salomonson et al. 1989), MERIS (Rast DEFINITIONS OF IMAGING et al. 1999)). Nevertheless, true imaging spectrom- SPECTROSCOPY TERMS eters in space, satisfying a strict definition of a con- tiguity criterion, are still sparse (CHRIS/PROBA Spectroscopy is defined as the study of light as (Barnsley et al. 2004, Cutter 2006), Hyperion/EO-1 a function of wavelength that has been emit- (Pearlman et al. 2003)). ted, reflected,or scattered from a solid, liquid, or Technological advances in the domain of focal gas. In remote sensing, the quantity most used plane (detector) development (Chorier and Tribolet is (surface) reflectance (expressed as a percent- 2001), readout electronics, storage devices, and age). Spectroradiometry is the technology for mea- optical design (Mouroulis and Green 2003) are suring the power of optical radiation in narrow, leading to a significantly better sensing of the contiguous wavelength intervals. The quantities Earth’s surface. Improvements in optical design measured are usually expressed as spectral irra- (Mouroulis et al. 2000) signal-to-noise, finer and diance (commonly measured in W m−2 nm−1) better defined bandwidths as well as contiguous and spectral radiance (commonly measured in spectral sampling combined with the goal of better Wsr−1 m−2 nm−1).

N M S Fig 18. d D B A a b G F g C c e E

Figure 12.1 Sir Isaac Newton’s ‘Treatise of Light’ discusses the concept of dispersion of light in 1704. He demonstrated that white light could be split up into component colors using prisms, and found that each pure color is characterized by a specific refrangibility (Newton 1704).

Figure 12.2 Schematic drawing of Hale’s spectroheliograph, which was used to image the sun (Wright et al. 1972).

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Spectroradiometric measurements remain one imaging spectrometry (imaging spectroscopy, or of the least reliable of all physical measurements also ) is a passive remote due to the multidimensionality of the problem, sensing technology for the simultaneous acqui- the instability of the measuring instruments and sition of spatially coregistered images, in many, standards used, and sparse dissemination of the spectrally contiguous bands, measured in cali- principles and techniques used for eliminating brated radiance units, from a remotely operated or reducing the measurement errors (Kostkowski platform (Schaepman et al. 2006). 1997). In the specific case of imaging spectrometry, the The term hyperspectral (alternatively also ultra- focus of the refined definition is on many, spectrally spectral) is used most often for spectroscopy and contiguous bands, de-emphasizing the need of spectrometry interchangeably and denotes usually ‘hundreds of contiguous bands’(Goetz, 2007). The the presence of a wealth of spectral bands without contiguity criteria or the proximity requirement of further specification. The variable use of the above spectral bands is usually poorly defined, in particu- terms expresses a variation in flavors, but usually lar since all imaging spectrometers in remote sens- not a fundamental physical difference. Hyperspec- ing undersample the Earth. The Nyquist–Shannon tral denotes many spectral bands, which potentially theorem requires that a perfect reconstruction of can be used to solve an n−1 dimensional prob- the signal is possible when the sampling frequency lem, where n represents the number of spectral is greater than twice the maximum frequency of bands. An imaging spectrometer with 200 spectral the signal being sampled, which is not the case bands (i.e., dimensions = 200) can theoretically in space based imaging spectrometers. The rate of solve a spectral unmixing based problem with 199 undersampling requires compromises to be made end members, or can be used in a model inversion in the resolution-acquisition-time domains, which approach with 199 unknowns. Practically, there are in turn has fostered the development of deconvolu- instrument performance limitations (e.g., signal-to- tion theories. Initially, instruments having at least noise ratio (SNR)), or strong correlations between 10 adjacent spectral bands with a spectral resolu- adjacent bands, as well as ill-posed problems in tion (or full width at half the maximum (FWHM)) model inversion, which reduce this dimensionality of 10 nm were considered as imaging spectrome- significantly. ters, however, nowadays the understanding is that The original definition for imaging spectrome- imaging spectrometers must be able to sample indi- try was coined by Goetz et al. (1985) as being vidual relevant features (absorption, reflectance, ‘the acquisition of images in hundreds of con- transmittance, and emittance) with at least three tiguous, registered, spectral bands such that for or more contiguous spectral bands at a spectral each pixel a radiance spectrum can be derived’ resolution smaller than the spectral width of the (Figure 12.3). A more detailed definition is that feature itself.

