Vol49_1_2006def 103 ANNALS OF GEOPHYSICS, VOL. 49, N. 1, February 2006 Key words Fourier transform imaging spectrome- ter – stationary Sagnac interferometer – spectral cal- ibration – hyperspectral remote sensing 1. Introduction Recent advances in optics technology and on-board pre-elaboration data facilities have driven the attention of the international scientif- ic community towards the development of the so called «stationary imaging interferometers» for Earth remote sensing purposes (Junttila, 1992; Horton, 1996). These interferometers do not employ any moving part to scan the imaged Field of View (FoV) and generate the entire in- terference pattern simply moving over the sur- face of the observed target (Junttila, 1991; De- scour, 1996; Horton et al., 1997). The launch of the first Fourier Transform HyperSpectral Imager (FTHSI) on board of US Department of Defence (DoD) technological satellite MightySat II.1 was an attempt to over- come the main drawbacks that limit the use of push-broom and whisk-broom imaging spec- trometers for environment investigation (Meigs et al., 1997; Otten III et al. 1997, 1998). These limitations concern sensor calibra- tion, electronics complexity and the circum- stance that spectral parameters such as spectral resolution and sampling step cannot be changed during the flight. System precursors of FTHSI had been the HyperCam and the IrCam developed by Kestrel Corporation (U.S.A.) for airborne applications, and the Spatially Modulated Fourier Transform Spectrometer (SMIFTS) developed by Hawaii University. The main properties of these instru- ments are listed in table I. In a stationary interferometer all the im- pinging radiation is collected by the detector for interferogram sampling (Jacquinot and Fellgett advantages) (Jacquinot, 1954; Bennett et al., 1993; Persky, 1995). This is a significant im- Recent advances in Earth remote sensing: Fourier Transform Stationary HyperSpectral Imagers Alessandro Barducci, Paolo Marcoionni and Ivan Pippi Istituto di Fisica Applicata «Nello Carrara» (IFAC), CNR, Firenze, Italy Abstract Future trends for the development of new remote sensing imagers have being defined since the launch of the first Fourier Transform HyperSpectral Imager (FTHSI) on board of DoD technological satellite MightySat II.1. Start- ing from the analysis of FTHSI optical configuration we have proposed an interesting modification which produces an image of the observed surface superimposed to a stationary interference pattern. This new optical arrangement together with the possibility to accommodate the spectral resolution by changing the device optical aperture and the sensor sampling step make the new instrument interesting for Earth remote sensing purposes. In this paper we present some preliminary results obtained from a laboratory prototype developed at our Institute. Some hints are discussed about the use of such an instrument on board of airborne and satellite platforms. Mailing address: Dr. Ivan Pippi, Istituto di Fisica Ap- plicata «Nello Carrara» (IFAC), CNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy; e-mail: I.Pip- pi@ifac.cnr.it 104 Alessandro Barducci, Paolo Marcoionni and Ivan Pippi provement of the Signal-to-Noise Ratio (SNR) because all the photons coming from a pixel of the source contribute to the interferogram pow- er, no matter what their wavelength is. On the contrary, filters, gratings and prisms of a stan- dard spectrometer reject most of the electro- magnetic radiation field. The principal critical points of an imaging interferometer are con- nected with the spreading of the observed target pixels originated by the not-entirely compensat- ed instrument motion, and the high data-rate re- quested. Starting from the analysis of FTHSI optical configuration we have developed a laboratory prototype of the imaging interferometer. In Section 2 we briefly discuss the adopted optical configuration and the experimental ac- tivity we have performed in order to calibrate the instrument response. In Section 3 we give our conclusions. 2. Experimental activity The laboratory device uses a new image plane interferometer geometry to produce «au- tocorrelation function modulation» in the im- age plane of the two dimensional array such as the phase offset of the modulation linearly varies across the images. The interferogram is imaged onto a CCD array as a spatial distribu- tion of intensity rather than as a function of time. The typical optical layout of the devel- oped interferometer, called Sagnac configura- tion, is shown in fig. 1. The characteristics of the CCD used to spa- tially sample the interferogram are listed in table II. As a two dimensional image is formed by the fore-optics, an entire frame is recorded for each autocorrelation phase offset. The three dimen- sional array of this data is processed to generate Table I. The main properties of FTHSI and its precursors. Instrument FTHSI HyperCam IrCam SMIFTS Spectral range 350-1050 nm 450-1050 nm 1700-5000 nm 1000-5200 nm 3000-5000 nm Spectral resol. 85.4 cm–1 87 cm–1 45 cm–1 95 cm–1 (1.7 nm@450 nm) (50 nm@3000 nm) 35 cm–1 Channels 256 180 55 256 Field-of-view 1.75° 13° 14° 13° Altitude 570 km, 3 km 3 km Sun-synchronous orbit Fig. 1. Layout of the stationary interferometer ar- ranged in the triangular geometry. The light from the object is first collimated by the objective L, it enters the device, it exits from it at 90° though two folding mirrors (M1 and M2) after being split into two coher- ent beams by the Beam-Splitter BS put at 45° with re- spect to the optical axes. P collocates the lens which focuses the energy on the CCD plane. 