Compensation of corneal aberrations by the ... - Journal of Vision

Artal, Guirao, Berrio & Williams 2combining these two sets of data, the aberrations of the internaloptics were estimated by direct subtraction. Then the WA of theeye was measured after neutralizing the cornea using swimminggoggles filled with saline water. The aberrations measured with thefilled goggles correspond approximately to those of the internalocular optics. The crystalline lens is the most important contributorto the aberrations of the internal ocular optics that also includesthe posterior surface of the cornea and the ocular media.MethodsFigure 1 shows a schematic diagram of the experimentalprocedure. From the aberrations of the complete eye andthe cornea, those of the internal optics are estimated bydirect subtraction. The aberrations of the internal opticsare directly measured after neutralizing the aberrations ofthe first corneal surface.internal optics = eye - cornea+ =internal optics (directly)optics to the overall ocular aberration were evaluated. Thegeometric center of the pupil was used as the reference pointfor the registration between H-S and corneal measurements.In a simple model with the two series of Zernike coefficientsfor the cornea and the eye, the aberrations of the internaloptics were obtained by direct subtraction of each pair ofcoefficients. It was assumed that the wave aberrations werethe same axially along small distances.In addition, the WA for the internal optics was directlymeasured with the H-S sensor when refraction andaberrations of the corneal surface were cancelled byimmersing the eye in saline water using swimming goggles(Millodot & Sivak, 1979). Light reached the eye through ahole along the optical axis of the goggles that was covered bya high-quality optical window. When the corneal surfacepower is cancelled, the eye becomes highly hyperopic. Toobtain H-S images adequate for processing, this large defocuswas compensated with a Badal optometer formed by twophotographic objectives (Figure 2B). By using thisprocedure, several problems may affect the estimates of theaberrations of the internal optics. When the H-S sensor isused at the extreme vergence required for correcting the eye’shyperopia, the apparatus itself introduces large aberrationsthat may affect the results. This issue was overcome by usinga reference image of the system, recorded in the sameconditions, to compute the aberrations of the internal ocularoptics. This removes any systematic aberrations present inthe apparatus. In addition, the optical window placed infront of the goggles and the water act as a plane parallel platethat introduces spherical aberration for those vergences. Thespherical aberration was calculated with a ray-tracingprogram and incorporated into the data analysis.+ =(b)hyperopiacorrection(46 D)salinewaterFigure 1. Schematic diagram of the experiments performed.The WA of the eye was measured with a Hartmann-Shack (H-S) sensor (Liang, Grimm, Goelz, & Bille, 1994; Liang& Williams, 1997; Prieto, Vargas-Martín, Goelz, & Artal, 2000).A narrow infrared beam that acts as a beacon source isproduced by a super-luminescent diode and is projectedinto the subject's retina. In the second pass, a microlensarray, conjugated with the eye pupil, produces an image ofspots on a charged-coupled device camera (CCD) (Figure2A). The relative displacements of the spots areproportional to the WA local slopes. Then the WA isreconstructed as a Zernike polynomial expansion (Noll,1976). The aberrations introduced by the anterior surfaceof the cornea were calculated from the corneal shape,measured by a videokeratographic device (MasterVuecorneal topography system; Humphrey Instruments, SanLeandro, CA). Once the anterior corneal surface wasmodeled, the difference in optical path between the chiefray and a marginal ray over the pupil yields the WA for thecornea (Guirao & Artal, 2000). From these two WA maps,the relative contributions of the cornea and the internalfocuscorrectormicrolensarrayCCD(a)SLDobjective(50 mm)objective(50 mm)gogglesFigure 2. A. Schematic diagram of the Hartmann-Shack wavefrontsensor used to measure the aberrations of the completeeye. SLD indicates super-luminescent source; CCD, chargedcoupleddevice camera. B. Diagram of the part of the set-upmodified to measure the aberration of the internal optics. Theeye was equipped with swimming goggles filled with salinewater. Defocus was compensated by the relative position of thetwo 50-mm objectives.