Figure 12.3 Original imaging spectrometry concept drawing as used by G. Vane and A. Goetz (courtesy of NASA JPL).

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Figure 12.4 Conceptual imaging spectrometer data cube with two spatial domains (x and y ) and the spectral domain (λ). Randomly distributed voxels each represent individual ‘radiometers’ (left) and a fully acquired data cube (right).

IMAGING SPECTROMETER PRINCIPLES

In imaging spectrometry a generalized data con- cept, called the data cube, is used to visualize the relation between the spatial and the spec- tral domains present. The spatial data is acquired by imaging a scene using techniques such as staring filter wheels, pushbroom, or whiskbroom scanner, amongst others. When acquiring data in only one spectral band (monochromatic acquisi- tion), each individual element may be referred to as a pixel with a spatial extent and a sin- gle wavelength. By adding many spectral bands Figure 12.5 Imaging spectrometer data all pixels can be represented conceptually as cube acquired by an airborne HyMap system voxels. A voxel (‘volume element’) is a three- on 26 August 1998 in Switzerland (Limpbach dimensional equivalent of a pixel, representing individual radiometers having 3D units of length Valley). The sides are color-coded spectra by (x, y) and wavelength (z). All these individual intensity. (See the color plate section of this radiometers represent the data cube as a 3D discrete volume for a color version of this figure). regular grid (Figure 12.4). Imaging spectrometer data is often visualized as a data cube formed by a series of image layers, series of lines, and staring systems, filter wheel each layer of which is an individual wavelength systems, or snap shot cameras series of monochro- interval. The sides of this cube are color-coded matic images at different spectral wavelengths spectra by intensity, and the top is a three-band (Figure 12.6). The data acquisition process is usu- color composite (Figure 12.5). ally performed until a complete data cube is filled with voxels. In the following sections, each of the major acquisition approaches is discussed in more detail. IMAGING SPECTROMETER DATA CUBE ACQUISITION Whiskbroom scanners The acquisition of the data cube is performed differ- Whiskbroom scanners are usually opto- or elec- ently by different imaging spectrometer technolo- tromechanical sensors that cover the field-of-view gies. In general, whiskbroom imaging spectrome- (FOV) by a mechanized angular movement using ters collect series of pixels, pushbroom scanners a scanning mirror sweeping from one edge of the

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swath to the other, or by the mechanical rota- much simpler than with other systems. In addi- tion of the sensor system. The inherent sensor tion, this technology supports in-flight calibration instantaneous field-of-view (IFOV) is therefore a with scanning reference sources located at the end single spatial pixel and its associated spectral com- or the beginning of each scan line. The disad- ponent. The image lines are collected using an vantages of this design include the presence of across-track scanning mechanism and the image a mechanical scanning system, the shorter inte- is acquired by the forward movement of the plat- gration time that is available than in pushbroom form used (Figure 12.7). The particular advantages based systems, and the image forming geometry of the whiskbroom scanning principle for imaging which is dependent on the scanning speed, scan spectrometers include a higher spectral uniformity mirror arrangement and the orthogonal platform since all pixels are recorded using the same detec- movement. Imaging spectrometers based on the tor line array and allowing the optical design to whiskbroom scanning principle include the air- accommodate a larger detector pixel size. borne AVIRIS (Green et al. 1998), DAIS (Chang Because the whiskbroom system can rely on et al. 1993), and HyMap (Cocks et al. 1998) single detectors, the calibration effort is usually instruments, as well as the spaceborne MODIS instrument (Salomonson et al. 1989).

