The optical set-up used in the experiments is shown in finaloptsetup. The two plano-convex lenses have focal lengths of respectively and . The diameter of the lenses are . The two lenses were already available in the laboratory, and other lenses would have been just as good.
The distance between the lenses and the size of the pin-hole was adjusted to optimize the homogeneity of the sheet. (We wanted a sheet with thickness around for reasons discussed in chapter ). The size of the pin-hole was chosen to . This size corresponds to about one half of the diameter of the beam from the laser. The hole is large enough for most of the laser light to pass through, and the irradiance of the light passing through is approximately constant. For this beam diameter the distance between the lenses was adjusted to minimize variations of the thickness. The distance between the two lenses was adjusted while using the video camera system to monitor the variations.
The laser sheet was measured to be wide over a range of around the center of the thin region. The distance between the two lenses then was . Using this value together with a desired length of for the thin region and a wavelength of in equation and gives and .
In our experimental set-up the center of the thin region was located behind the last lens. Using this value in equation the limiting thickness is , while the thickness was measured to . We conclude that the theoretical estimates seem fairly good.