Matthew T. Sheehan

Reconstruction of the optical system of the human eye with reverse ray-tracing

We present a practical method for reconstructing the optical system of the human eye from off-axis wavefront measurements. A retinal beacon formed at different locations on the retina allows probing the optical structure of the eye by the outgoing beams that exit the eye through the dilated pupil. A Shack-Hartmann aberrometer measures the amount of wave aberrations in each beam at the exit pupil plane. Wavefront data obtained at different oblique directions is used for tomographic reconstruction by optimizing a generic eye model with reverse ray-tracing. The multi-configuration system is constructed by tracing pre-aberrated beams backwards from each direction through the exit pupil into the optical system of the aberrometer followed by the generic eye model. Matching all wave aberrations measured at each field point is equivalent to minimizing the size of the beacon spots on the retina. The main benefit of having a personalized eye model is the ability to identify the origin of the ocular aberrations and to find the optimal way for their correction.

Population study of the variation in monochromatic aberrations of the normal human eye over the central visual field

We present data analysis for ocular aberrations of 60 normal eyes measured with a Hartmann-Shack (HS) wavefront sensor (WFS). Aberration measurements were made on-axis and at 5 degree field angles in the nasal, inferior, temporal and superior semi-meridians. Particular attention is given to aberration distributions and possible strategies for aberration correction are discussed. A versatile HS WFS was designed and constructed with features of simultaneous pupil centre determination, off-axis capability, real-time data displays, and efficient lenslet sampling orientation. The subject alignment is achieved by the use of a parallel channel that is recombined with the sensing channel to simultaneously image the eye and the HS spots onto a single CCD. The pupil centre is determined using this image of the eye (iris edge), rather than the HS spots. The optical design includes a square lenslet array positioned with its diagonals aligned with the most typical principal astigmatic meridians of the eye. This favourable orientation helps to enlarge the dynamic range of the WFS. The telecentric re-imaging of the HS spots increases the robustness of the system to defocus in the event of CCD misalignment.

Shape of the anterior cornea : Comparison of height data from 4 corneal topographers

Purpose To compare the ability of clinical corneal topographers to describe the shape of the anterior cornea for optical modeling. Setting University Medical Center Groningen, Groningen, Netherlands. Design Cross‐sectional study. Methods The anterior corneal shape of healthy subjects was assessed with 4 topographers (Atlas Placido disk, Galilei dual Scheimpflug, Orbscan scanning slit, Pentacam single Scheimpflug). Exported height data were fit with Zernike polynomials. Mean values with the standard deviation, interdevice variability, and test–retest variability were determined for the defocus Z(2,0), astigmatism Z(2,−2) and Z(2,2), coma Z(3,−1) and Z(3,1), and spherical aberration Z(4,0) coefficients for 5.5 mm and 8.0 mm diameters. Results At 5.5 mm, the single Scheimpflug topographer showed the smallest coefficient of repeatability; 0.31 &mgr;m for Z(2,0); 0.40 and 0.34 &mgr;m for Z(2,−2) and Z(2,2), respectively; 0.15 and 0.11 &mgr;m for Z(3,−1) and Z(3,1), respectively; and 0.08 &mgr;m for Z(4,0); the other topographers showed up to 10 times larger coefficients of repeatability. The (unsigned) mean differences between the topographers were in the range of 0.20 to 1.21 &mgr;m for Z(2,0); 0.02 to 0.31 &mgr;m and 0.06 to 0.42 &mgr;m for Z(2,−2) and Z(2,2), respectively; 0.03 to 0.18 &mgr;m and 0.03 to 0.35 &mgr;m for Z(3,−1) and Z(3,1), respectively; and 0.00 to 0.14 &mgr;m for Z(4,0). The Placido‐disk topographer and single Scheimpflug topographer data corresponded best. Similar trends were found at 8.0 mm. Conclusion Test–retest variability hampered a detailed description of the anterior corneal shape at the level of individual subjects; interdevice variability compromises the exchangeability of the devices. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned.

