We have designed a nd built a phase-measuring LUPI interferometer to use pre-aligned custom CGH nulls for high accuracy figure metrology of deep aspherics. The CGH nulls operate in double pass, first producing an aspheric test wavefront and then recollimating the return wavefront. This eliminates any need to locate the CGH at an image of the test pupil THe CGH is common to both test and reference paths, allowing the use of photomask quality substrates. Tho enable the CGH-LUPI to test a wider variety of aspheres, we have designed and built a set of 100 mm aperture accessory optics for use in combination with CGH nulls. These accessory optics consist of five singles, each approximately F/3, which may be kinematically stacked in numerous combinations and permutations to produce test wavefronts ranging from nearly collimated to F/0.75 A CGH null compensates for asphericity of the test optic and design aberrations of the accessory optics. The interferometer and accessory optic designs permit independent verification of all aspects of system accuracy and calibration without the need for disassembly. Designing a custom CGH null involves raytracing the accessory optics but not the interferometer mainframe optics. Depending on the phase measuring algorithm selected, known system aberrations due to manufacturing tolerances may be software compensated in real time.
Many electronic fingerprinting devices are required to have a distortion-free high-contrast imaging capability, with diffraction-limited resolution. That is, no software correction of image distortion is allowed, and all fingerprint scaling correction must be accomplished with optical components. For such systems, a minimum of four key requirements must be satisfied simultaneously in the optical design: (1) Total internal reflection (TIR) at the fingerprinting surface; (2) Optical compensation for producing zero distortion; (3) Variable anamorphic scaling capability in two axes for producing correct image sizing; (4) Diffraction-limited imagery across the entire field of view. In the mechanical design for manufacture and assembly of the imaging system, generally a number of mechanical implementations are needed to provide for each of alignment. Thus a strong interaction between the optical designer, the mechanical engineer, and the marketing company which dictates the system specification is essential all throughout the design process. In this paper we present several optical design principles involved with electronic fingerprinting. A discussion of the Scheimpflug condition, its attendant keystone distortion, tilted object and image planes to assist the correction of distortion, tilted lens elements to assist in the correction of defocus, and variable anamorphic prism pairs (or cylinders) will illustrate how a unified design solution is arrived at for a complex imaging system. Illumination concepts involving TIR and non-TIR approaches are also discussed.
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