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W Neil Charman

Publications and source records attributed to W Neil Charman.

At least 19 recordsLinked to original sources
PubMed2006

The Charles F. Prentice Award Lecture 2005: optics of the human eye: progress and problems.

The history of measurements of ocular aberration is briefly reviewed and recent work using much-improved aberrometers and large samples of eyes is summarized. When on-axis, higher-order, monochromatic aberrations are averaged, undercorrected, positive, fourth-order spherical aberration dominates; other Zernike wavefront aberration coefficients have average values near zero. Individually, however, many eyes show substantial amounts of third-order and other fourth-order aberrations; the value of these varies idiosyncratically about zero. Most normal eyes show only small amounts of axial monochromatic aberration for photopic pupils up to around 3 mm; the limits to retinal image quality are then usually set by diffraction, uncorrected or imperfectly corrected spherocylindrical refractive error, accommodation error, and chromatic aberration. Longitudinal chromatic aberration varies very little across the population. With larger mesopic and scotopic pupils, monochromatic aberration plays a more important optical role, but overall visual performance is increasingly dominated by neural factors. Some remaining problems in measuring and modeling the eye's optical performance are discussed.

Aging↗
PubMed2006

Peripheral refraction in orthokeratology patients.

PURPOSE: The purpose of this study is to measure refraction across the horizontal central visual field in orthokeratology patients before and during treatment. METHODS: Refractions were measured out to 34 degrees eccentricity in both temporal and nasal visual fields using a free-space autorefractor (Shin-Nippon SRW5000) for the right eyes of four consecutively presenting myopic adult patients. Measurements were made before orthokeratology treatment and during the course of treatment (usually 1 week and 2 weeks into treatment). Refractions were converted into mean sphere (M), 90 degrees to 180 degrees astigmatism (J180), and 45 degrees to 135 degrees astigmatism (J45) components. RESULTS: Before treatment, subjects had either a relatively constant mean sphere refraction across the field or a relative hypermetropia in the periphery as compared with the central refraction. As a result of treatment, myopia decreased but at reduced rate out into the periphery. Most patients had little change in mean sphere at 30 degrees to 34 degrees . In all patients, the refraction pattern altered little after the first week. CONCLUSION: Orthokeratology can correct myopia over the central +/- 10 degrees of the visual field but produces only minor changes at field angles larger than 30 degrees . If converting relative peripheral hypermetropia to relative peripheral myopia is a good way of limiting the axial elongation that leads to myopia, orthokeratology is an excellent option for achieving this.

Adult↗
PubMed2005

Off-axis refraction and aberrations following conventional laser in situ keratomileusis.

PURPOSE: To investigate off-axis refraction and aberrations following conventional laser in situ keratomileusis (LASIK) for myopia and hypermetropia. SETTING: School of Optometry, Queensland University of Technology, Australia. METHODS: Using an autorefractor, off-axis refractions were analyzed along the horizontal visual field between 35 degrees nasally and 35 degrees temporally in 1 eye each of 15 emmetropic subjects (-0.50 to +0.50 diopters [D]), 6 myopic subjects (-2.25 to -6.50 D), 6 hyperopic subjects (+1.50 to +3.00 D), 6 myopic LASIK patients (presurgical refraction -2.75 to -9.00 D), and 6 hyperopic LASIK patients (presurgical refraction +0.75 to +2.00 D). Wavefront sensing measured off-axis higher-order aberrations in 2 myopic LASIK patients. RESULTS: In myopic LASIK, the mean spherical components of refraction M became highly myopic away from the center of the visual field; in emmetropic and untreated myopic eyes, there were relatively small myopic shifts and hyperopic shifts, respectively. Off-axis 90-degree to 180-degree astigmatisms J180 in myopic LASIK subjects were greater than in untreated subjects. In hyperopic LASIK, there were mainly hyperopic shifts in M, opposite the direction in emmetropic and untreated hyperopic subjects. Off-axis J180 was less than in emmetropic and untreated hyperopic subjects. Some hyperopic LASIK patients had greater off-axis 45-degree to 135-degree astigmatisms J45 than patients in the other groups. In 2 myopic LASIK patients, Zernike root-mean-square 4th-order aberrations were higher than in the near-emmetropia group because of higher levels of positive spherical aberration. CONCLUSIONS: Off-axis aberrations can be dramatically affected by conventional myopic and hyperopic LASIK. In myopic LASIK, the increased off-axis refractive errors may have adverse effects on peripheral visual tasks that are dependent on off-axis refractive errors. The relatively low off-axis refractive errors in hyperopic LASIK patients may improve peripheral visual tasks.

