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L N Thibos

Publications and source records attributed to L N Thibos.

At least 19 recordsLinked to original sources
PubMed2001

Power vector analysis of the optical outcome of refractive surgery.

PURPOSE: To demonstrate the power vector method of representing and analyzing spherocylindrical refractive errors. SETTING: School of Optometry, Indiana University, Bloomington, Indiana, USA. METHODS: Manifest and keratometric refractive errors were expressed as power vectors suitable for plotting as points in a 3-dimensional dioptric space. The 3 Cartesian coordinates (x, y, z) of each power vector correspond to the powers of 3 lenses that, in combination, fulfill a refractive prescription: a spherical lens of power M, a Jackson crossed cylinder of power J0 with axes at 90 degrees and 180 degrees, and a Jackson crossed cylinder of power J45 with axes at 45 degrees and 135 degrees. The Pythagorean length of the power vector, B, is a measure of overall blurring strength of a spherocylindrical lens or refractive error. Changes in refractive error due to surgery were computed by the ordinary rules of vector subtraction. RESULTS: Frequency distributions of blur strength (B) clearly demonstrate the effectiveness of refractive surgery in reducing the overall blurring effect of uncorrected refractive error. CONCLUSIONS: Power vector analysis also revealed a reduction in the astigmatic component of these refractive errors. Paired comparisons revealed that the change in manifest astigmatism due to surgery was well correlated with the change in keratometric astigmatism. Power vectors aid the visualization of complex changes in refractive error by tracing a trajectory in a uniform dioptric space. The Cartesian components of a power vector are mutually independent, which simplifies mathematical and statistical analysis of refractive errors. Power vectors also provide a natural link to a more comprehensive optical description of ocular refractive imperfections in terms of wavefront aberration functions and their description by Zernike polynomials.

Astigmatism↗
PubMed2001

Optics of aberroscopy and super vision.

This paper (1) reviews the fundamental limits to visual performance imposed by optical imaging and photoreceptor sampling to determine the limits to the potential gains offered by ideal corrections; (2) examines the predicted losses in vision induced by chromatic aberration, phase shifts, typical ocular aberrations, and the gains possible by correcting the monochromatic aberrations of the eye; (3) discusses the principles of aberration measurement in the eye; and (4) presents methods for measuring and classifying monochromatic aberrations of the eye.

Eye↗
PubMed2001

Use of a Shack-Hartmann aberrometer to assess the optical outcome of corneal transplantation in a keratoconic eye.

We report the optical outcome of corneal transplantation treatment on a keratoconic eye as measured with a Shack-Hartmann aberrometer, and we compare the results with the recovery of visual acuity after surgery. Before surgery, the naked keratoconic eye exhibited extremely large aberrations that could not be measured unless the patient wore a rigid gas-permeable contact lens. With the lens, the computed point-spread function of the eye was large and multimodal, and simulated retinal images confirmed the patient's subjective report of multiple, overlapping images. After full recovery from surgery, aberrations of the corrected eye were much smaller compared with the presurgical eye, which implied a more compact point-spread function and clearer retinal images. These optical changes were mirrored by an improvement in uncorrected visual acuity from 1.3 logarithm of the minimum angle of resolution (logMAR) before surgery to 0 logMAR with spectacle correction after full recovery. We conclude that the Shack-Hartmann aberrometer provides an objective, quantitative assessment of the optical outcome of penetrating keratoplasty that allows the clinician to measure retinal image quality objectively and to accurately simulate the complex visual distortions associated with keratoconus.

Cornea↗
PubMed2001

On-eye evaluation of optical performance of rigid and soft contact lenses.

A Shack-Hartmann aberrometer was used to assess the optical performances of eyes corrected with rigid or soft contact lenses compared with spectacles. Metrics of optical quality derived from the measured wave aberrations were consistent with the subjective rating of visual clarity by subjects. Optical aberration analysis illustrated the differences in aberration structures of eyes wearing different optical corrections. For our subjects, correction with a rigid gas-permeable lens yielded significantly better optical quality than did the soft contact lens or spectacle lens. This was due to a reduction in the eye's asymmetric (odd-order) aberrations and a reduction in the amount of the eye's positive spherical aberration. These observations can be explained by theoretical calculations of the aberrations of the eye plus lens optical system. We conclude that aberrometry provides a better understanding of the optical effects of contact lenses in situ and could be useful for optimizing future designs of contact lenses.

Contact Lenses, Hydrophilic↗
PubMed2000

Optical and visual impact of tear break-up in human eyes.

