FARLY RESULTS FROM AN IMAGING INTERFEROMETER PROTOTYPE
OPERATING IN THE SAGNAC CONFIGURATION
Paolo Marcoionni**
*Parma University — Earth Sciences Department, Parco Area delle Scienze 157A, 43100 Parma, ITALY
“National Research Council — “Nello Carrara” Applied Physics Institute, via Panciatichi 64, 50147 Firenze, ITALY
P.Marcoionni@bifac.enr.it
Submit to: YOUNG FORUM SESSIONS on Remote Sensing
KEYWORDS: Imaging interferometer, hyperspectral remote sensing, calibration methods, image processing.
ABSTRACT
Recent advances in Earth remote sensing imagers have being attempted with the launch of the first Fourier Transform Hyperspectral
Imager placed on board of U.S. Department of Defence. From the analysis of alternative optical layouts for a stationary
interferometer, we have developed a laboratory imaging interferometer operating in the Sagnac configuration. This instrument does
not employ any moving part to optically scan the instrument field-of-view, and acquires the image of the observed target modulated
from a staring interference pattern of vertical fringes. Despite of traditional Michelson-like interferometers, the intensity .
autocorrelation functions are generated by moving the concerned scene with respect to the imaging device. In this paper we present
some laboratory measurements obtained by this new stationary imaging interferometer. In order to calibrate the instrument response
and to determine its spectral resolution, we have executed a set of measurements illuminating a planar double diffuser with a red He-
Ne laser. The dependence of the optical-path-differences on the source spectral content has been assessed observing a 600W halogen
lamp with interference filters having 10nm of bandwidth. In order to retrieve at-sensor radiance spectrum a calibration procedure of
the acquired data has been implemented, which also takes into account dark signal subtraction, instrument spatial response
compensation, geometrical and radiation distortion correction, DC-offset subtraction, and inverse cosine transform. Some hints are
given about the use of such an instrument on board of airborne platforms for remote sensing of the Earth.
imaging interferometer developed by Applied Spectral Imaging
(Israel) for laboratory microscopy applications (Cabib, 1996).
Many advantages arise from using these systems regarding high
1. INTRODUCTION optical efficiency (Jacquinot and Felgett effects) and the
circumstance that the detectable spectral range and resolution
The launch of the first Fourier Transform Hyperspectral Imager can be changed by acting on the sensor sampling step and the
(FTHSI) on board of U.S. Air Force Research Laboratory instrument FOV (Descour, 1997; Herring et al, 1993; Junttila et
technological satellite MightySat II.] has been an attempt to al, 1991). :
overcome the main drawbacks that limit the use of push-broom From a radiometric point of view this circumstance provides a
and whisk-broom imaging spectrometers for Earth remote significant improvement of the signal-to-noise ratio (SNR).
sensing applications (Otten et al, 1998; Otten et al, 1995). However, due to the nature of the acquired interferogram it is
The FTHSI optical concept is derived from the so-called necessary to employ detectors with high accuracy of
“stationary imaging interferometers” which don't use any digitalization (Junttila, 1991).
moving part to optically scan the instrument Field-Of-View Other critical points are connected with the high data-rate
(FOV). In the FTHSI instrument the complete interferogram of ~~ requested and the need to spectrally pre-filter the incoming
the light from a slit is obtained by means of a cylindrical lens radiation in order to avoid aliasing artefacts ‘in the retrieved
which spatially disperses the physical information on one axis spectrum (Bracewell, 1965; Goodman, 1968).
of the employed detector array. Movement of the satellite with Starting from the analysis of FTHSI optical configuration, a
respect to the target allows the instrument to scan all the laboratory prototype imaging interferometer operating in the
observed surface. Sagnac configuration has been developed. -
Differently from more conventional time-scanning instruments, In Section two the instrument concept is presented and Section
in a stationary interferometer the range of Optical Path three is devoted to physical background. The description of the
Difference (OPD) between the recombined beams generated by experimental activities we have performed in order to calibrate
amplitude splitting, is obtained as a pattern of stationary fringes the instrument response is depicted in Section four. In Section
of “equal thickness” onto the detector array plane (Geneste et al, five some preliminary results are presented and discussed and in
1998). Section six open problems and conclusion are drawn.
System precursors of FTHSI have been the HyperCam and the :
IrCam developed by the Kestrel Corporation (USA) for airborne 2. OPTICAL CONFIGURATION
applications, the Spatially Modulated Fourier Transform
Spectrometer (SMIFTS) developed by Hawaii University The typical layout of the developed instrument, called Sagnac
(Lucey et al, 1992), the High Etendue Imaging Fourier ^ configuration, is shown in Figure 1.
Transform Spectrometer (HEIFTS) by Science Application
International Corporation (USA) (Horton, 1996), and the
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