Full text: Proceedings, XXth congress (Part 8)

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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B-YF. Istanbul 2004 
Figure 1. Conceptual layout of the stationary interferometer 
arranged in the Sagnac (triangular or cyclic path) geometry 
The light from the object is first collimated by the objective L, 
and, by means of a beam-splitter BS and two folding mirrors 
M1 and M2, is focused onto a CCD plane by the lens P. Like 
the Michelson device, this Sagnac interferometer also uses a 
beam-splitter (as source-doubling technique) but inside of a 
common (triangular) path with a tilt introduced between the two 
interfering image wavefronts to form a fixed (stationary) pattern 
of interference fringes of “equal thickness”. 
The BS is the fundamental component, which provides phase- 
delay between the two coherent interfering rays so that the OPD 
changes linearly with varying the angle of the entering ray with 
respect to the instrument optical axis. Therefore, the device 
acquires the image of an object superimposed to a fixed pattern 
of across-track interference fringes. Then, introducing a relative 
motion between the sensor and the object, each scene pitch 
experiences the same interference pattern and an entire image- 
frame is recorded for each autocorrelation phase offset. 
The 3-D array of these frames has to be “realigned” in order to 
extract the complete autocorrelation function (interferogram) of 
each scene pixel, then this record is inverse cosine transformed 
to yield a wavenumber hyperspectral data cube. 
Differently from the FTHSI optical configuration, our prototype 
interferometer doesn't employ the slit nor the cylindrical lens in 
front of the detector array. In fact, in the developed version a 
complete 2-D image of the observed scene is acquired with 
superimposed the autocorrelation modulation pattern. 
The main feature is that we need acquire as many frames as the 
number of photosensitive elements. For example, if we utilize a 
detector array with 512 pixels, it is necessary to acquire a set of 
512 records to reconstruct the complete autocorrelation function 
of each scene pixel. It is clear that the requested data-rate is 
very high. 
Figuré 2 shows a picture of the developed instrument. 
Figure 2. Picture of the developed instrument 
The characteristics of the CCD camera used to spatially resolve 
the interferogram are listed in Table 3. 
Type: CCD  frame-transfer with 
anti-blooming, TH7888A 
1024 x 1024 
Number of pixel: 
Pixel size 14 um x 14 um 
430 nm-1000 nm @QE>3% 
Spectral range: 
Responsivity: 23DN/(nJ em^) @450 nm 
18% @680 nm 
Dynamic range: 3200:1 
Maximum frame/rate: | 60 fps 
Digitalization: 12 bit 
Table 3. Characteristics of the frame grabber used to spatially 
resolve the interferogram record 
In our case, the data cube of a typical measurement contains 
more than 1 million data points for a total size of 2 MB before 
cosine transform. 
In a Fourier Transform Spectrometer (FTS) the acquired. 
physical information is the interferogram /(OPD), which 
depends on OPD values according to the following fundamental 
law (Barnes, 1985; Okamoto et al, 1984): 
19 (k 
I(OPD) = | ) [1+ cos(2nkOPD dk (1) 
where [1 (k) 
interferometer, OPD is the optical path difference introduced 
by the BS in the considered propagation direction, k =14A is 
the wavenumber and the integration takes into account for a 
non-monochromatic light spectrum. 
is the intensity of the ray entering the 

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