Gregory A. Williby

Interferometric testing of soft contact lenses

Rapid growth in the contact lens industry towards higher levels of customization has precipitated the need for advances in the metrology techniques and instrumentation used to evaluate soft contact lenses. By measuring the transmitted wavefront, the information needed to evaluate a wide range of lens types (spherical, toric, bifocal) is obtained. A Mach-Zehnder interferometer is used with the lenses tested in saline solution. The lenses must be tested in saline solution to prevent dehydration of the lens, which results in an index change. The lenses are mounted in a cuvette, or water cell, that circulates fresh saline. Calibration of the instrument is complicated by the aspheric wavefronts produced by the lenses and the inherent aberrations picked up by the wavefront as it is imaged from immediately behind the lens to the detector. Simply removing a baseline, no test optic measurement from the measured wavefront does not satisfactorily remove the induced aberrations. Instead, removal of the induced aberrations is achieved by reverse raytracing. In reverse raytracing, the wavefront at the detector is traced back through the system to immediately behind the lens. The use of raytracing code enables theoretical wavefronts to be generated and expected-to-calculated performance evaluations to be made at the transmitted wavefront level.

Optical testing using Shack-Hartmann wavefront sensors

The basic problem associated with aspheric testing without the use of null optics is to obtain increased measurement range while maintaining the required measurement accuracy. Typically, the introduction of a custom-designed and fabricated null corrector has allowed the problem of aspheric testing to be reduced to that of spherical testing. Shack-Hartmann wavefront sensors have been used for adaptive optics, but have seen little application in optical metrology. We will discuss the use of a Shack-Hartmann wavefront sensor as a means of directly testing wavefronts with large aspheric departures. The Shack-Hartmann sensor provides interesting tradeoffs between measurement range, accuracy and spatial resolution. We will discuss the advantages and disadvantages of the Shack-Hartmann wavefront sensor over more conventional metrology tests. The implementation of a Shack-Hartmann wavefront sensor for aspheric testing will be shown.

Interferometry and ophthalmics at the College of Optical Sciences

A long-term research program has been in place at the College of Optical Sciences to apply interferometry to ophthalmic applications. These unique systems have been developed in response to industrial need. The first system is a transmission Mach-Zehnder interferometer used to measure the transmitted wavefront of a contact lens while it is submersed in saline. This interferometer allows the refractive power distribution of the lens to be measured. A second system makes use of a low-coherence interferometer to measure the index of refraction of contact lens materials. This task is complicated by the fact that the material is only available in very thin, flexible samples, and because the sample must remain hydrated in saline during the measurement. A third system also makes use of low-coherence interferometry to characterize the surface profile of both surfaces of a contact lens. Combined with index information, a complete model of the contact lens can be produced. Two additional interferometers examine the dynamics of fluid layers on the surface of a contact lens (in vitro) and of the tear film on the surface of the cornea (in vivo). Both systems are instantaneous phase shifting Twyman-Green interferometers. The evolution and changes to the fluid surface is measured at video rates with sub-wavelength precision. This paper tells the story of this research program.

Aspheric metrology with a Shack-Hartmann wavefront sensor

The basic problem associated with aspheric testing without the use of null optics is to obtain increased measurement range while maintaining the required measurement accuracy. Typically, the introduction of a custom-designed and fabricated null corrector has allowed the problem of aspheric testing to be reduced to that of spherical testing. Shack-Hartmann wavefront sensor have been sued for adaptive optics, but have seen little application in optical metrology. We will discus the use of a Shack-Hartmann wavefront sensor as a means of directly testing wavefronts with large aspheric departures. The Shack-Hartmann sensor provides interesting tradeoffs between measurement range, accuracy and spatial resolution. We will discus the advantages and disadvantages of the Shack-Hartmann wavefront sensor over more conventional metrology tests. The implementation of a Shack-Hartmann wavefront sensor for aspheric testing will be shown.

Rock and roll polishing: a process for optical surface polishing

We describe and present experimental results of a nonconventional process that can be used for the polishing of optical surfaces. In particular, we present data regarding polishing uniformity, removal rate, surface microroughness, and subsurface damage.