Adult↗
PubMed2005

Dynamic changes in the tear film in dry eyes.

PURPOSE: To examine the dynamics of the tear film in patients with dry eye by measuring the wavefront aberrations of the anterior surface of the film. METHODS: Anterior surface aberrations for a 7-mm pupil were determined in 13 patients with dry eye at 1-second time intervals, for 15 seconds after a blink. The aberrations were calculated from the elevations provided by corneal topography. All data were decomposed using Zernike polynomials. Total, spherical, and comalike aberrations terms were studied separately. Results were compared with those in normal eyes. Outcome measures included comparison with clinical tear breakup time measurements. RESULTS: The total root mean square (RMS) wavefront aberration in patients with dry eye passed through in a minimum of 2.9 +/- 0.4 seconds after a blink in comparison to the minimum at 6.1 +/- 0.5 seconds in normal patients. In both groups, the minimum in total aberration appeared to be associated with similar changes in comalike aberrations, rather than in spherical aberrations, which increased monotonically with time. The time at which minimum RMS aberration occurred correlated reasonably well with the measured tear breakup times. CONCLUSIONS: Measurements of the dynamic changes in the optical aberrations introduced by the anterior tear film surface give valuable insights into tear film changes and may provide a convenient objective method for the diagnosis of dry eye.

Adult↗
PubMed2005

Postblink changes in the ocular modulation transfer function measured by a double-pass method.

PURPOSE: To examine the temporal changes in the modulation transfer function (MTF) of the eye after a blink. METHODS: The distance MTF of a 5-mm pupil was derived from double-pass retinal images in 20 healthy young subjects at various intervals after a blink (1 second up to 15 seconds). Such measurements include the effects of wavefront aberration, scattered light, and errors in focus and are thus more relevant to real life than are estimates of MTF based on wavefront errors alone. RESULTS: Optimal MTF by a variety of criteria was found some 6 seconds after a blink. CONCLUSIONS: Inclusion of the effects of stray light and errors of focus does not affect the finding that optimal retinal image quality occurs some time after a blink. It does not appear that a loss in optical quality is the trigger of normal blinking.

Accommodation, Ocular↗
PubMed2005

Influences of reference plane and direction of measurement on eye aberration measurement.

We explored effects of measurement conditions on wave aberration estimates for uncorrected, axially myopic model eyes. Wave aberrations were initially referenced to either the anterior corneal pole or the natural entrance pupil of symmetrical eye models, with rays traced into the eye from infinity (into the eye) to simulate normal vision, into the eye from infinity and then back out of the eye from the retinal intercepts (into/out of the eye), or out of the eye from the retinal fovea (out of the eye). The into-the-eye and out-of-the-eye ray traces gave increases in spherical aberration as myopia increased, but the into/out-of-the-eye ray trace showed little variation in spherical aberration. Reference plane choice also affected spherical aberration. Corresponding residual aberrations were calculated after the models had been optically corrected, either by placing the object or image plane at the paraxial far point or by modifying corneas to simulate laser ablation corrections. Correcting aberrations by ablation was more complete if the original aberrations were referenced to the cornea rather than to the entrance pupil. For eyes corrected by spectacle lenses, failure to allow for effects of pupil magnification on apparent entrance pupil diameter produced larger changes in measured aberrations. The general findings regarding choice of reference plane and direction of measurement were found to be equally applicable to eyes that lacked rotational symmetry.

Computer Simulation↗
PubMed2004

Postblink changes in total and corneal ocular aberrations.