Investigation of the isoplanatic patch and wavefront aberration along the pupillary axis compared to the line of sight in the eye

Conventional optical systems usually provide best image quality on axis, while showing unavoidable gradual decrease in image quality towards the periphery of the field. The optical system of the human eye is not an exception. Within a limiting boundary the image quality can be considered invariant with field angle, and this region is known as the isoplanatic patch. We investigate the isoplanatic patch of eight healthy eyes and measure the wavefront aberration along the pupillary axis compared to the line of sight. The results are used to discuss methods of ocular aberration correction in wide-field retinal imaging with particular application to multi-conjugate adaptive optics systems.

Illumination system for corneal collagen crosslinking.

Purpose. We have designed and constructed an ultraviolet (UV) illumination system for use in corneal collagen crosslinking (CXL) procedures. CXL is a recently developed procedure to treat Keratectasia such as Keratoconus and Pellucid Marginal Degeneration. The role of the illumination system is to safely provide UV light at the absorption peak of the Riboflavin photosensitizer, which generates reactive oxygen species and subsequently increases the number of covalent bonds between collagen fibrils. Methods. Raytracing simulations were used to compare various optical designs for the illumination system. Alternative layouts are presented and design compromises between competing goals are discussed in reference to safety and practical considerations for clinical use. The mechanical mounting system and the beam output power and beam diameter testing system are briefly described. Results. The illumination system has several features that help provide uniform effectivity of the applied treatment over the desired treatment diameter, assuming Riboflavin has uniform absorption in the treatment zone. The optical design offers substantially greater beam control and safety than rudimentary devices relying on direct illumination. The prototype was relatively inexpensive to construct and includes a mounting system and beam testing setup. Conclusions. The UV illumination system was found to be successful in providing a balanced solution regarding clinical use and the eye safety issues relevant to CXL.

Design of a versatile clinical aberrometer

We have designed an ocular aberrometer based on the Hartmann-Shack (HS) type wavefront sensor for use in optometry clinics. The optical system has enhanced versatility compared with commercial aberrometers, yet it is compact and user-friendly. The system has the capability to sense both on-axis and off-axis aberrations in the eye within an unobstructed 20 degree field. This capability is essential to collect population data for off-axis aberrations. This data will be useful in designing future adaptive optics (AO) systems to improve image quality of eccentric retinal areas, in particular, for multi-conjugate AO systems. The ability of the examiner to control the accommodation demand is a unique feature of the design that commercial instruments are capable of only after modification. The pupil alignment channel is re-combined with the sensing channel in a parallel path and imaged on a single CCD. This makes the instrument more compact, less expensive, and it helps to synchronize the pupil center with the HS spot coordinate system. Another advantage of the optical design is telecentric re-imaging of the HS spots, increasing the robustness to small longitudinal alignment errors. The optical system has been optimized with a ray-tracing program and its prototype is being constructed. Design considerations together with a description of the optical components are presented. Difficulties and future work are outlined.

Unwanted reflections during slit lamp assisted binocular indirect ophthalmoscopy

Binocular indirect ophthalmoscopy is a routine ophthalmic examination procedure. Two different apparatus setups are commonly employed; the head/spectacle mounted designs of Schepens and slit lamp assisted ophthalmoscopy, both typically performed through a fundus lens of high positive power. It can be difficult for clinicians to avoid unwanted back reflections primarily emanating from the fundus lens and the pre-corneal tear film, particularly when initially learning the skills required to perform the examination. In this investigation the illumination system of a slit lamp was modified to include a variety of obscuration designs optically conjugate to surfaces responsible for creating unwanted reflections. The modified apparatus was then used to perform binocular indirect ophthalmoscopy on an artificial eye and on real eyes. Clinicians used questionnaires to score the appearance of reflections. The mean scores were similar across all trials, including the control unmodified trial, indicating general consensus that the modified illumination system provided no substantial effect on the perception of these unwanted reflections.

Wavefront tomography of the human eye assisted with corneal topography and optical path measurements

We present a new method of reconstructing the optical system of the eye based on combined data from an aberrometer, corneal topographer and a partial coherence interferometer (IOL Master), which is used for customized eye model optimization.