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Measurement of Monochromatic Ocular Aberrations of Human Eyes As a Function of Accommodation by the Howland Aberroscope Technique DAVID A

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Measurement of Monochromatic Ocular Aberrations of Human Eyes As a Function of Accommodation by the Howland Aberroscope Technique DAVID A View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Vision Res. Vol. 35, No. 3, pp. 313 323, 1995 ~ Pergamon Copyright © 1995 Elsevier Science Ltd 0042-6989(94)00139-1 Printed in Great Britain. All rights reserved 0042-6989/95 $7.00 + 0.00 Measurement of Monochromatic Ocular Aberrations of Human Eyes as a Function of Accommodation by the Howland Aberroscope Technique DAVID A. ATCHISON,* MICHAEL J. COLLINS,* CHRISTINE F. WILDSOET,* JAMES CHRISTENSEN,* MICHAEL D. WATERWORTHt Received 24 November 1995. in revised form 14 April 1994 Further development of the objective version of the Howland and Howland [(1976) Science, 193, 580-582; (1977) Journal of the Optical Society of America, 67, 1508-1518] aberroscope technique for measuring ocular aberrations is described. Compensation for refractive corrections and calibration is discussed. The technique was used to investigate the effect of accommodation upon the mono- chromatic aberrations of the right eyes of 15 subjects. Coma and coma-like aberrations were the dominant aberrations for most people at different accommodation levels, thus confirming previous findings. Variations in aberrations were considerable between subjects. About half the subjects showed the classical trend towards negative spherical aberration with accommodation. Changes in spherical aberration with accommodation in this study were less than that found in previous studies where all monochromatic aberration was considered to be spherical aberration. Aberration Aberroscope Accommodation Coma Spherical aberration INTRODUCTION dation, which reduced with accommodation and in one case became negative. Changes in longitudinal spherical It has been generally considered that for foveal vision in aberration, relative to that for a zero stimulus, for the monochromatic light, the quality of the eye optics is subjects at 5 mm pupil size were -1.6 D (for 4.6D limited only by diffraction effects for pupil diameters up stimulus), -0.5 D (4.6 D stimulus) and -0.7 D (2.7 D to 2mm (Campbell & Green, 1965) and thereafter is stimulus). predominantly affected by spherical aberration (e.g. Ivanoff (1956) used a vernier acuity method to Charman, Jennings & Whitefoot, 1978). Most eyes have measure longitudinal "spherical aberration" along the been considered to suffer from positive (undercorrected) horizontal meridian. On changing accommodation spherical aberration when unaccommodated, with a stimulus from 0 to 3 D, a trend from positive aberration trend to negative spherical aberration being observed to negative aberration on both sides of the pupil was with accommodation. For positive spherical aberration, obvious in four eyes, a similar trend but limited to one rays passing through the periphery of the pupil intercept side of the pupil only was observed for three eyes, and the optical axis in front of rays passing nearer the centre three eyes showed no dependence of the aberration upon of the pupil (Fig. 1). accommodation. Ivanoff did not measure beyond a pupil Koomen, Tousey and Scolnik (1949) used a method diameter of 4 mm. The average change in longitudinal which determined longitudinal spherical aberration spherical aberration for a 3.0 mm pupil size was from "averaged" over all meridians of the pupil by the use of 0.25 D for a 0 D accommodation stimulus to 0.0 D for annular apertures. The three subjects showed positive a 3 D accommodation stimulus. spherical aberration for a zero stimulus to accommo- Jenkins (1963) used a similar technique to Ivanoff, but measured along both horizontal and vertical meridians. *Centre for Eye Research, School of Optometry, Queensland The results differed qualitatively and quantitatively University of Technology, Kelvin Grove Campus, Locked Bag between subjects and also between meridians. From a No. 2, Red Hill, Queensland 4059, Australia [Fax 61 7 864 5665]. tCentre for Medical and Health Physics, School of Physics, 0D stimulus to a 2.5D stimulus, the eyes generally Queensland University of Technology, GPO Box 2434, Brisbane, decreased in positive aberration and usually changed to Queensland 4001, Australia. negative values. For all meridians and all subjects, the 313 314 DAVID A. ATCHISON et al. mean changes in longitudinal spherical aberration for a Campbell et al.'s (1990) refinement of Ivanoff's vernier change in accommodation stimulus from 0 to 2.5 D were acuity technique to measure aberrations as a function of -0.71 D (2.0 mm pupil diameter), - 1.04 D (4.0 mm), accommodation stimulus between 0 and 4 D. They fitted - 1.08 D (6.0 mm), and - 1.02 D (8.0 mm). aberration terms up to the sixth power in horizontal Berny (1969) used a knife-edge test method to assess pupil position. Most aberrations changed considerably spherical aberration, and also found a trend of decreas- as a function of accommodation. Two of four eyes ing positive spherical aberration