Coherent Backscattering from Multiple Scattering Systems - KOPS ...

3.3 Small Angle Setup
Figure 3.6: The optical setup. Backscattered light with a certain scattering angle θ
forms a cone-shaped shell, which is converged in the focal plane on a circle with radius
r by the lens with focal length f .
this ring of light on a circle in the focal plane, the radius of which is given by
r = f · tan θ (3.1)
where f is the focal length of the lens.
Like in the wide angle setup, single scattering is blocked by a circular polarizer. In the small
angle setup it is unnecessary to have a bendable polarizer foil, so a high-quality circular
polarizer from an industrial optics manufacturer (AUC circular polarizer from B+W) can be
used. These are available with larger diameters than usual polarizers offered by laboratory
suppliers and provide extinction ratios up to 4000:1 [1].
3.3.3 Sample average
Solid samples like teflon or the titania powders have to be moved during the measurement to
average over the speckle pattern. The method used in the wide angle setup, where the sample
is simply rotated, however turned out to be inconvenient as the high-resolving camera picked
up this rotational motion as a circular structure in the images. To have a larger ensemble to
average over in the small angle experiments, the sample is therefore pulled on Lissajous loops
by two motors with a frequency of several Hertz on each axis (fig. 3.7).
3.3.4 Data evaluation
For the evaluation of the backscattering data the exact position of the tip of the coherent
backscattering cone at θ = 0 has to be obtained from the CCD image. We do this by converting
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