Brian C. Primeau

Imaging Freeform Optical Systems Designed with NURBS Surfaces

Abstract. The designs of two imaging freeform systems using nonuniform rational basis-spline (NURBS) optical surfaces are described. The first system, a 10  deg×9  deg f/2 three-mirror anastigmat has four times higher spatial resolution over the image plane compared with the equivalent conventional rotational aspheric design, and 2.5 times higher resolution compared with a 10th-order XY polynomial freeform design. The mirrors for the NURBS freeform design have more than twice the asphericity than the conventional rotational and XY polynomial designs. In the second system, a Ritchey–Chretien telescope followed by a two-mirror NURBS freeform corrector is compared to a four-mirror Korsch telescope, for imaging to a visible-infrared imaging spectrometer. The freeform corrector design had 70% smaller spot sizes over the field and eliminated the large tertiary required in Korsch type design. Both of these NURBS freeform designs are possible due to a custom optical design code for fast accurate NURBS optimization, which now has parallel raytracing for thousands of NURBS grid points.

Interferometer for measuring the dynamic surface topography of a human tear film

The anterior refracting surface of the eye is the thin tear film that forms on the surface of the cornea. Following a blink, the tear film quickly smoothes and starts to become irregular after 10 seconds. This irregularity can affect comfort and vision quality. An in vivo method of characterizing dynamic tear films has been designed based upon a near-infrared phase-shifting interferometer. This interferometer continuously measures light reflected from the tear film, allowing sub-micron analysis of the dynamic surface topography. Movies showing the tear film behavior can be generated along with quantitative metrics describing changes in the tear film surface. This tear film measurement allows analysis beyond capabilities of typical fluorescein visual inspection or corneal topography and provides better sensitivity and resolution than shearing interferometry methods. The interferometer design is capable of identifying features in the tear film much less than a micron in height with a spatial resolution of about ten microns over a 6 mm diameter. This paper presents the design of the tear film interferometer along with the considerations that must be taken when designing an interferometer for on-eye diagnostics. Discussions include eye movement, design of null optics for a range of ocular geometries, and laser emission limits for on-eye interferometry.

Interferometer and analysis methods for the in vitro characterization of dynamic fluid layers on contact lenses

The anterior refracting surface of the eye when wearing a contact lens is the thin fluid layer that forms on the surface of the contact lens. Under normal conditions, this fluid layer is less than 10 μm thick. The fluid layer thickness and topography change over time and are affected by the material properties of the contact lens and may affect vision quality and comfort. An in vitro method of characterizing dynamic fluid layers applied to contact lenses mounted on mechanical substrates has been developed by use of a phase-shifting Twyman-Green interferometer. This interferometer continuously measures light reflected from the surface of the fluid layer, allowing precision analysis of the dynamic fluid layer. Movies showing this fluid layer behavior can be generated. Quantitative analysis beyond typical contact angle or visual inspection methods is provided. Different fluid and contact lens material combinations have been evaluated, and variations in fluid layer properties have been observed. This paper discusses the interferometer design and analysis methods used. Example measurement results of different contact lens are presented.

Dynamic measurement of the corneal tear film with a Twyman-Green interferometer

Abstract. 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.

Laser exposure analysis for a near- infrared ocular interferometer

Ocular interferometry has potential value in a variety of ocular measurement applications, including measuring ocular thicknesses, topography of ocular surfaces or the wavefront of the eye. Of particular interest is using interferometry for characterizing corneal shape and irregular corneal features, making this technology attractive due to its inherent accuracy and spatial resolution. A particular challenge of designing an ocular interferometer is determining safe laser exposure levels to the eye, including both the retina and anterior segment. Described here are the laser exposure standards relevant in the interferometer design and the corresponding calculations and results. The results of this work can be used to aid in the design of similar laser-based systems for ocular evaluation.

Imaging with NURBS Freeform Surfaces

New developments in the application of NURBS freeform surfaces in imaging systems are described, including their implementation and initial results from next generation designs using the complex shapes that can be modeled. Article not available.

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.

Illumination system design in a project-based course

This past spring a new for-credit course on illumination engineering was offered at the College of Optical Sciences at The University of Arizona. This course was project based such that the students could take a concept to conclusion. The main goal of the course was to learn how to use optical design and analysis software while applying principles of optics to the design of their optical systems. Projects included source modeling, displays, daylighting, light pollution, faceted reflectors, and stray light analysis. In conjunction with the course was a weekly lecture that provided information about various aspects of the field of illumination, including units, étendue, optimization, solid-state lighting, tolerancing, litappearance modeling, and fabrication of optics. These lectures harped on the important points of conservation of étendue, source modeling and tolerancing, and that no optic can be made perfectly. Based on student reviews, future versions of this course will include more hands-on demos of illumination components and assignments.

In-vitro interferometric characterization of dynamic fluid layers on contact lenses

The anterior refracting surface of the eye when wearing a contact lens is the thin fluid layer that forms on the surface of the contact lens. Under normal conditions, this fluid layer is less than 10 microns thick. The fluid layer thickness and topography change over time and are affected by the material properties of the contact lens, and may affect vision quality and comfort. An in vitro method of characterizing dynamic fluid layers applied to contact lenses mounted on mechanical substrates has been developed using a phase-shifting Twyman-Green interferometer. This interferometer continuously measures light reflected from the surface of the fluid layer, allowing precision analysis of the dynamic fluid layer. Movies showing this fluid layer behavior can be generated. The fluid behavior on the contact lens surface is measured, allowing quantitative analysis beyond what typical contact angle or visual inspection methods provide. The interferometer system has measured the formation and break up of fluid layers. Different fluid and contact lens material combinations have been used, and significant fluid layer properties have been observed in some cases. The interferometer is capable of identifying features in the fluid layer less than a micron in depth with a spatial resolution of about ten microns. An area on the contact lens approximately 6 mm wide can be measured with the system. This paper will discuss the interferometer design and analysis methods used. Measurement results of different material and fluid combinations are presented.

Tolerancing, Alignment and Test of the Transiting Exoplanet Survey Satellite (TESS) Optical Assembly

The Transiting Exoplanet Survey Satellite (TESS) will carry four visible waveband seven-element refractive f/1.4 lenses, each with a 34 degree diagonal field of view. This paper describes the tolerancing, assembly and alignment methods developed during the build of the TESS Risk Reduction Unit optical system. Lens assembly tolerances were derived from a sensitivity analysis using an image quality metric customized for mission performance. The optomechanical design consists of a two-stage lens housing that provides access for active alignment of each lens using a Trioptics OptiCentric measurement system. Thermal stresses and alignment shifts are mitigated by mounting the optics with cast RTV silicone spacers into individually aligned bezels, and custom fixtures were developed to aid in RTV bonding with reduced alignment error. The lens assembly was tested interferometrically over the field of view at room temperature and results were used to successfully predict lens performance and compensator adjustments and detector shim thickness for the -75C operational temperature and pressure.