with increasing accom- showed the classical trend of decreasing spherical aber- modation stimulus in three subjects. ration (the fourth-order term) with increasing accommo- Spherical aberration as a contributor to mono- dation. The higher order terms (fifth and sixth) were chromatic aberrations is generally smaller than values generally opposite in sign to their primary terms (third- obtained on the premise that all monochromatic aber- and fourth-order, respectively). ration is spherical aberration (Howland & Howland, Most methods previously used to measure the mono- 1977; Charman & Walsh, 1985; Campbell, Harrison & chromatic aberrations of the human eye are limited by Simonet, 1990). From the early studies of Ivanoff (1956) one or more of the following: and Jenkins (1963), it is obvious that few eyes show (i) reliance on subjective responses; symmetry of aberrations about the chosen reference (ii) the need for time consuming measurement, as position in the pupil. More recent research indicates that in measurement at many pupil positions in coma-like aberrations are the predominant aberrations vernier alignment techniques (Ivanoff, 1956; for foveal vision with few eyes showing typical spherical Jenkins, 1963; Campbell et al., 1990), which aberration (Howland & Howland, 1976, 1977; Walsh, have limited measurement to only one or two Charman & Howland, 1984; Walsh & Charman, 1985; meridians; Santamaria, Artal & B6scos, 1987). For rotationally (iii) averaging measurements over all meridians symmetrical systems coma only occurs for off-axis object (Koomen et al., 1949); and points. Campbell et al. (1990) were able to identify (iv) the need for time consuming analysis, as for subjects for whom coma occurred because of decen- the Foucault knife-edge method (Berny, 1969; tration of the pupil as well as other subjects for whom Berny & Slansky, 1970); this problem has coma occurred because of asymmetry of refracting declined in importance with improvements in surfaces. electronic photography and computing. There is evidence that significant changes in mono- chromatic aberrations other than spherical aberration An aberroscope technique, developed by Howland also occur with change in accommodation. For example, and Howland (1976, 1977) and subsequently modified by Howland and Buettner (1989) have recently reanalysed Walsh et al. (1984) and Walsh and Charman (1985), the data of Van Den Brink (1962), who had made overcomes these problems. The method employs a dis- subjective measurements of focus with a telescopic tant light source and an aberroscope placed close to the apparatus through various parts of the pupil for one eye. The aberroscope consists of a nearly square grid subject, and found that a large reduction in coma-like mounted between the plane surfaces of + 5 and -5 D aberration occurred from 0 to 1 D accommodation. piano-cylindrical lenses, with the cylindrical axes More recently, Lu, Munger and Campbell (1993) used mutually perpendicular and at 45 deg to the vertical (Fig. 2). The grid is distorted slightly so that it projects squares onto the entrance pupil of the eye. A shadow of Positive Spherical Aberration the grid is formed on a subject's retina in the interval of Ideal Wavefront Sturm, where aberrations are revealed by distortions away from a square pattern. The initial version of this technique was subjective. Howland and Howland analysed sketches of grid shadows drawn by their sub- jects to obtain wave-aberration polynomials and optical transfer functions. Walsh et al. (1984) and Walsh and Charman (1985) made the technique objective by insert- ing a beam splitter between the aberroscope and the eye Gaussian Image Plane so that the retinal grid shadow could be photographed. Negative Spherical Aberration This was done with a simple camera. We report further refinements to the latter objective version of the aberroscope technique. These are the use of a fundus camera for photography, a stimulus system for accommodation, and a method for correction for grid projection at the entrance pupil of the eye. We used the technique to measure the effect of accommodation upon monochromatic aberrations. A summary of FIGURE 1. Positive and negative spherical aberration of optical aspects of this work has been previously published systems. (Atchison, Collins & Wildsoet, 1992). ABERRATIONS AND ACCOMMODATION 315 Grid -- He-Ne Laser (633 nm ) ~ - 8 D lens -5D x 135" . ~-....... I 1.0 N.D. Filter I Correcting I Lens ~ I Aberroscope Fundus Camera i Beam ~ Left i Splitter ~ ~ Eye Autoref R-1 c -1-~4irror Target Slide ~ i!E Accommodation :$: Badal Lens Target ~ | ..... SUBJECT VIEW RIGHT EYE LEFT EYE BINOCULAR H " o F Q B Y G J FIGURE 2. Components of aberroscope and technique for measuring aberrations in right eye while inducing accommodation via the left eye. METHODS and focussing of the grid shadow is managed by an observer viewing through the camera eye-pieces. The Experimental set-up optical path distance between the grid and the cornea is The apparatus is shown in Fig. 2. The light source set at approx. 27 mm (corresponding to 30 mm between is a 0.9 mW He-Ne laser (2 = 633 nm). A -8 D lens the grid and the entrance pupil).
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