Journal of the Korean Physical Society, Vol. 49, No. 1, July 2006, pp. 121 125 Customized Correction of Wavefront Aberrations in Abnormal Human Eyes by Using a Phase Plate and a Customized Contact Lens Tae Moon Jeong Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712 Geunyoung Yoon Center for Visual Science, University of Rochester, Rochester, New York 14627, U.S.A. (Received 6 February 2006) Wavefront aberrations in human eyes were measured with a Shack-Hartmann wavefront sensor. Based on the measured wavefront aberrations, customized optics, such as phase plates and a customized contact lens, were designed and fabricated to compensate for higher-order wavefront aberrations in human eyes. The fabricated customized optics could successfully correct higherorder aberrations in human eyes and improve retinal image quality, and compensating for such aberrations with customized optics might be a promising technique to improve human vision. PACS numbers: 42.15.Fr, 87.62.+n, 42.66.-p Keywords: Eye, Wavefront, Shack-Hartmann wavefront sensor, Phase plate, Customized contact lens I. INTRODUCTION The deep understanding of the effect of wavefront aberration on visual performance immediately led to objective measurement of wavefront aberrations in human eyes [1]. In most human eyes, second-order aberrations, such as defocus and astigmatism, were dominantly observed when the wavefront aberration was measured [2]. However, some human eyes, which had abnormal corneal conditions, such as keratoconus and penetrating keratoplasty (PK), revealed a large amount of higher-order aberrations (Coma, spherical aberration, trefoil, etc.), as well as the second-order aberrations. Although the second-order aberrations can be easily corrected with conventional glasses and contact lenses, the remaining higher-order aberrations still distort retinal image quality in an extreme way. The best way to improve retinal images for human eyes having higher-order aberrations is to compensate for the higher-order aberrations through customized correction. To date, methods using adaptive optics (AO) system [3], phase plates [4,5], customized contact lenses [6], and customized refractive surgery [7] have been available for correcting higher-order aberrations in human eyes. Although many scientific achievements have been obtained by using AO system [8 10], AO systems are not practically applicable for vision improvement because of their size. Corneal tissues can be removed by customized re- E-mail: jeongtm@apri.gist.ac.kr; Fax: +82-62-970-3388 fractive surgery to give a certain shape compensating for higher-order aberrations of eyes. However, refractive surgery is nonreversible and its availability is restricted by factors such as cornea thickness and the amount of higher-order aberration. A phase plate is a simple optical device, and its ability to correct higher-order aberrations has already been demonstrated with normal eyes [5]. However, the practical use of phase plates can be restricted by pupil decentration due to changes in the line of sight. For these reasons, the use of a customized contact lens is being considered for correcting higher-order aberrations in human eyes. The customized contact lens can be obtained by fabricating a particular surface profile on the front side of a contact lens to correct higher-order aberrations in an eye. Although the customized contact lens is considered as a practical method for correcting higher-order aberrations, its ability to correct higherorder aberrations has not yet been demonstrated. In this paper, we present a technique for correcting higher-order aberrations in human eyes, especially abnormal eyes, by use of a phase plate and a customized contact lens. In the following sections, the measurement of an ocular wavefront aberration for fabricating a phase plate and a customized contact lens is described. Then, the fabrication and the evaluation of a customized optics, i.e. a phase plate and a customized contact lens, are explained. Finally, the performance of the customized optics is discussed. The customized optics successfully corrected higher-order aberrations in human eyes to improve vision. The successful correction of higher-order aberrations with a customized optics shows promise as -121-
-122- Journal of the Korean Physical Society, Vol. 49, No. 1, July 2006 Fig. 1. Optical layout for measuring wavefront aberrations in human eyes. a customized correction technique for improving human vision. II. MEASUREMENT OF WAVEFRONT ABERRATION IN HUMAN EYES This research was approved by the University of Rochester Research Subject Review Board, and all subjects signed an informed consent before their participations in the study. The wavefront aberration in a human eye was measured with a Shack-Hartmann wavefront sensor. Figure 1 shows the optical layout used in the measurement of wavefront aberrations in a human eye. Two image-relay systems having a magnification of 1 were used for relaying the pupil image onto the surface of a microlens array in the Shack-Hartmann wavefront sensor. The use of two image-relay systems allowed a conjugate pupil plane in between the two image-relay systems for placing a phase plate to correct higher-order aberrations. The microlens array in the Shack-Hartmann wavefront sensor had a spacing of 400 µm and a focal length of 5.2 mm. This microlens array