1. Application of computed tomography for measuring three dimensional refractive index inhomogeneity
- Author
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Stamper, Brian L. and Stamper, Brian L.
- Abstract
Manufacturers of optical glass strive to make a product that is homogeneous, isotropic, and free of any bubbles or mechanical strain. Glass used in forming images is very good, but the process of mixing the constituent materials, and melting them into a glass is limited. The index of refraction varies based on the lack of uniformity remaining after the manufacturing process. Transmitted wavefronts will have errors due to this inhomogeneity. The most common method currently used to quantify the homogeneity of a glass sample is to measure in one direction through the glass. Variations along the test axis are integrated resulting in loss of positional information in this direction. Homogeneity is then reported by using the peak-to-valley wavefront error reducing the three dimensional nature of glass to a single value. Not only have we lost the longitudinal information, but we have also lost any knowledge of the transverse texture of the sample. We present in this research a method for retrieving three dimensional information about the inhomogeneity of a glass test piece. Computed tomography provides a well developed methodology for constructing a three dimensional measurement from two dimensional data. Common interferometric measurements, or projections, taken at multiple angles has sufficient information to estimate the full three dimensional structure of the test piece. Important differences from computed tomography used for medical diagnoses are explored. Refraction at the interfaces of the sample limits the number of angles over which projections can be made. The angular distance between projections also influences the accuracy of the reconstructed object.
- Published
- 2004