Pushbroom scanners A pushbroom scanner is a sensor typically with- Line out mechanical scanning components for the data acquisition. The image formation is solely based Pixel on the (forward) movement of the sensor. A push- l Spectral dimension broom sensor is an imaging system which acquires Image a series of one-dimensional samples orthogonal to the platform line of flight with the along-track spa- tial dimension constructed by the forward motion of the platform. The spectral component is acquired y Spatial dimension by dispersing the incoming radiation onto an area (along track) x array. Translated to the concept of the data cube, a pushbroom scanner records the across track dimen- Spatial dimension sion x and the spectral dimension λ, representing (across track) lines, simultaneously (Figure 12.8) and the along track (y) component is acquired with the platform Figure 12.6 Data cube schematic depicting movement. the three major acquisition principles of Pushbroom scanners have the advantage that imaging spectrometers: Pixels are acquired they allow a longer integration time for each by whiskbroom systems, lines by pushbroom individual detector element in comparison with systems, and images by filter wheel systems whiskbroom based instruments (e.g., the inverse of (or staring cameras). the line frequency is equal to the pixel dwell time).

Figure 12.7 Whiskbroom scanning and its representation in the data cube (left single spectrum (one pixel), middle one scan line, right full data cube).

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l l

y y x x

Figure 12.8 Pushbroom scanning and its representation in the data cube (a single scan line is composed out of the across track pixels x and the spectral bands λ (left), resulting in a full data cube (right)).

l l

y y x x

Figure 12.9 Filter wheel acquisition and its representation in the data cube (left single monochromatic image, right full data cube).

In addition, there are distinct and fixed geometric Filter wheel cameras relations between the pixels within a scan line. Since area arrays are used as focal planes in The filter wheel camera is an opto-mechanical sen- these systems, the uniform calibration of the detec- sor that changes the spectral sensitivity of various tor response is critical. In a combined analysis of channels using a turnable filter wheel in the optical SNR, uniformity, and stability, pushbroom scan- path. The field-of-view (FOV) therefore represents ners might not necessarily outperform whiskbroom a full monochromatic frame, represented in the systems even though they have a longer integra- data cube by the x and y axis. The spectral com- tion time. ponent is collected by rotating the filter wheel Examples of pushbroom based imaging spec- to different band pass filters, which have differ- trometers include the airborne CASI (Babey and ent transmissions for different spectral regions. Anger 1989) and ROSIS (Kunkel et al. 1991) The data cube is filled by ‘stacking’ individual instruments and the spaceborne MERIS (Rast et al. (monochromatic) images on top of each other 1999) and Hyperion (Pearlman et al. 2003). (Figure 12.9).