made it possible to measure the defocus in the range from 9 to +9 diopters (D) for a 7.09-mm pupil. A He-Ne laser was used as a light source to form a laser beam spot on the retina. The input laser power on the cornea was approximately 10 µw. The human pupil was dilated with a tropicamaide 1 % solution. A pupil camera was installed to monitor the pupil position of a human eye, and the pupil was aligned with the optic axis of the Shack- Hartmann wavefront sensor. The reflected light from the retina was collimated with a crystalline lens and other optics in a human eye. The pupil image formed by the human pupil was relayed into the microlens array in the wavefront sensor for measurement of the wavefront aberration. A spot array pattern was captured by the CCD camera in the Shack-Hartmann wavefront sensor, and a homemade software was used for calculating the Zernike coefficients from the captured spot array pattern. We measured the wavefront aberration for a 7.09-mm pupil and computed the 66 Zernike coefficients corresponding to up to the tenth-order Zernike polynomials. The measured Zernike coefficients were theoretically renormalized for a 6-mm pupil or smaller pupil size for further analyses. The double-index scheme established by the Vision Science and Its Applications Standard Taskforce team [11] was used to label the Zernike coefficients. Three normal (subject initial: GY, JP, and IC) and two abnormal (subject initial: FE and SC) eyes were used in our experiments. For the normal eyes, the dominant aberrations were the second-order aberrations, showing a root-mean-square (rms) value of 0.86 ± 0.08 (mean ± standard deviation) µm on average. The rms value of higher-order aberrations up to the fifth order was 0.39 ± 0.08 µm for a 6-mm pupil. The two abnormal eyes used in our experiments were keratoconus and had a conic corneal shape. In one abnormal eye (FE), the condition was mild, and in the other (SC), it was advanced. Coma was the most dominant aberration out of the higher-order aberrations for these abnormal eyes. Comas of 1.67 and 3.93 µm were measured for FE and SC, respectively. The rms values of higher order aberrations up to the fifth order were 1.75 and 4.52 µm for FE and SC, respectively. III. FABRICATION OF CUSTOMIZED OPTICS Four phase plates and a customized contact lens were fabricated as customized optics for correcting higherorder aberrations in human eyes. The fabrication of a phase plate is simple. The wavefront aberrations in human eyes were measured with the Shack-Hartmann wavefront sensor described above. The particular surface profile for correcting the higher-order aberrations was directly calculated from the measured wavefront aberration and was produced on the side of an optical PMMA block by using a lathe machine installed at the Bausch and Lomb contact lens company. On the other hand, the fabrication of a customized contact lens was somewhat complicated. A contact lens is usually decentered and rotated on a human cornea. The amount of decentration and rotation of a contact lens is related with the radii of the cornea and of the back surface of the contact lens. Thus, the position and the orientation that a customized contact lens might have on a human cornea should be predetermined with a given radius for the back surface of a contact lens. In order to minimize this positioning effect of the customized contact lens, we determined the wavefront aberrations while a subject wore a blank contact lens having a proper radius for the back surface. At the same time, the data on the position and the orientation of the blank contact lens were taken from the pupil camera. The blank contact lens had a spherical refraction of 1.5 D. The new decentered and rotated wavefront aberration data for the customized contact lens was calculated from the measured position and orientation of the blank contact lens on the human cornea. An expansion factor was also considered because a contact lens
Customized Correction of Wavefront Aberrations in Abnormal Tae Moon Jeong and Geunyoung Yoon -123- Fig. 2. Zernike coefficients for the design and the measured wavefront aberrations for a phase plate. Open bars and solid bars are the Zernike coefficients for the design and the measured wavefront aberrations, respectively. Fig. 3. Zernike coefficients for the design and the measured wavefront aberrations for a customized contact lens. Open bars and solid bars are the Zernike coefficients for the design and the measured wavefront aberrations, respectively. undergoes a wetting process before its final production. The surface profile obtained from the above processes was produced on a front side of a contact lens. The customized contact lens was also manufactured at the Bausch and Lomb contact lens company. When higher-order aberrations are corrected with a customized optics, the manufacturing error for the customized optics is an important parameter for determining the performance of the customized optics. In our study, the manufacturing errors of the customized optics were examined by measuring wavefront aberrations and comparing them with design values. The alignment of the customized optics was optimized by minimizing the differences between the measured and the design aberrations. The wavefront aberration of a phase plate was measured with the Shack-Hartmann wavefront sensor used for measuring