PURPOSE: The purpose of this study was to examine the optical and visual impact of tear break-up. METHODS: Optical quality of the eye was assessed during periods of nonblinking by quantifying vessel contrast in the fundus image and by monitoring the psychophysical contrast sensitivity and the spatial distribution of tear thickness changes by retroillumination. All measures were obtained from three eyes either with or without a soft contact lens. RESULTS: A noticeable decrease in retinal vessel contrast and contrast sensitivity were observed soon after a blink. Both of these measures of optical quality of the eye showed a similar pattern of image degradation both with and without a soft contact lens. Although trial-to-trial variability was considerable, sample means show that image contrast in the low spatial frequency range can drop to between 20% and 40% of initial values after 60 seconds of nonblinking. Retroillumination of the tear film showed local intensity fluctuations that progressively spread across the pupil with increasing time after the blink. CONCLUSIONS: Optical aberrations created by tear break-up contribute to the decline in image quality observed objectively and psychophysically. The decline in image quality that accompanies tear break-up may be a direct cause of the blurry vision complaints commonly encountered in dry-eye patients.

Adult↗
PubMed1999

Clinical applications of the Shack-Hartmann aberrometer.

The efficacy of the Shack-Hartmann technique for measuring the optical aberrations of the eye was evaluated for four classes of clinical conditions associated with optically abnormal eyes. These categories (with specific examples) are: anomalies of the tear film (dry eye), corneal disease (keratoconus), corneal refractive surgery [laser-assisted in situ keratomileusis (LASIK)], and lenticular cataract. We show that in each of these cases, it is possible to obtain at least a partial topographic map of the refractive aberrations of the patient's eyes, but severe losses of data integrity can occur. We further show that the Shack-Hartmann aberrometer provides additional information about the eye's imperfections on a very fine spatial scale (< 0.4 mm) which scatter light and further degrade the quality of the retinal image. Taken together, spatial maps of the variation of optical aberrations and scatter across the eye's entrance pupil represents an improved description of the optical imperfections of the abnormal eye.

Cataract↗
PubMed1999

Relationship between acuity for gratings and for tumbling-E letters in peripheral vision.

Earlier studies have reported that grating resolution is sampling-limited in peripheral vision but that letter acuity is generally poorer than grating acuity. These results suggest that peripheral resolution of objects with rich Fourier spectra may be limited by some factor other than neural sampling. To examine this suggestion we formulated and tested the hypothesis that letter acuity in the periphery is sampling-limited, just as it is for extended and truncated gratings. We tested this hypothesis with improved methodology to avoid the confounding factors of target similarity, alphabet size, individual variation, peripheral refractive error, and stimulus size. Acuity was measured for an orientation-discrimination task (horizontal versus vertical) for a three-bar resolution target and for a block-E letter in which all strokes have the same length. We confirmed previous reports in the literature that acuity for these targets is worse than for extended sinusoidal gratings. To account for these results quantitatively, we used difference-spectrum analysis to identify those frequency components of the targets that might form a basis for performing the visual discrimination task. We find that discrimination performance for the three-bar targets and the block-E letters can be accounted for by a sampling-limited model, provided that the limited number of cycles that are present in the characteristic frequency of the stimulus is taken into account. Quantitative differences in acuity for discriminating other letter pairs (e.g., right versus left letters E or characters with short central strokes) could not be attributed to undersampling of either the characteristic frequency or the frequency of maximum energy in the difference spectrum. These results suggest additional tests of the sampling theory of visual resolution, which are the subject of a companion paper.

Discrimination, Psychological↗
PubMed1999

Sampling limits and critical bandwidth for letter discrimination in peripheral vision.

We develop and test two functional hypotheses based on the sampling theory of visual resolution that might account for letter acuity in peripheral vision. First, a letter smaller than the acuity limit provides insufficient veridical energy for performing the task, and, second, the available veridical energy is masked by increased amounts of visible but aliased energy. These two hypotheses make opposite predictions about the effect of low-pass filtering on letter acuity, which we tested experimentally by using filtered letters from the tumbling-E alphabet. Our results reject the masking hypothesis in favor of the energy insufficiency hypothesis. Additional experiments in which high-pass-filtered letters were used permitted the isolation of a critical band of spatial frequencies, which is necessary and sufficient for achieving maximum visual acuity. This critical band varied with the particular pair of letters to be discriminated but was in the range 0.9-2.2 cycles per letter.

