2) Experimental Set Up

Figure 1: (Original figure.) Experimental Set up.

A polarized beam of light from the 10mW HeNe laser was constricted by an iris and then focused by a bi-convex lens of focal length 62.9mm onto the suspension sample stored in a cuvette. Because the HeNe laser used was not one that produced a perfect beam of a single wavelength, it was observed that there was blue light visible at the edges of the red laser beam. The iris was used to cut off the edges of the beam produced in an effort to reduce the variance in wavelength of the light in the laser beam. The laser beam was scattered in all directions by the spheres in the sample, and a photodiode was used to collect intensity data of the light scattered at 90 degrees to the incoming beam. The 90-degree angle was chosen to make the set-up convenient. Another iris was used between the cuvette and the photodiode, to optimize the number of coherence areas being observed.

In simple terms, a coherence area is a building block of the area being observed for intensity fluctuations. This is mathematically defined as:

for a scattering volume of radius a, with a detector of radius b at a distance R away [1]. The number of such "blocks" being analyzed simultaneously determines the number of coherence areas. This needs to be experimentally optimized because for a low number of coherence areas the fluctuations in the intensity are high, which is desired, but the overall signal strength is quite low. If the signal strength is too low, the fluctuations cannot be analyzed properly. If the number of coherence areas is too high, then the signal strength is high but the magnitude of the fluctuations is low when compared to the signal itself. This again makes it difficult to extract sufficient information from the signal.

Once the photodiode detects the scattered light, it converts the measured intensity data into a photocurrent. A low noise current pre-amplifier was used to amplify this current and a second low noise pre-amplifier convert this current into a voltage. The amplified output voltage was read in by a LabVIEW program, designed by us for this experiment.