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The particular advantages of the filter wheel directly to an area array, avoiding the use of often camera consist in the coherent spatial coregistra- bulky and complex optics required for imaging tion if not used on a moving platform. This makes spectrometers that use gratings or prisms. This this technology very suitable for staring telescope approach acquires a 2D FOV, consisting of x · y applications in astronomy. Most filter wheel cam- lines (corresponding to the number of x · y pix- eras use area arrays for the simultaneous coverage els of the area array). The difference compared of the spatial extent. If operated from a moving to a filter wheel is that here the y pixels (in the platform, mosaicing the individual frames is the along-track direction) record y different spectral most important feature. The calibration of the filter channels but for different adjacent spatial swaths. wheel camera can be performed by using a cali- With the movement of the platform along-track, the brated spectrometer or band pass filter/detector to across-track ground images are sequentially sam- test the sensitivity and non-uniformity of the detec- pled at the range of wavelengths supported by the tor elements. The nonuniformity calibration of the wedge filter. detector array is the most challenging issue with The major advantage of a wedge spectrometer this technology. The disadvantages of this design is the compact design because many optical ele- include the presence of the mechanical turning fil- ments can be avoided. The major disadvantage is ter wheel, which necessitates a fast change of the accommodating the Earth rotation (or the platform individual filters on moving platforms. Even so, movement), which can generate spectral smearing individual spectral images may not be aligned sat- (or spectral mis-registration). isfactorily. A major advantage is that it is easy The calibration of wedge spectrometers is com- to change the spectral bands by replacing the fil- parable to pushbroom imagers in static environ- ter wheel for different applications. In general, ments, although the wider FOV in the along-track very few airborne or spaceborne imaging spec- direction may introduce different challenges. An trometers are based on the filter wheel camera example of a wedge spectrometer flow in space approach, mostly due to its limitation in simultane- is the LAC instrument onboard of EO-1 (Reuter ously collecting many spectral bands. However, the et al. 2001), others are in planning (Puschell et al. concept has been demonstrated in airborne instru- 2001), but the concept has not yet seen significant ments (e.g., Airborne POLDER (Leroy and Bréon data distribution and use. 1996)), spaceborne (e.g., STRV-2 MWIR imager Other interesting imaging spectrometer concepts (Cawley 2003) and in astronomy staring telescopes include the computed tomography imaging spec- using a filter wheel approach. trometer (CTIS) (Descour et al. 1997). CTIS is a non-scanning instrument capable of simulta- neously acquiring full spectral information from Other, less frequently used imaging every position within its FOV. The raw image col- spectrometer concepts lected by the CTIS consists of 49 diffraction orders. The 0th diffraction order is located at the center of Wedge spectrometers (Figure 12.10) are based on the image. This order represents a direct view of a linear spectral wedge filter, which can be mated the spatial radiance distribution in the field stop

l l

y y x x

Figure 12.10 Wedge spectrometer and its representation in the data cube (left single scan, right optimal filled cube).

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and exhibits no dispersion. The remaining diffrac- spectrograph at this time named Spectrovisor – was tion orders exhibit dispersion increasing with order tilted to different view directions to increase the number. Reconstruction of the data cube from the angle of observation. raw data requires knowledge of how individual Inherent limitations of computer I/O perfor- voxels map to the imaging array. Each voxel corre- mance and – at these times – tape or band recorder sponds to an object volume, measuring xyλ, capacity, resulted in first designs of spectrome- where x, y, and λ are the spatial and spectral ters that were not capable of imaging the full sampling intervals, respectively. swath width continuously. Consequently they were Another emerging technology for imaging spec- called profiling systems, having across-track gaps trometers is the use of acousto-optical tunable in the spatial coverage. One of the first profil- filters (AOTF) allowing a rapid change of spectral ing instruments that was deployed on an aircraft bands (Calpe-Maravilla et al. 2004). Conceptually was the GER (Geophysical and Environmental AOTF based systems are similar to filter wheel Research Corp., of Millbrook, NY, USA, a com- instruments. pany that is no longer in business) MARK II Airborne Infrared Spectroradiometer (Chiu and Collins 1978, Figure 12.12). Much of the technology development for imag- EVOLUTION FROM AIRBORNE TO ing spectrometers took place in the 1970s and 1980s SPACEBORNE IMAGING SPECTROMETERS at the NASA Jet Propulsion Laboratory (JPL) in Pasadena (USA). At that time, Alex Goetz and This section presents an overview of selected Gregg Vane proposed successfully to use NASA instruments which have had an impact on the evo- internal funds to use a hybrid focal plane array lution and development of imaging spectrometers. with 32×32 elements, allowing the construction Comprehensive and detailed overviews are diffi- of an imaging spectrometer that covered the spec- cult to generate; however, Kramer (2002) presents tral region beyond the 1100 nm cutoff of silicon a very complete list of existing and planned arrays (Goetz 2007). These efforts resulted in the instruments. Airborne Imaging Spectrometer (AIS) (Vane et al. Early concepts of acquiring spectral (and direc- 1984) (Figure 12.12). tional) information from natural targets were dis- Following the successful deployment of AIS, cussed already in 1958 in the former Soviet Union the spectroscopists at JPL proposed a fully fledged (Arcybashev and Belov, 1958). The idea was to imaging spectrometer program that would range acquire a scene – a forest in this case – under from the airborne AIS1 and AIS2, the Airborne various view angles by using a complex flight pat- Visible/Infrared Imaging Spectrometer (AVIRIS), tern (Figure 12.11). In addition, the camera – a as well as two orbiting sensors, the Shuttle Imaging

Figure 12.11 Spectro-directional airborne acquisition pattern assessing forest angular spectral reflectance (left) and the Spectrovisor imaging spectrograph (right) (Arcybashev and Belov 1958, Kol’cov 1959) (Reprinted with permission of Juris Druck + Verlag AG).

