Daniel G. Smith

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.

Graphical approach to Shack-Hartmann lenslet array design

The performance of a Shack-Hartmann wavefront sensor is largely determined by the design of the lenslet array. In its simplest form, there are two available design parameters: the number of lenslets across the diameter of the wavefront, and the focal length. Spatial resolution, sensitivity, accuracy, and dynamic range are all affected by one or both of these parameters. For example, by virtue of increased spot motion under a fixed wavefront slope, longer focal lengths provide greater slope sensitivity. Simultaneously, longer focal lengths reduce the dynamic range, since spots may overlap for smaller wavefront curvatures. This paper presents a graphical approach to lenslet array design where various considerations constrain the possible solutions of the design problem.

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.

Calibration issues with Shack-Hartmann sensors for metrology applications

A long-standing goal of optical metrology is testing aspherics without the need for part specific nulls lenses. The problem involves increasing the measurement dynamic range while preserving accuracy. The Shack-Hartmann wavefront sensor offers an interesting alternative to interferometry where the dynamic range is tied to the wavelength of light. Because the Shack-Hartmann wavefront sensor is a geometric test, the lenslet array can be designed in a way that trades sensitivity for dynamic range making it possible to test, without a null, aspheres that would otherwise require null optics. However, a system with this much dynamic range will have special calibration issues. Shack-Hartmann wavefront sensors are widely used in feedback control systems for adaptive optics. In that application, calibration is not a serious problem as the system drives the correction to a null; calibration errors slow the rate of convergence. For metrology applications, the calibration of the Shack-Hartmann wavefront sensor must be absolute. This presentation will discuss issues related to the design and calibration of a Shack-Hartmann metrology system including the design of an appropriate lenslet array, methods for dealing with induced aberrations, vignetting and spatial resolution limitations.

Important considerations when using the Shack-Hartmann method for testing highly aspheric optics

The Shack-Hartmann (S-H) method is a good candidate for general aspheric metrology because the lenslet array can be designed to accommodate the dynamic range associated with wildly aspheric wavefronts. However, when the S-H method is used in this fashion several issues must be taken into consideration. First, while the sensitivity and dynamic range of the instrument can be increased by allowing the spots to shift several lenslet sub-apertures, real lenslets are not thin lenses with zero aperture so the spots will not shift in exact proportion to the average phase gradient across the lenslet as is commonly expected. Second, if the wavefront is sufficiently aspheric, any relay optics will induce additional aberrations, which can be accounted for with proper calibration and reverse raytracing. Another limitation of the S-H method is that spots cannot overlap or cross. While this is a limitation on the divergence of the phase distribution or wavefront curvature the problem can be avoided if we guarantee that the beam has no caustic between the lenslet array and detector. Finally, the single biggest problem in aspheric metrology is losing the light or vignetting. One general way to address this problem is to image the part onto the lenslet array with a large numerical aperture. In this way, rays leaving the part can have some range of angles that are guaranteed to make it through the system. This presentation will discuss these issues and methods for overcoming them. Experimental results will also be presented to demonstrate the effects.

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.

Simulation of laser radar tooling ball measurements: focus dependence

The Nikon Metrology Laser Radar system focuses a beam from a fiber to a target object and receives the light scattered from the target through the same fiber. The system can, among other things, make highly accurate measurements of the position of a tooling ball by locating the angular position of peak signal quality, which is related to the fiber coupling efficiency. This article explores the relationship between fiber coupling efficiency and focus condition.

A method for associating spots in a Shack-Hartmann wavefront sensor with a hexagonal lenslet array

Historically, the spots produced by a Shack-Hartmann wavefront sensor are expected to stay in a region immediately behind their respective lenslets. Valuable sensitivity and dynamic range can be recovered if the spots are allowed to wander. A new algorithm is presented for resolving this issue for a hex-packed lenslet geometry.