Coherent Backscattering from Multiple Scattering Systems - KOPS ...

3 Setups
Figure 3.7: Sample shaker. The two motor-driven eccentric wheels M (red) pull the
sample S back and forth between the levers and the two rubber bands (brown), so that
the samples moves on Lissajous loops.
the greyscale image into a binary image, based on a threshold value of 95% of the maximum
intensity in the picture. The center of the cone is then identical with the center of mass of
the white region. The angular intensity distribution of the backscattered light can then be
obtained either by a profile section through the image or by an azimuthal average.
The angular resolution of the setup can be tested by replacing the multiple scattering sample
with a mirror. The image on the CCD chip is then that of a single scattering angle. If the lens
in front of the camera were able to focus the light on a single pixel, the angular resolution of
the setup would be given by the pixel size, which is 7.4 µm for the camera in use. However, in
the present setup configuration the focus spot is 4 − 8 pixels wide, depending on the diameter
of the incoming laser beam, which is between 0.5 cm and 2 cm.
To account for the structure of the focus spot, the data are therefore fitted with theory curves
that are convoluted with the intensity profile of the spot. a The resulting angular resolution of
the measured backscattering data is then close to the maximal resolution of 0.00085 ◦ , which
is given by the pixel size of the CCD chip.
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