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Figure 12.12 Airborne imaging spectrometers. From left, top: GER MARK II Airborne Infrared Spectroradiometer, Airborne Imaging Spectrometer (AIS) instrument assembly, Airborne Visible/Infrared Imaging Spectrometer (AVIRIS); Middle: Fluorescence Line Imager/ Programmable Multispectral Imager (FLI/PMI), Digital Airborne Imaging Spectrometer (DAIS7915), Reflective Optics Imaging Spectrometer (ROSIS); Bottom: Shortwave Infrared Full Spectrum Imager (SFSI), Hyperspectral Digital Imagery Collection Experiment (HYDICE) detector assembly, and Hyperspectral Mapper (HyMap). (See the color plate section of this volume for a color version of this figure). Source: Photos courtesy of: S.-H. Chang, G. Vane, R. Green, R. Baxton, A. Müller, H. van der Piepen, B. Neville, M. Landers, and M. Schaepman.

Spectrometer Experiment (SISEX) and a satellite- 380 nm and 2,500 nm. AVIRIS uses a scanning borne instrument, the High Resolution Imaging mirror to sweep back and forth (whiskbroom fash- Spectrometer (HIRIS). ion), producing 614 pixels for the 224 detectors The development of AVIRIS started in 1984 and each scan. The pixel size and swath width of the the imager first flew aboard a NASA ER-2 aircraft AVIRIS data depend on the altitude from which at 20 km altitude in 1987 (Vane 1987). Since then the data is collected. When collected by the ER-2 it has gone through major upgrades as technology from 20 km above the ground, the so-called ‘high changed in detectors, electronics, and computing. altitude option,’ each pixel produced by the instru- AVIRIS can be seen as the major driver for the ment covers an area approximately 20 × 20 m development of imaging spectrometry. on the ground (with some overlap between pix- The AVIRIS instrument (Figure 12.12) con- els), thus yielding a ground swath about 11 km tains 224 different detectors, each with a wave- wide. When collected by the lower flying Twin length sensitivity range (also known as spectral Otter ata4kmaltitude, the ‘low altitude option,’ bandwidth) of approximately 10 nanometers (nm), each ground pixel is 4 × 4 m, and the swath is allowing it to cover the entire range between 2 km wide.

