| The advancement of optical systems arises from furthering at least one of the three fields of optical development: design, fabrication, and testing. One example of such advancement is the growth in customization of contact lenses, which is occurring in part due to advances in testing. Due to the diverse quantities that can be derived from it, the transmitted wavefront is the tested parameter. There are a number of tests that can evaluate a transmitted wavefront, including moiré deflectometry, Shack-Hartmann wavefront sensing, and interferometry. Interferometry is preferred for its sensitivity and spatial resolution. The dynamic range issue is mitigated by the required immersion of the contact lenses in saline due to the complex nature of the lens material. The partial index-match between the lens and surrounding saline reduces the measured power of the lens and enables testing in an absolute, or non-null, configuration. Absolute testing allows for the generation of ophthalmic prescriptions and power maps from the transmitted wavefront.; Designing a non-null interferometer is based on three principles. The transmitted light must be collected, the resulting interference must be resolved, and the imaged wavefront must be calibrated. The first two principles are fulfilled by proper choices for the imaging lens and detector. Calibration comes from removing the wavefront-dependent induced aberrations via reverse raytracing. Reverse raytracing demands an accurate model of the interferometer. With such a model, theoretical wavefronts can be produced and compared to measured wavefronts. The difference between measured and modeled wavefronts quantifies the answer to the fundamental question in transmitted wavefront testing: does the optic perform as desired?; Immersion in index-matching fluid provides an adjustable increase in the dynamic range of the interferometer. The increase comes at the expense of sensitivity. The tradeoff between dynamic range and sensitivity can be quantified by the dimensionless ratio between the two numbers. This ratio is interpreted as a degree of difficulty for a measurement. Combined with absolute testing, immersion provides the ability to measure fast cylindrical lenses, which are notoriously difficult to test. Understanding the parameters of the interferometer provides a simple condition for determining the gain from immersion. |
