12 Imaging Spectrometers Michael E. Schaepman Keywords: imaging spectroscopy, 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 sun 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 remote sensing (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 [17:24 2/3/2009 5270-Warner-Ch12.tex] Paper Size: a4 paper Job No: 5270 Warner: The SAGE Handbook of Remote Sensing Page: 166 166–178 IMAGING SPECTROMETERS 167 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 imaging spectrometer 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). [17:24 2/3/2009 5270-Warner-Ch12.tex] Paper Size: a4 paper Job No: 5270 Warner: The SAGE Handbook of Remote Sensing Page: 167 166–178 168 THE SAGE HANDBOOK OF REMOTE SENSING Spectroradiometric measurements remain one imaging spectrometry (imaging spectroscopy, or of the least reliable of all physical measurements also hyperspectral imaging) 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). [17:24 2/3/2009 5270-Warner-Ch12.tex] Paper Size: a4 paper Job No: 5270 Warner: The SAGE Handbook of Remote Sensing Page: 168 166–178 IMAGING SPECTROMETERS 169 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.