105 Recent advances in Earth remote sensing: Fourier Transform Stationary HyperSpectral Imagers an autocorrelation function data-cube which is Fourier transformed to yield a wavenumber hy- perspectral data-cube. The fundamental law, which describes the intensity I(OPD) falling on the photosensitive element (neglecting the optical losses) is (2.1) where I0(m) is the intensity of the ray before en- tering the interferometer, OPD is the optical path difference introduced by the beam splitter for every direction of propagation, and k = 1/m is the wavenumber relative to m. As can be seen from eq. (2.1), the interferogram is an oscillat- ing function which has a maximum at OPD = 0 and decays for large OPD’s. Actually, an interferogram is constituted by a pair of values p, DN(p) which respectively indicate the pixel position on the matrix and the corresponding electronic signal expressed in digital number. As shown in eq. (2.2), the spatial coordinate p of the pixel can be linked to the sampling step determined as the distance p (pitch) between the centres of two adjacent pixels (2.2) j being the index of the acquired interferogram and j0 the position of the pixel corresponding to the null OPD. As we have stated, the relation between OPD and the propagation direction j of a single ray entering the interferometer is lin- ear, as long as the imaged FoV is small enough ( )j j p0= -p ( ) ( ) ( )OPD cos OPDI I k k dk 2 1 2 0 = + r6 @# (2.3) f being the focal length of the lens which focuses the interference image, and a expresses the direct proportionality between OPD and j. In order to calibrate in wavenumber (or in OPD values) the response of the interferometer we employed a He-Ne laser (mexc = 632.8 nm) illuminating a dou- ble planar diffuser to produce uniform (homoge- ( ) ( )OPD a f a f a j j p0= = = -j j p . Table II. Principal characteristics of the CCD array used to sample the interferogram. Type CCD frame-transfer with anti-blooming, TH7887A Number of pixel 1024 × 1024 Pixel size 14 nm × 14 nm Spectral range 430 nm-1000 nm @QE>3% Responsivity 23DN/(nJ cm2) @450 nm 18% @680 nm Dynamic range 3200:1 Maximum frame/rate 60 fps Digitalization 12 bit Fig. 2. Raw (level 0) image (in gray-scale) obtained illuminating a double planar diffuser with a He-Ne laser. We observe a pattern of vertical fringes super- imposed to the target image. 106 Alessandro Barducci, Paolo Marcoionni and Ivan Pippi neous and isotropic) radiation field at the instru- ment entrance. Figure 2 shows a single image- frame obtained with the laser source. As can be seen, the instrument FoV is entire- ly filled with straight stripes (also called «fringes of equal thickness»), due to electromagnetic in- terference overlapping the flat input image. The shape of this pattern is due to the circumstance that all pixels on a vertical line go through the same OPD, so that they undergo the same amount of interference for the same wavelength of light. Conversely, the high number of these fringes is due to the high intrinsic coherence-de- gree of the employed source. The image here presented was pre-processed according to the following scheme: Fig. 4. Un-calibrated spectral radiance retrieved from the interferogram shown in fig. 3 after offset subtracting. Fig. 3. Interferogram acquired onto the CCD plane relative to an impulse-like source, having a bandwidth far below the spectral resolution of the imaging system. The employed source is a He-Ne laser. The solid line indi- cated the fitting function used to retrieve the pixel position corresponding to the null OPD. 107 Recent advances in Earth remote sensing: Fourier Transform Stationary HyperSpectral Imagers – dark signal subtraction; – instrument spatial response compensation; – geometrical distortion correction (vi- gnetting, spatial shift of the fringes pattern). Then, from the single frame an interfero- gram averaged over all pixels was extracted. Its central part was fitted with a sinc-like function in order to estimate and subtract the DC offset as expressed in eq. (2.1) by the term I0(k) / 2. An ideal interferogram is symmetric with respect to zero path difference and contains cosine contri- butions only. However, additional phase errors caused by the misaligned sampling grid, which does not match the region of zero path difference, result in an evident asymmetry in a real interferogram shape. Figure 3 shows an average interferogram and the fitting function, which best fits its cen- tral part. In this plot a strong asymmetry is evi- dent with respect to the pixel scale. After offset subtracting, the new interfero- gram is Fourier transformed to retrieve the un- calibrated spectral radiance of the employed source. Figure 4 shows the result of inverse co- sine transform obtained without adopting any apodization. 3. Conclusions In this paper we have presented preliminary images acquired with a new stationary imaging interferometer operating in Sagnac configura- tion. We have discussed the experimental activ- ity carried out in order to calibrate the interfer- ometer response. In addition we have analyzed a methodology to pre-process the acquired raw interferogram for dark-signal subtraction, in- strument spatial response compensation and geometrical distortion correction (vignetting, spatial shift of the fringes pattern). 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