Michael Morag edited Experimental Setup.tex  almost 10 years ago

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\section{Experimental Setup}  \par The schematic for the experimental setup is shown in figure 1. Our setup uses a 635 nm laser as the light source, which is attenuated to 1.6 by a combination of neutral density filters in order to prevent over saturation. The light is then projected onto a 100 $\mu$m pinhole, generating a high quality uniform wavefront. To create a collimated beam, a 75 mm converging lens is placed one focal length after the pinhole, generating a 1.5 mm beam spot. To eliminate higher order lobes, an iris is placed after the converging lens to only allow the central disk of the beam. After a creating a collimated plane wave, it is illuminated onto the sample, which generates a diffraction pattern. A rotating stage, seen in figure 2, was installed in order to be able to rotate our sample in and take multiple images at various measurable angles. A 300 mm objective lens is placed after the sample to focus the image onto the Charged Coupled Device detector. The objective lens also increases the optical distance of the image, allowing us to acquire a far-field diffraction pattern within the size limitations of our workspace. The detector is connected to a computer where we will use built in software to control the device. A light image and a dark image are taken so that contamination from the device can be subtracted from the final image.\\  \vspace  \par In order to acquire a satisfactory image, and due to requirements of the Matlab program, a high oversampling ratio was required. Calculating an over sampling ratio is not difficult, however in the experimental setup used, a lens is placed between the sample and the detector, which complicates the calculations. Instead, the oversampling ratio was measured after taking the image and finding the distance between each peak, which is correlated to the oversampling ratio. With the setup used, the oversampling ratio came out to be \textasciitilde 20 along the x-axis and \textasciitilde 15 along the y-axis. All images were taken in the dark, limiting the exposure of ambient light onto the detector.   \par The sample used was a \textit{C. elegans} larvae, which was approximately 0.6 mm in size. The \textit{C. elegans} was a good sample because it was small, easy to grow and maintain, transparent, and weakly scattering. The worm was mounted onto a glass slide using water droplets that were allowed to dry, keeping the worm in place on the slide. Originally, a fly-wing was used instead of the worm. We used the larger fly-wing to improve our optical setup and our image reconstruction techniques. The results for both can be seen in the data analysis section.