Discrimination, Psychological↗
PubMed1998

Entoptic image quality of the retinal vasculature.

Spatial details of entoptically visible retinal vessels were investigated using transcleral and Maxwellian-view stimulators. Nine normal subjects provided detailed drawings of the entoptic images which were digitized and superimposed onto digitized fundus photographs and fluorescein angiograms from the same eyes. Subjects also used a tracing method to locate visible entoptic features. The trans-scleral method provided images similar in detail to standard fundus photography (lacking capillary detail, but capturing larger arteries, veins, arterioles and venules) in the macula and around the disk. The Maxwellian-view method illuminated the fovea (7.7 degree field) and provided foveola capillary detail (capillaries traversing the foveola, the capillary arcade forming the FAZ) as well as the larger foveal vessels supplying the foveola, and often contained more foveal detail that available with fluorescein angiography.

Adult↗
PubMed1998

Acuity perimetry and the sampling theory of visual resolution.

Visual resolution is limited by neural sampling of the retinal image everywhere in the visual field except the fovea, where optical limitations usually dominate. Consequently, measurements of resolution acuity may be interpreted as estimates of the spatial density of the mosaic of photoreceptors (in parafoveal retina) or ganglion cells (in peripheral retina) of the living human eye. Recent experiments in the basic visual sciences have paved the way for clinical applications of this approach to the problem of detecting the loss of photoreceptors and ganglion cells during aging and in a variety of retinal diseases.

Humans↗
PubMed1998

Comparison of the eye's wave-front aberration measured psychophysically and with the Shack-Hartmann wave-front sensor.

The Shack-Hartmann wave-front sensor offers many theoretical advantages over other methods for measuring aberrations of the eye; therefore it is essential that its accuracy be thoroughly tested. We assessed the accuracy of a Shack-Hartmann sensor by directly comparing its measured wave-front aberration function with that obtained by the Smirnov psychophysical method for the same eyes. Wave-front profiles measured by the two methods agreed closely in terms of shape and magnitude with rms differences of approximately lambda/2 and approximately lambda/6 (5.6-mm pupil) for two eyes. Primary spherical aberration was dominant in these profiles, and, in one subject, secondary coma was opposite in sign to primary coma, thereby canceling its effect. Discovery of an unusual, subtle wave-front anomaly in one individual further demonstrated the accuracy and sensitivity of the Shack-Hartmann wave-front sensor for measuring the optical quality of the human eye.

Adult↗
PubMed1997

Aliased frequencies enable the discrimination of compound gratings in peripheral vision.

Although gratings beyond the Nyquist limit of the peripheral retina are visible as aliased percepts when presented in isolation, the reported lack of aliasing for targets with complex spectra (edges, lines, letters) suggests that aliased frequency components are invisible in the presence of sub-Nyquist image components. We tested this hypothesis by systematically exploring a range of stimulus parameters in search of conditions which would enable subjects to detect the supra-Nyquist components of a compound grating. A three-alternative forced-choice masking paradigm was used, which required subjects to discriminate a 2.5 deg patch of compound grating (mask + test) from a simple grating (mask only). Using a 2 c/deg grating as the masking component, which is well below the 4 c/deg Nyquist limit to veridical perception at 20 deg in the horizontal nasal field, we varied the spatial frequency of the test grating over a range extending above and below the Nyquist frequency. We found that aliased, supra-Nyquist components are reliably detected in the presence of high contrast, sub-Nyquist gratings, provided they have sufficient contrast. Contrast threshold for detecting the aliased gratings was higher when presented as a component of the compound grating than when presented in isolation. This masking effect of the sub-Nyquist component was not specific to supra-Nyquist test targets, however, since a similar masking effect also occurred for sub-Nyquist targets. These results suggest that the invisibility of aliasing described previously for edges or square wave gratings is due primarily to the combination of the low amplitudes of supra-Nyquist harmonics in such stimuli and a high contrast threshold of the peripheral retina. The additional masking effects of the sub-Nyquist, fundamental component of a square wave on the supra-Nyquist harmonics make the detection of aliasing impossible even for very high contrast square wave gratings presented in the periphery.

Contrast Sensitivity↗
PubMed1997

Interaction between sub- and supra-Nyquist spatial frequencies in peripheral vision.

