WIDE SPECTRAL RANGE IMAGING INTERFEROMETER Alessandro Barducci, Donatella Guzzi, Cinzia Lastri, Paolo Marcoionni, Vanni Nardino, Ivan Pippi CNR IFAC Sesto Fiorentino, ITALY ICSO 2012 Ajaccio 8-12/10/2012
Overview Imaging interferometry is an alternative technology to obtain hyperspectral images. Instruments of this kind has been developed and embarked in technological space mission in the recent past (FHTSI), and also in the present on Chinese satellites. The Italian Space Agency has granted our Institute for the development of ALISEO, an imaging interferometer in the VNIR spectral region, for MIOSAT, a small technological mission, for which Phase B is ended on 2011. In 2012 Regione Puglia has financed our project called SHIRA, which comprises the development of an airborne prototype of a wide band imaging interferometer, working in the MIR and TIR regions. This technology has advantages and drawbacks, depending also from the spectral region of interest.
Advanced Remote Sensing Imaging spectrometers Imaging interferometers SAGNAC INTERFEROMETER PUSH-BROOM SPECTROMETER WITH DISPERSIVE GRATING (by Sira) MICHELSON INTERFEROMETER
The Sagnac configuration Sagnac Inteferomter Focusing optics 2D Image sensor The two beams travel on opposite paths, and exhibit exactly phase-delay (for a point source on the optical axis) due to a difference of reflections between the two rays. The Optical Phase Difference (OPD) linearly increases with increasing the slope on the optical axis of the input ray. Due to the absence of entrance slit the device acquires the image of the target superimposed to a fixed pattern of interference fringes. Imaging interferometer prototype
Leap-frog operating mode Every frame contains the scene superimposed on the stationary fringe pattern Standard acquisition: spectrometers and interferometers The interferogram of a given pixel is dispersed along a slant direction of the acquired data cube Sensor OPD Sensor λ or OPD y x y x Leap-frog technique Standard techniques Energy from each pixel is dispersed spectrally and ever y plane of a data-cube is a monochromatic image of the scene. In push-broom sensors a single frame is a (x, ) 2-dim domain.
Laboratory set-up employed for testing the airborne prototype performances: nine calibrated ceramic tiles have been placed on a motorized sledge and observed under a field of view equal to 11. Laboratory set-up
Laboratory set-up Nine frames of a short sequence of measurements portraying ceramic tiles that were observed for assessing the instrument spectral calibration.
Retrieval of spectral radiance and reflectance Single frame from the re-assembled data cube RGB of the spectral reflectance hypercube
Advantages & Drawbacks Advantages Absence of moving parts, very compact and stable instrument. Sampled spectral range and resolution can be adjusted simply changing the inclination of one of the mirror. Simple optical layout, no generation of multiple orders. Drawbacks Heavy pre-processing, which comprises alignment of the frames, vignetting removal, Dc offset estimation and subtraction. Slow spectral estimation, obtained by a modified IDFT, taking into account the spectral dispersion of the OPD, which prevents from using optimized FFT, and that is relate to the refractive index of the beam splitter material. SNRinterferometric SNRdispersive considering the same spectral interval.
Optical Layout The shortest wavelength that can be interpolated (reconstructed) without aliasing is related to the Optical Path Difference (OPD). These quantities are linked together by the geometry and dimensions of the optical components of the interferometer. The spectral resolution d and range of a stationary interferometer are linked to the maximum OPD and to the number of samples used for the interferogram reconstruction: A careful design of the optical system, which take into account both optical materials and beam-splitter dimensions, allows the use of the same Sagnac layout both for the realization of an instrument working in the VNIR (400 nm 1000 nm), or in the medium and thermal infrared range (3000 nm -- 5000 nm and 8000 nm --13000 nm) of the spectrum. 2 2OPD max Imaging interferometry in the MIR-TIR region is appealing because maintains the advantage seen for the VNIR, but overcome some drawbacks, like the OPD dependencies from the refractive index of the beam splitter material min 2 ( OPD) OPD N OPD max
VNIR Optical layout Lens based optical system for the imaging interferometer operating in the range 400 nm 1000 nm.
MIR -TIR Optical layout Mirror based optical system for the imaging interferometer operating in the range 3000 nm 5000 nm and 8000 nm 11000 nm. TIR interferometry can easily achieve high spectral resolution, with good SNR performance
TIR Performance simulation Simulated interferogram observed by an instrument equipped with a zinc selenide beam splitter when imaging a 2-deg wide field of view in the TIR spectral region Simulated Interferogram @ 300K 4500000 4000000 70000 60000 Black Body 1400 K Spectrum obtained from T-ALISEO interferogram 3500000 50000 Intensity (a.u.) 3000000 2500000 2000000 1500000 Intensity (a.u.) 40000 30000 20000 10000 1000000 500000 0 150000 130000 110000 90000 70000 50000 30000 10000 10000 OPD 35 30 0 1 3 5 7 9 11 13 15 Wavelength (micron) Spectrum obtained from T-ALISEO interferogram Black Body 300 K Intensity (a.u.) 4500 4000 3500 3000 2500 2000 1500 1000 Black Body 800 K Spectrum obtained from T-ALISEO interferogram Intensity (a.u.) 25 20 15 10 5 500 0 1 3 5 7 9 11 13 15 17 Wavelength (micron) 0 1 3 5 7 9 11 13 15 Wavelength (micron)
Conclusions Laboratory measurements performed with a Sagnac imaging interferometer realized in the visible-near infrared have been shown. The analysis of the obtained results confirms the goodness of the processing chain developed for the elaboration of an image acquired by an imaging interferometer in a leap-frog configuration. The feasibility of a Sagnac imaging interferometer in the thermal infrared has been simulated. Advantages in using imaging interferometers are their compactness, and the possibility to easily adjust the sampled spectral range and resolution. Critical points concerns the high data-rate requested to the detectors, the heavy and slow data processing for obtaining the pixel spectra, and SNR performances at least comparable to a dispersive instrument. In the MIR and TIR region some drawbacks can be overcame, and the use of imaging interferometry becomes more appealing.
For further informations: http://www.ifac.cnr.it/soasar i.pippi@ifac.cnr.it