OBJECTIVE: The purpose of this study was to determine objectively the changes in optical aberrations induced by the progressive tear film irregularity after a blink and their effects on retinal image quality. DESIGN: Prospective, consecutive, nonrandomized comparative trial (self-controlled). PARTICIPANTS: Twenty healthy subjects. MAIN OUTCOME MEASURES: Corneal and total ocular aberrations were determined in 20 subjects at various time intervals (approximately 0, 10, and 20 seconds) after a blink. Corneal and total aberrations were measured with an Orbscan II topographer (Orbtek Inc, Salt Lake City, UT) and a Zywave aberrometer (Bausch & Lomb, Irvine, CA), respectively. All data were decomposed using Zernike polynomials to yield the root mean square wavefront deviations, in micrometers (microm), for different pupil diameters (3.5, 4.5, 5.5, and 6.5 mm). A merit function (Mf), defined as the volume under the 2-dimensional modulation transfer function as computed from the total wavefront error, was taken as the image quality metric. RESULTS: Both corneal and total aberrations (third order and higher) showed a statistically significant increase with time after the blink for all pupil diameters except 3.5 mm. The magnitude of the increase was greater at larger pupil diameters. For a 6.5-mm pupil, the aberrations increased on average by a factor of 2.5 (total) and 2.5 (corneal). Increases in total aberration were related closely to increases in corneal aberration (P<0.0001). For a 6.5-mm pupil, after 20 seconds, the reduction in optical quality (Mf) among the subjects studied was 21+/-8%. CONCLUSIONS: After a blink, the gradual increase in optical aberration associated with the increasingly irregular tear film may cause a progressive reduction in the optical quality of the eye. These changes in aberration with time may partly limit the improvements in visual performance that are achievable by customized corneal ablation.

Adult↗
PubMed2004

Visual performance with multifocal intraocular lenses: mesopic contrast sensitivity under distance and near conditions.

OBJECTIVE: To evaluate distance and near visual performance under bright (photopic) and dim (mesopic) conditions in patients who had undergone uncomplicated cataract extraction with multifocal or monofocal intraocular lens (IOL) implantation. DESIGN: Prospective, nonrandomized, masked, comparative, observational case series. PARTICIPANTS: Thirty-two eyes of 32 patients after zonal-progressive multifocal IOL implantation (Allergan Medical Optics Array SA-40N) and 32 eyes of 32 age-matched patients after monofocal IOL implantation (Allergan Medical Optics SI-40NB). INTERVENTION: All eyes underwent phacoemulsification and IOL implantation. MAIN OUTCOME MEASURES: At 18 months after surgery, the monocular contrast sensitivity (CS) function was measured with sinusoidal grating charts at distance and near, at one photopic luminance level and 2 mesopic luminance levels (85, 5, and 2.5 candelas per square meter). RESULTS: Under bright conditions, CS at distance in the multifocal group was not statistically different (P>0.01) from that in the monofocal group at any tested grating spatial frequency (1.5, 3, 6, 12, and 18 cycles per degree [cpd]). At low luminances, distance CS for the multifocal group was worse than that for the monofocal group at the highest test spatial frequencies (12 and 18 cpd; P<0.01). At near, photopic CS in the multifocal group was lower than at distance; patients with only a monofocal distance correction, however, could not detect the test gratings, even at the highest available contrast. With optimal near spectacle additions (i.e., using the distance correction of the multifocal IOL), there were no significant differences between the photopic near CS values for the multifocal and monofocal groups. When the luminance was decreased, near CS at all spatial frequencies was reduced in both groups. Contrast sensitivity in the near-corrected, multifocal group was significantly worse than in the near-corrected, monofocal group at high spatial frequencies (12 and 18 cpd). CONCLUSIONS: This work supports the findings of earlier authors that the Array multifocal IOL, with its center-distance design, is distance biased. Distance CS is within normal limits under bright photopic conditions but shows deficits at higher spatial frequencies (more than approximately 12 cpd) under dim mesopic conditions. Near CS obtained with the multifocal IOL is below that which can be achieved by an appropriate monofocal near correction, for all spatial frequencies and illumination conditions.

Aged↗
PubMed2004

Assessment of just-noticeable differences for refractive errors and spherical aberration using visual simulation.