the aberrations in the human eyes. However, unlike for phase plates and other solid optics, the measurement of the wavefront aberrations in a customized contact lens is not straightforward because of surface deformation and desiccation of the contact lens during the measurement. To prevent surface deformation and desiccation, we emerged the customized contact lens in a wet cell. A conversion factor was used to compensate for the reduction in the measured magnitude of the aberration in the wet cell. The detailed procedures for measuring higher-order aberrations in a customized contact lens are presented elsewhere [12]. Figure 2 shows the Zernike coefficients for the design and the measured wavefront aberrations of a phase plate for subject FE. The pupil size for these Zernike coefficients was 6 mm. In this case, the difference between the design and the measured higher-order aberrations was 0.22 µm rms. For the three normal eyes, all the phase plates were well fabricated to have a difference between the design and the measured wavefront aberrations of 0.16 µm on average. These differences were assumed to be due to manufacturing error. From the measure- ments, the manufacturing error ranged from 0.10 to 0.22 µm rms, and partially depended on the rms values of higher-order aberrations. A customized contact lens was fabricated for subject SC. Subject SC was chosen with the expectation of experiencing a large amount of vision improvement with a customized contact lens. Figure 3 shows the Zernike coefficients for the design and the measured wavefront aberrations of a customized contact lens for a 6-mm pupil size. In this case, the difference between the design and the measured wavefront aberration was 0.59 µm for a 6-mm pupil. As Figure 3 shows, a large manufacturing error was observed in fabricating the customized contact lens. The wetting process in fabricating the customized contact lens and the large amount of higher-order aberrations in the design values might have caused this large manufacturing error. Thus, the manufacturing processes should be optimized to minimize the error in manufacturing a customized contact lens. IV. CORRECTION OF WAVEFRONT ABERRATIONS IN HUMAN EYES 1. Wavefront Correction with a Phase Plate The residual wavefront aberration was measured for correcting higher-order aberrations in a human eye with a phase plate placed in the conjugate pupil plane. As for the three normal eyes with a 6-mm pupil, the rms value of the higher-order aberrations was reduced from 0.39 ± 0.09 to 0.15 ± 0.02 µm with phase plates. Vision improvement was also demonstrated when correcting higher-order aberrations with a phase plate. The visual acuities before and after correcting higher-order aberrations were measured to demonstrate vision improvement. The detailed method for measuring visual
-124- Journal of the Korean Physical Society, Vol. 49, No. 1, July 2006 Fig. 4. Zernike coefficients before and after correcting higher-order aberrations with a phase plate. Open bars and solid bars are the Zernike coefficients before and after correcting higher-order aberrations with a phase plate, respectively. Fig. 6. Typical movement of a contact lens on the human cornea. (a) Horizontal (solid diamonds) and vertical (solid squares) movement of the contact lens on the human cornea. (b) Rotational movement of the contact lens on the human cornea. Fig. 5. Zernike coefficients before and after correcting higher-order aberrations with a customized contact lens. Open bars and solid bars are the Zernike coefficients before and after correcting higher-order aberrations with a customized contact lens, respectively. acuity is described elsewhere [5]. When using phase plates, the visual acuity for normal eyes was reduced from 0.21 to 0.24 logmar (logarithm of Minimum Angular Resolution) with a high contrast (100 %) letter, which means approximately a 0.5 line improvement on the visual acuity chart (eye chart). The correction of higher-order aberrations with a phase plate was much clearer for the abnormal eye (subject FE). Figure 4 shows the wavefront aberrations before and after correcting higher-order aberrations with a phase plate for subject FE. As Figure 4 shows, coma, as well as other higher-order aberrations, was successfully reduced with the phase plate. The wavefront aberration in this abnormal eye decreased from 1.75 to 0.14 µm with a phase plate. The visual acuity also decreased from 0.12 to 0.20 logmar. This means a vision improvement of about 1 line on the eye chart. In summary, the phase plate could successfully compensate for higherorder aberrations in normal and abnormal eyes, resulting in vision improvement through correction of higher-order aberrations. 2. Wavefront Correction with a Customized Contact Lens The correction of wavefront aberration with a customized contact lens was accomplished by just wearing the customized contact lens. Because the position and the orientation of a customized contact lens determined the performance of the contact lens, the position and the orientation of the customized contact lens were measured after the customized contact lens had been stabilized on the cornea. The measurement of the position and the orientation showed that the customized contact lens was placed at almost the same position and orientation as the design values. The decentrations of the customized contact lens from the design value were 70 µm and 40 µm in the horizontal and the vertical directions, respectively. A rotation of 2 degrees from the design value was observed for the customized contact lens. Figure 5 shows the measured residual higher-order aberrations before and after correction with a customized contact lens. As Figure 5 shows, vertical and horizontal comas (Z 1 3 and Z 1 3) and spherical aberration (Z 0 4) were the dominant higher-order aberrations for this abnormal
