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.
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.
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.