PURPOSE: The aim of this study was to evaluate the threshold levels of aberration change that a typical reference eye is able to detect. METHODS: The method involved simulation of the foveal vision of a typical eye in polychromatic light through optics affected by different levels of the various chosen monochromatic aberrations. The reference eye had the following monochromatic wavefront characteristics based on the aberrations of a population of young adults: no spherical defocus, astigmatism -0.37D oriented at 0 degrees celsius, coma -0.17 D/mm oriented at 270 degrees celsius, and spherical aberration -0.12 D/mm. Average amounts of longitudinal and transverse chromatic aberration were assumed, and allowance was made for the Stiles-Crawford effect. The pupil diameter of the simulated eye was kept fixed at 6 mm. Three observers each compared, 100 times, a simulated image as seen through the standard reference eye with a variant "aberrated" image. The varying parameter was the value of a chosen additional aberration affecting the variant image in the reference eye. The test was repeated for varying amounts of spherical defocus, astigmatic defocus, and spherical aberration. For each of these aberrations and each observer, the discrimination probability as a function of the aberration level in the variant image was determined. The just-noticeable difference in aberration (JNDA) was derived from each discrimination curve as the difference between the aberrations corresponding to discrimination probabilities of 75% and 25%. The JNDA values obtained were expressed in the form of root mean square (RMS) wavefront error thresholds. RESULTS: It was found that 0.04 microm of RMS aberration should be considered as the threshold of just-noticeable image change, in good agreement with the Maréchal criterion. CONCLUSIONS: The results imply that in normal viewing conditions (e.g., a 3-mm pupil size), optical corrections should be in the range of +/-0.15 D in sphere and cylinder from the target prescription if perceptible change in the quality of the perceived images is to be avoided. The design of conventional soft contact lenses of high negative power or positive power should aim to produce -0.07 D/mm of spherical aberration, with a tolerated interval between -0.15 to +0.01 D/mm for a 6-mm pupil size.

Adult↗
PubMed2004

A method for simulation of foveal vision during wear of corrective lenses.

PURPOSE: The aim was to simulate the visual appearance of images viewed through corrective lenses having known, arbitrary types and amounts of monochromatic aberration, so that the visual effect of changing the design parameters of the lens could be explored. METHODS: We first calculate the optical response of the eye and any corrective lens using a numerical model eye. We then use this response as a filter, which we convolve with a selected original (unaberrated) image, to obtain an initial simulated retinal image. This image is then deconvolved by a second filter, which is calculated as the optical response of the eye of the observer who views the final image displayed on a video monitor. The originality of our approach to visual simulation is to take the aberrational characteristics of the observer's eye into account in the calculation. We validated our simulation by comparing images degraded by simulated dioptric blur with real defocused images seen through corresponding optical lenses. RESULTS: When using a small (2.5 mm) pupil size and a "typical" observer wavefront aberration model, there was a close resemblance between optical and simulated blurs. Although it was not necessary to consider the measured aberrations of the subject when simulating vision with a small pupil size, this requirement could not be ignored when vision through a larger pupil was simulated. With a 5.7-mm pupil diameter, use of Shack-Hartmann measurements of the ocular aberrations of the individual observers rather than "typical" levels of aberrations for the entire population gave excellent agreement between the effects of simulated and real defocus blur in monochromatic and polychromatic light. A Bland-Altman analysis of the differences between matching simulated and real blurs for a 5.7-mm pupil in polychromatic light with the model including allowance for individual measured aberrations gave mean differences close to zero and 95% confidence limits of about +/-0.25 D over a defocus range of -2.00 to +2.00 D. CONCLUSION: The simulation technique can be expected to be a useful tool to evaluate the potential performance of an eye that wears various designs of corrective lens.

Adult↗
PubMed2004

Temporal changes in optical quality of air-tear film interface at anterior cornea after blink.