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In Canada, the development of imaging spec- detector elements. This gives 240 spectral bands trometers has been intensive and G. Borstad propo- for each of 496 pixels in the across-track dimen- sed the Fluorescence Line Imager/Programmable sion for each integration period, which is hardware Multispectral Imager (FLI/PMI) instrument with selectable by various clock speeds (40–67 ms). The 288 spectral bands (‘spectral mode’) and 512 data are digitized to 13 bits and recorded as 16 bits pixels swath width (‘spatial mode’). The first data (Figure 12.12). were acquired with this instrument around 1985 The HYDICE instrument is also worth mention- (Figure 12.12) (Borstad et al. 1985). ing. HYDICE (Rickard et al. 1993) was a program An interesting further development of the to develop a state-of-the-art imaging spectrometer FLI/PMI instrument is considered to be the CASI to support utility studies of high spectral resolu- (CompactAirborne Spectrographic Imager), devel- tion measurements in the 400–2500 nm range. The oped as a combination of a spectral or spatial program was initiated by the U.S. Congress to imager similar to the FLI/PMI but subsequently investigate the application of hyperspectral data to enhanced to an operational and commercially flown the needs of federal agencies (forest assessment for instrument for many years (Babey andAnger 1989, the U.S. Department ofAgriculture, mineral explo- Gower et al. 1992). ITRES Corp., the manufacturer ration for the U.S. Geological Survey, and so forth). of the CASI line of instruments, can be seen as an The sensor was built by Hughes Danbury Optical offspring of Moniteq, who produced the FLI/PMI Systems, Inc., and integrated into a Convair 580 instrument. aircraft operated by the Environmental Research In Europe, airborne imaging spectrometers were Institute of Michigan (ERIM). The HYDICE sen- first primarily flown by leasing instruments from sor made its first data collection flight on 26 January the US or Canada. However, thanks to the efforts of 1995. The HYDICE sensor is a pushbroom imag- the German Aerospace Centre (DLR, Oberpfaffen- ing spectrometer that uses a biprism dispersing hofen (GER)) two instruments became available element and a two-dimensional focal plane array on a broader basis for the user community. First, to enable a single optical path design. The array a European Commission supported purchase of a is a 320 × 210 element InSb array fabricated by GER imaging spectrometer (Collins and Chang, Hughes Santa Barbara Research Center, with mul- 1990) with particular features such as the inclusion tiple gain regions to support operation over the of a thermal range (Chang et al. 1993) prompted a full 400–2500 nm spectral range. The array is European proposal for anAirborne Remote Sensing electronically shuttered with a fixed read time of Capability (EARSEC) (Carrère et al. 1995). The 7.3 ms. The frame rate can be adjusted from 8.3 instrument was named GER DAIS 7915 (Digital to 50 ms, allowing one to use nearly the full range Airborne Imaging Spectrometer) and incorporated of velocity to height (V/H) ratios within the flight 72 solar reflective and 7 mid-infrared/thermal envelope of the CV 580. In particular, the altitude bands (79 in total). Its operation was eventually dis- range from 5,000 to 25,000 feet can be used to continued in 2005, after having served 10 years in achieve spatial resolutions from 0.8 to 4 meters Europe fostering the use of imaging spectrometers (Figure 12.12). (Figure 12.12). A widely available instrument is the HyMap. The second development by DLR can be con- Manufactured by Integrated Spectronics of sidered an airborne forerunner instrument for the Australia, and operated by HyVista Corp., also spaceborne MERIS on and was named of Australia, HyMap (and its predecessor called ROSIS (Kunkel et al. 1991). The Reflective Optics Probe1) became operational in 1996 (Cocks et al. System Imaging Spectrometer was tested first in 1998). The HyMap series of airborne hyperspectral 1989 (van der Piepen et al. 1989) and featured scanners have been deployed in a large number a CCD–based pushbroom design. The instrument of countries, undertaking hyperspectral remote included a choice of selectable bands (32 out of sensing surveys in support of a wide variety of 128) covering the wavelength range significant for applications ranging from mineral exploration coastal zones and oceans (400–1100 nm). A totally to defense research to satellite simulation. The revised version of ROSIS was presented under evolution of the HyMap series continues with the name ROSIS-03 in 1998 (Gege et al. 1998) the development of a system providing hyper- (Figure 12.12). spectral coverage across the solar wavelengths Two other interesting airborne instruments were (0.4–2.5 µm) and 32 bands in the thermal infrared also developed. One was the SWIR Full Spec- (8–12 µm) (Figure 12.12). trum Imager (SFSI) (Neville and Powell 1992). Spaceborne imaging spectrometers are currently SFSI employs a two-dimensional platinum sili- still only sparsely available. Following a strict cide Schottky barrier CCD array with 488 rows interpretation of the definition of an imaging spec- of 512 detector elements. In operation, a region trometer, only Hyperion on EO1 (Pearlman et al. of 480 lines by 496 columns is used; two adja- 2003) and CHRIS on PROBA (Barnsley et al. cent lines are summed together on the detector 2004) can be considered true imaging spectrom- array to yield an effective array of 240 by 496 eters. MERIS on ENVISAT (Rast et al. 1999) is

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