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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
3. EXPERIMENTAL ACTIVITY 
3.1 Calibration procedure 
Preliminary measurements have been carried out in order to 
calibrate the interferometer response. Two dyed He-Ne lasers 
( A red = 632.and A 8reen =543.2nm ) have been 
employed for illuminating a double planar diffuser in order to 
obtain a homogeneous and isotropic radiation distribution inside 
the instrument FOV. Figure 4 shows a single image-frame 
obtained with the green laser source. The high number of these 
interference fringes is related to the high intrinsic coherence- 
degree of the employed radiation source. 
rawLaserGreen 
raw LaserRed 
clOOO 
<u 
i! 1 : 
200 
400 pixel 600 800 1000 
Figure 4: Raw image (grey scale) obtained illuminating a 
double planar diffuser with a green He-Ne laser. The image is 
filled with a pattern of across-track interference fringes of equal 
thickness 
The pre-processed interferogram should have a null-mean, 
starting and ending tails approaching to zero, and any optical 
artefact removed. In order to achieve these characteristics, we 
have elaborated a general scheme which is based on the 
following main steps (Barducci, 2001a): 
- dark signal subtraction to account bias and noise in the 
detector electronic stage; 
- instrument spatial response compensation to remove saturated 
pixels, hot and cold pixels, and fixed-pattern noise; 
- geometrical and radiometric distortion correction to remove 
effects of vignetting and spatial shift of the fringes; 
- DC-offset subtraction 
- apodization to avoid “Gibbs effect” (ringing phenomenon); 
- cosine inverse transform to retrieve the un-calibrated at-sensor 
radiance spectrum; 
Figure 5. Interferograms averaged over all the columns of 
image of Figure 3 for the two He-Ne measurements. 
The spectral dispersion of optical path differences, which is 
due to the dispersion law of the refractive index of the beam 
splitter makes the calibration of the OPD axis really a complex 
task. When a broad spectral range is exploited, the OPD values 
may significantly depend upon the spectrum of the observed 
target. Thus the inversion model of the interferogram as well as 
the spectral calibration of the sensor might be difficult and their 
interpretation ambiguous. 
The uncalibrated radiance, retrieved applying gaussian 
apodization are plotted versus wavelengths in Figure 6. Due to 
the circumstance that the employed spectral source may be 
approximated to an impulse-like radiation source, this 
measurement is also a good test to estimate the instrument 
spectral resolution: roughly 23 nm at 632 nm. Let us note that 
the spectral resolution has been lowered due to apodization. and 
OPD spectral dispersion. 
150 
LasaOmi 
wavelength (mri) 
Figure 6. Uncalibrated spectra of at-sensor radiance retrieved 
by cosine transforming the interferograms plotted in Figure 4. 
3.2 SNR estimates 
We have executed 40 measurements employing a 600W 
halogen lamp, each measurement being constituted by 30 
frames. The mean and the standard deviation have been 
computed for each pixel-sample, hence obtaining a set of 1024 
mean and standard deviation values interferograms. The SNR 
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