PURPOSE: To examine temporal changes in the optical quality of the air-tear film interface at the anterior cornea after a blink. METHODS: Corneal aberrations were determined in fifteen healthy subjects at 1 second time intervals after a blink, up to a total elapsed time of 15 seconds. Corneal aberrations were obtained from corneal elevation maps measured using a Tomey TMS-2N topographer and custom software. All data were decomposed using Zernike polynomials to yield the root mean square (RMS) wavefront deviations, in micrometers, for two pupil diameters (3 and 7 mm). RESULTS: Total wavefront aberration decreased slightly with time in the first few seconds after a blink for both pupil diameters, reaching a minimum after approximately 6 seconds. Thereafter aberrations increased steadily, exceeding the immediate postblink level after approximately 10 seconds. CONCLUSIONS: In normal subjects, the contribution of the anterior cornea to the overall ocular aberration remains reasonably stable over the normal interblink interval (approximately 4 seconds) but rises to levels which could perceptibly degrade retinal image quality under circumstances where the interblink interval is increased to exceed 10 seconds, as may occur during the use of visual display screens or when performing difficult tasks.

Adult↗
PubMed2003

Hartmann-Shack technique and refraction across the horizontal visual field.

We compared refractions across the horizontal visual field, based on different analyses of wave aberration obtained with a Hartmann-Shack instrument. The wave aberrations had been determined for 6-mm-diameter pupils up to at least the sixth Zernike order in five normal subjects [J. Opt. Soc. Am. A 19, 2180 (2002)]. The polynomials were converted into refractions based on 6-mm pupils and second-order Zernike aberrations (6 mm/2nd order), 3-mm pupils and second-order aberrations (3 mm/2nd order), 1-mm pupils and second-order aberrations (1 mm/2nd order), and 6-mm pupils with both second- and fourth-order aberrations (6 mm/4th order). The 3-mm/2nd-order and 6-mm/2nd-order refractions differed by as much as 0.9 D in mean sphere on axis, but the differences reduced markedly toward the edges of the visual field. The cylindrical differences between these two analyses were small at the center of the visual field (<0.3 D) but increased into the periphery to be greater than 1.0 D for some subjects. Much smaller differences in mean sphere and cylinder were found when 3-mm/2nd-order refractions and either the 1-mm/2nd-order refractions or the 6-mm/4th-order refractions were compared. The results suggest that, for determining refractions based on wave aberration data with large pupils, similar results occur by either restricting the analysis to second-order Zernike aberrations with a smaller pupil such as 3 mm or using both second- and fourth-order Zernike aberrations. Since subjective refraction is largely independent of the pupil size under photopic conditions, objective refractions based on either of these analyses may be the most useful.

Adult↗
PubMed2002

Theoretical analysis of peripheral imaging after excimer laser corneal refractive surgery for myopia.

PURPOSE: To explore theoretically the retinal point images in the peripheral fields of eyes that have had excimer laser refractive surgery. University research laboratory. METHODS: Model eyes were based on Navarro's finite schematic eye, the eyes being made myopic by an increase in axial length. To simulate photorefractive keratectomy (PRK), the anterior shape and thickness of the cornea were modified. Variables included pupil size, ablation zone size, preexisting refractive error, and the addition of a blending zone. Image-quality criteria for each retinal point image were its size and the angular separation of the centroids of those parts of the image produced by rays passing through ablated and unablated corneal zones. RESULTS: In the peripheral visual field, the boundary between the ablated and unablated cornea caused a separation of the retinal image of a single point into 2 parts. The separation increased with the preexisting refractive error. Image quality was correspondingly reduced by ablation. As pupil size increased, the field angle at which the retinal image doubling first occurred decreased. Increasing the diameter of the ablation zone or using a blending zone increased the angle at which the doubling first occurred, and the blending zone improved image quality considerably. Chromatic effects appeared to be relatively unimportant. CONCLUSIONS: This analysis provides further evidence of the disadvantages of small central ablation zones in excimer laser refractive surgery and of the advantages of well-designed blending zones in improving postsurgical peripheral image quality. Image quality in the peripheral field of the pseudoemmetropic post-PRK eye is generally worse than in a naturally emmetropic eye, even though the axial image quality may be similar.

Humans↗
PubMed2002

Some possible longer-term ocular changes following excimer laser refractive surgery.

