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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
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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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