Abstract
This paper describes a method for probing a nonrefracting phase object confined by a cylindrical tube and quantifies the errors introduced into the measurement. The method is applicable to a phase object with a refractive index that is much less than that of the tube material; such as a gas contained in a glass or plastic tube. Double-exposure holographic interferometry with plane-wave illumination and real-image reconstruction was used to produce the interferogram, and ray-tracing computations were used when reconstructing the refractive-index distribution to correct the distortions in the interferogram caused by the tube. An error analysis was performed for a specific tube using both experimental results and numerical simulations. The experiments were conducted using a 52-mm long acrylic tube with a 51.3-mm i.d. and a 56.3-mm o.d. The tube contained CO2 during the first holographic exposure and air during the second; both exposures were made at room temperature and pressure. Therefore, the refractive-index change between exposures was known accurately. For the experimental conditions used in this study, it was possible to image the interferogram and still probe to within 1.1 mm of the tube wall. The average error in the refractive-index distribution reconstructed from the experimental interferogram was <0.5%, even though the concentricity and eccentricity of the tube surfaces were 0.010 and 0.013 mm, respectively. The numerical simulations demonstrated that the average error in the reconstructed refractive-index distribution due to error in the tube refractive index and error in the tube-wall thickness was <0.15%.
© 1985 Optical Society of America
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