While the short- and medium-term refractive and acuity results of excimer laser refractive surgery may now be generally satisfactory, the relatively brief history of the procedures involved (around 10 years or less) means that those concerned with eye care must remain vigilant to the possibility of longer-term problems. This paper reviews some relevant studies of potential post-surgical effects, including imperfect corneal healing and recovery of innervation, reduced corneal sensitivity and dry eye problems, changes in corneal rigidity leading to slow refractive change, possibly misleading reductions in measured intraocular pressure, and retinal and vitreous pathology.

Cornea↗
PubMed2002

Diffraction haloes resulting from corneal oedema and epithelial cell size.

Corneal oedema was induced by exposing the corneas of nine subjects, covering an age range from 20 to 64 years, to distilled water for approximately 25 min. Measurement of the diameters of the associated diffraction haloes observed around a small 633 nm source allowed the size of the diffracting elements within each cornea to be estimated as about 10 microm, in good agreement with direct measurements by in vivo confocal microscopy of the subject's basal epithelial cells.

Adult↗
PubMedDate not supplied

Mesopic contrast sensitivity function after excimer laser photorefractive keratectomy.

PURPOSE: To evaluate contrast sensitivity under mesopic conditions in patients who had undergone uncomplicated excimer laser photorefractive keratectomy (PRK) for myopia. METHODS: Monocular contrast sensitivity function was measured with the Stereo Optical F.A.C.T. chart in 26 patients who had received PRK using the Nidek EC-5000 excimer laser system. Mean preoperative refractive error was -6.23 +/- 1.69 D (range, -4.00 to -8.25 D); postoperatively, mean refractive error was -0.36 +/- 0.58 D (range, -0.75 to +0.50 D). Contrast sensitivity function was measured 6 months after surgery using four different chart luminances: 85, 5.0, 2.5, and 0.1 cd/m2, the first being a photopic level and the rest mesopic. A control group of eight emmetropic subjects was also studied to allow comparison of results for statistical purposes. RESULTS: Logarithmic values of contrast sensitivity at each spatial frequency were used for statistical analysis and normalized values were used for graphical representation. The results showed a statistically significant reduction (P < .01) in contrast sensitivity for the PRK patients in comparison with the control group under mesopic conditions for each spatial frequency tested (1.5, 3, 6, 12, and 18 c/deg), although no significant contrast sensitivity differences were observed between PRK and control groups at the photopic (85 cd/m2) level (P > .01 for all frequencies). CONCLUSION: Photorefractive keratectomy can induce significant reductions in contrast sensitivity under mesopic conditions, even though the photopic contrast sensitivity function is normal.

Adult↗
PubMedDate not supplied

Image quality and visual performance in the peripheral visual field following photorefractive keratectomy.

PURPOSE: A theoretical and experimental study was performed to assess the possible effects of photorefractive keratectomy (PRK) on retinal image quality and thresholds in the peripheral visual field. METHODS: Simple optical calculations suggest that although the quality of the retinal image at the fovea of the postoperative PRK eye may be comparable to that in an emmetropic eye, images in the peripheral field may be markedly worse, since peripheral ray bundles may pass partly through ablated and partly through unablated cornea, giving a simultaneous-vision bifocal effect. This would be expected to create an annular zone of confusion, so that light from two different directions in object space arrives at the same point on the retina. The position of this zone and its width are a function of the ablation zone geometry, the attempted correction, and the pupil diameter, but the major effects typically occur at field angles between about 40 degrees and 60 degrees. To explore this effect, Goldmann static quantitative perimetry was carried out along the 0 degree to 180 degrees meridian in six patients who had undergone myopic PRK and eight emmetropes. RESULTS: The results showed that thresholds for the PRK group at field angles from 40 degrees to 60 degrees were significantly higher than those of the emmetropic group (P < .01). These findings arise because the retinal images in the peripheral field of originally myopic, postoperative PRK patients are, in general, significantly degraded in comparison with those of emmetropes. CONCLUSION: Although under photopic conditions, retinal image quality close to the visual axis in patients who have been corrected by PRK is similar to that in emmetropes, it may be markedly worse in the peripheral visual field.

Adult↗