The cornea is the anterior most surface of the eye and plays a critical role in vision. A thin fluid layer, the tear film, coats the outer surface of the cornea and serves to protect, nourish, and lubricate the cornea. At the same time, the tear film is responsible for creating a smooth continuous surface, where the majority of refraction takes place in the eye. A significant component of vision quality is determined by the shape of the cornea and stability of the tear film. A dual interferometer system for measuring the dynamic corneal topography is designed, built, verified, and qualified by testing on human subjects. The system consists of two coaligned simultaneous phase-shifting polarization-splitting Twyman–Green interferometers. The primary interferometer measures the surface of the tear film while the secondary interferometer tracks the absolute position of the cornea, which provides enough information to reconstruct the absolute shape of the cornea. The results are high-resolution and high-accuracy surface topography measurements of the in vivo tear film and cornea that are captured at standard camera frame rates.
Complex surface forms are becoming increasingly prevalent in optical designs, requiring advances in manufacturing and surface metrology to maintain the state of the art. Non-null interferometry extends the range of standard interferometers to test complex shapes without the need for complicated and expensive compensating elements. However, non-null measurements will accumulate significant retrace errors, or interferometer-induced errors, which can be difficult to isolate from surface figure errors. Methods discussed in the literature to correct for retrace errors in a reflection-based interferometer are computationally intensive and limited in spatial resolution. A method is presented for reconstructing complex surface shapes in a reflection-based non-null interferometer configuration, which is computationally efficient, easy to implement, and can produce high spatial resolution surface reconstructions. The method is verified against simulated surfaces that contain more than 200 μm of surface departure from a null configuration. Examples are provided to demonstrate the effects of measurement noise and interferometer model uncertainties, as well as an experimental validation of the method.
An interferometer for measuring dynamic properties of the in vivo tear film on the human cornea has been developed. The system is a near-infrared instantaneous phase-shifting Twyman-Green interferometer. The laser source is a 785 nm solid-state laser, and the system has been carefully designed and calibrated to ensure that the system operates at eye-safe levels. Measurements are made over a 6 mm diameter on the cornea. Successive frames of interferometric height measurements are combined to produce movies showing both the quantitative and qualitative changes in the topography of the tear film surface and structure. To date, measurement periods of up to 120 s at 28.6 frames per second have been obtained. Several human subjects have been examined using this system, demonstrating a surface height resolution of 25 nm and spatial resolution of 6 μm. Examples of features that have been observed in these preliminary studies of the tear film include postblink disruption, evolution, and stabilization of the tear film; tear film artifacts generated by blinking; tear film evaporation and breakup; and the propagation of foreign objects in the tear film. This paper discusses the interferometer design and presents results from in vivo measurements.
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.
An interferometer for measuring dynamic properties of the in vivo tear film on the human cornea has been developed. The
system is a near-infrared instantaneous phase-shifting Twyman-Green interferometer. The laser source is a 785 nm solidstate
laser; the system has been carefully designed and calibrated to ensure that the system operates at eye safe levels.
Measurements are made over a 6 mm diameter on the cornea. Successive frames of interferometric height measurements
are combined to produce movies showing both the quantitative and qualitative changes in the topography of the tear film
surface and structure. To date, measurement periods of up to 120 seconds at 28.6 frames per second have been obtained.
Several human subjects have been examined using this system, demonstrating a surface height resolution of 25 nm and
spatial resolution of 6 μm. Examples of features that have been observed in these in preliminary studies of the tear film
include: post-blink disruption, evolution, and stabilization of the tear film; tear film artifacts generated by blinking; tear
film evaporation and break-up; and the propagation of foreign objects in the tear film. This paper discusses the
interferometer design and presents results from in vivo measurements.
Raytheon Vision Systems (RVS) has developed scanning, high-speed (<3klps), all digital, with on-chip Analog-to-Digital Conversion (ADC), mid-wave infrared (MWIR 3-5mm) focal plane arrays (FPA) with excellent modulation transfer function (MTF) performance. Using secondary ion mass spectrometry (SIMS) data and detailed models of the mesa geometry, RVS modeled the predicted detector MTF performance of detectors. These detectors have a mesa structure and geometry for improved MTF performance compared to planar HgCdTe and InSb detector structures and other similar detector structures such as nBn. The modeled data is compared to measured MTF data obtained from edge spread measurements and shows good agreement, Figure 1. The measured data was obtained using a custom advanced test set with 1µm precision alignment and automatic data acquisition for report generation in less than five minutes per FPA. The measured MTF values of 83 unique parts, Figure 2, had a standard deviation of 0.0094 and a mean absolute deviation of 0.0066 at half Nyquist frequency, showing excellent process repeatability and a design that supports high MTF with good repeatability.
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