Coherent Backscattering from Multiple Scattering Systems - KOPS ...

3 Setups
Figure 3.5: Small angle setup. A CCD camera captures the the backscattered radiation
around backscattering direction.
to 45 K below ambient temperature to reduce electronic noise. It turned out that the glass
window that covers the CCD chip needs to be wedge shaped to avoid interferences in the glass,
which cause a disturbing ripple structure on the CCD image. The camera offers exposure
times between 30 ms and 183 min; to improve the signal-to-noise ratio, the backscattering data
of our experiments are an average over a series of usually 100 pictures, taken with an exposure
time of 0.5 s.
The camera replaces an older 8-bit model with 580 × 740 pixels, which did not offer enough
angular and intensity resolution for the planned experiments. The exchange of the camera
improves the maximum intensity resolution by more than two orders of magnitude and the
maximum angular resolution or respectively the maximum angular range by a factor of three.
As the direct backscattering direction is blocked by the camera, a beamsplitter directs the
laser beam onto the sample. The remaining part of the beam that passes straight through the
beamsplitter must be dumped completely, as even weak backreflection will disturb the image
on the camera. For similar reasons, a pellicle beamsplitter instead of a glass beamsplitter has
to be used, as even anti-reflection coated surfaces of a glass plate would cause distinct ghost
images.
In contrast to the wide angle setup described in the previous section, where the sample is
illuminated by a slightly divergent light beam, the incoming laser beam in the small angle
setup is exactly parallel. Backscattered light with a certain scattering angle θ then forms a
cone-shaped shell with a thickness that is determined by the diameter of the sample area
illuminated by the incoming laser beam (fig. 3.6). The lens above the beamsplitter converges
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