In peripheral vision, high-frequency gratings beyond the Nyquist limit are visible as aliased patterns but, as shown previously, their visibility can be masked by superimposed sub-Nyquist gratings. Is the converse also true? Can supra-Nyquist gratings affect the detectability of sub-Nyquist gratings? In this study, we investigated the masking effect of high contrast, supra-Nyquist components of a compound grating on the contrast detection of sub-Nyquist components by employing a temporal three-alternative, forced-choice (3AFC) masking paradigm. We found that high-frequency, aliased gratings with contrast just 2 or 3 times above threshold can have a powerful masking effect on low-frequency, resolved gratings in peripheral vision. This result was surprising because prior results from sub-Nyquist masking studies in the fovea and the periphery have indicated that masking occurs only when the mask contrast is at least 5 times greater than threshold. Strong masking by supra-Nyquist gratings that are only just visible may be accounted for by an irregular sampling model in which the alias of the mask is distributed over a band of frequencies in the sub-Nyquist range. Furthermore, if undersampling is the explanation for the results of this study, then masking must occur after spatial sampling.

Contrast Sensitivity↗
PubMed1997

Power vectors: an application of Fourier analysis to the description and statistical analysis of refractive error.

The description of sphero-cylinder lenses is approached from the viewpoint of Fourier analysis of the power profile. It is shown that the familiar sine-squared law leads naturally to a Fourier series representation with exactly three Fourier coefficients, representing the natural parameters of a thin lens. The constant term corresponds to the mean spherical equivalent (MSE) power, whereas the amplitude and phase of the harmonic correspond to the power and axis of a Jackson cross-cylinder (JCC) lens, respectively. Expressing the Fourier series in rectangular form leads to the representation of an arbitrary sphero-cylinder lens as the sum of a spherical lens and two cross-cylinders, one at axis 0 degree and the other at axis 45 degrees. The power of these three component lenses may be interpreted as (x,y,z) coordinates of a vector representation of the power profile. Advantages of this power vector representation of a sphero-cylinder lens for numerical and graphical analysis of optometric data are described for problems involving lens combinations, comparison of different lenses, and the statistical distribution of refractive errors.

Computer Graphics↗
PubMed1997

Spherical aberration of the reduced schematic eye with elliptical refracting surface.

We extend the single-surface schematic-eye model of ocular chromatic aberration to account for spherical aberration of the eye. This extension is accomplished by allowing the model's single refracting surface to be a member of the family of ellipses with variable shape parameter (eccentricity). The resulting model, dubbed the "Indiana Eye," may have either positive or negative spherical aberration of varying degree, depending upon the numerical value of the shape parameter. Spherical aberration of the model eye is well described by third-order optical theory for shape parameters in the range 0 < or = p < or = 0.7, but requires fifth-order theory for an accurate description over the parametric range 0.7 < p < or = 1.0. An improved technique was devised for fitting the model to published measurements of ray aberrations while avoiding errors of estimation of the degree of spherical aberration present in eyes which also manifest odd-symmetric aberrations, such as coma. A shape parameter value of approximately p = 0.6 provided the best fit of the model to selected data from the literature.

Eye↗
PubMed1997

Oblique (off-axis) astigmatism of the reduced schematic eye with elliptical refracting surface.

The oblique (off-axis) astigmatism of the Indiana Eye, an aspheric reduced-eye model of ocular chromatic aberration and spherical aberration, is computed across the visual field by using Coddington's equations for nonspherical surfaces of revolution. Our results show that the amount of astigmatism varies significantly with the shape of the refracting surface and with the axial location of the pupil. For a pupil located 1.91 mm from the apex of the refracting surface (as originally specified for the model), the calculated Sturm's interval was larger than that reported in the literature. However, by moving the model's pupil 0.84 mm axially away from the apex toward the nodal point, a close match was achieved between Sturm's interval of the model eye and published data from human eyes for eccentricities up to 60 degrees. These results demonstrate that the aspheric reduced-eye model is capable of simultaneously accounting for the chromatic, spherical, and oblique astigmatic aberrations typically found in human eyes.

Astigmatism↗
PubMed1997

Wavelength-dependent magnification and polychromatic image quality in eyes corrected for longitudinal chromatic aberration.

Theoretical calculations using a simple model eye in combination with achromatizing lenses or artificial pupils show that correcting wavelength-dependent refractive error or its effects can exaggerate wavelength-dependent magnification by up to a factor of 7. These calculations are confirmed experimentally, and their effects on retinal image quality are modeled. Because of the increased wavelength-dependent magnification, gains in polychromatic image quality produced by correcting wavelength-dependent refractive error (or minimizing its effects with small pupils) are generally restricted to a small region of the retina.

Color Perception↗