Customized Correction of Wavefront Aberrations in Abnormal Tae Moon Jeong and Geunyoung Yoon -125- eye (SC). The customized contact lens successfully corrected a large amount of coma. However, spherical aberration and secondary astigmatism (Z 2 4 and Z 2 4) were not perfectly corrected with the customized contact lens. The rms value of higher-order aberrations was reduced from 3.89 to 1.28 µm for this eye. Although the visual acuity was not measured with the customized contact lens, subject SC reported he could see much clearer images with the customized contact lens than he could with the conventional correction method prescribed in an eye clinic. The slight decentrations and rotation of the customized contact lens could not explain the large amount of residual higher-order aberrations after correction. Another possible explanation for the large amount of residual higher-order aberrations is a change in the states in tear films between a blank contact lens and a customized contact lens. The investigation of the effect of a tear film between the cornea and a contact lens is the next step to be studied for a complete understanding of correcting higher-order aberrations by using a customized contact lens. Although the customized contact lens can successfully compensate for higher-order aberrations in a human eye, the movement of the customized contact lens on the cornea can induce residual wavefront aberrations after correction [13]. Figure 6 shows the typical movement of a contact lens on the cornea when a subject wears a contact lens. The movement of the contact lens on the human cornea was recorded with the pupil camera in Figure 1. As Figure 6 shows, the contact lens dominantly moved in a vertical direction when blinking. After blinking, the contact lens was stabilized to a certain position within about 1.5 seconds. The standard deviation for the vertical movement of the stabilized position from an average position was 0.18 mm. The measurement of the orientation showed a standard deviation for the rotation of 3.6 degrees. These values agreed well with the typical movement of a contact lens on a human cornea reported by Guirao et al. [14]. For the practical use of a customized contact lens, vision improvement should be investigated with this typical movement of the contact lens. The theoretical vision improvement with decentration and rotation of a customized optics is under investigation. V. CONCLUSION In this paper, a customized technique for correcting ocular wavefront aberrations has been described. As customized optics, phase plates and a customized contact lens were designed and fabricated for correcting higher-order aberrations in normal and abnormal human eyes. The phase plates successfully compensated for the higher-order aberrations in human eyes, resulting in vision improvement in normal and abnormal eyes. A customized contact lens also showed promising results in improving human vision by correcting higher-order aberrations. Although the customized optics successfully compensated for the higher-order aberrations, the effects of decentration and the roation of the customized optics on the vision improvement are not clearly investigated yet, especially for human eyes having abnormal corneal conditions. The effects of decentration and rotation of the customized optics on vision improvement should be investigated for improving human vision through the correction of higher-order aberrations by using customized optics. REFERENCES [1] J. Liang, B. Grimme, S. Goelz and J. Bille, J. Opt. Soc. Am. A 11, 1949 (1994). [2] J. Porter, A. Guirao, I. G. Cox and D. Williams, J. Opt. Soc. Am. A 18, 1793 (2001). [3] J. Liang and D. Williams, J. Opt. Soc. Am. A 14, 2873 (1997). [4] R. Navarro, E. Moreno-Barriuso, S. Bara and T. Mancebo, Opt. Lett. 25, 236 (2000). [5] G. Yoon, T. M. Jeong, I. G. Cox and D. R. Williams, J. Refract. Surg. 20, S553 (2004). [6] N. Lopez-Gil, A. Benito, J. Castejon-Mochon, J. Marin, G. Lo-a-Foe, G. Marin, B. Fermigier, D. Joyeux, N. Chateau and P. Artal, Invest. Ophthalmol. Vis. Sci. 43, U213 (2002). [7] S. MacRae, J. Schwiegerling and R. Snyder, J. Refract. Surg. 16, S230 (2000). [8] J. Liang, D. Williams and D. Miller, J. Opt. Soc. Am. A 14, 2884 (1997). [9] D. Williams, G. Yoon, J. Porter, A. Guirao, H. Hofer and I. Cox, J. Refract. Surg. 16, S554 (2000). [10] H. Hofer, L. Chen, G. Y. Yoon, B. Singer, Y. Yamauchi and D. R. Willimans, Opt. Express 8, 631 (2001). [11] L. Thibos, R. Applegate, J. Schwiegerling, R. Webb and VSIA Standard Taskforce Members, OSA Trends in Optics and Photonics Series, Vol. 35 (Optical Society of America, Washington D.C., 2000), p. 232. [12] T. M. Jeong, M. Menon and G. Yoon, Appl. Opt. 44, 4523 (2005). [13] S. Bara, T. Mancebo and E. Moreno-Barriuso, Appl. Opt. 39, 3413 (2000). [14] A. Guirao, D. Williams and I. Cox, J. Opt. Soc. Am. A 18, 1003 (2001).