Improved Computer Generated Holograms for Testing Aspheric and Freeform Optics

The most practical industrial method for measuring aspheric and free-form parts includes the use of Computer-Generated Holograms, diffractive elements that approximate the ideal method of using Kinoform surfaces. Computer Generated Holograms approximate Kinoforms by using binary diffraction maps, however this approximation leads to errors bleeding into the ideal fringe map when using a CGH for aspheric correction. Traditionally these errors are removed by using processes that decrease aspheric correction capability and increase alignment difficulty, and thus time. We propose a method that would allow us to use Computer Generated Holograms with lower required resolutions than the traditional Computer Generated Hologram test methods. This would decrease the alignment times, and increase the possible aspheric departure of the parts under test, decrease costs and increase capability. The outcome of this will decrease the time and cost of producing aspheric optics, and increase the availability of higher departure aspheric and free-form optics which require even more specialized testing equipment, reducing their cost up to the limits of the CGH. Our plan is to extend analytical simulations of an improved CGH, print a CGH to test the effectiveness of this method, and compare the results to traditional methods for CGH correction.

NASA has displayed continued interest in large, highly aspheric and free-form mirrors and lenses, and a method to test more aspheric surfaces while also decreasing alignment costs would be highly advantageous to the manufacturing of these surfaces.

CGHs are a key technology for testing aspheric and free-form optics. This method would provide the industry with an easier method for measuring aspheric surfaces, at lower cost, and providing the capability to measure more challenging aspheres and free-forms. This will assist NASA programs, DoD programs, IC programs, and the US commercial earth observing satellite market.

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