Abstract
Abstract: :
Purpose: While the effect of contact lens translation on aberration has been studied (Guirao et al, 2001), the results were not directly applicable to rigid gas permeable (RGP) contact lenses decentration due to lens tilt and the influence of the tear lens. We modelled aberrations with RGP lens decentration taking into account the tear lens separately, and in combination with the lens-eye. Methods: Bi-monocurve RGPs (t=0.1mm; LD=9mm; n=1.49) were modelled on the Navarro model eye (Navarro et al, 1985). Four lens designs were tested: bi-spherical, front surface aspheric, back surface aspheric (BSA) and bi-aspheric. Steep, alignment and flat fittings (BOZR=7.4, 7.8, 8.2mm respectively) were tested. A Matlab routine was developed to model lens tilt, axial position, and the resultant tear lens with decentration from 0 to 2 mm at 0.25 mm steps. At each combination of design, fitting and decentration, the RMS wavefront error was computed (4 mm pupil) as well as Zernike coefficients up to the 20th term. Results: RMS error increased up to approximately 3 waves at 2 mm decentration (spherical front surface designs having the greatest RMS error). As decentration increased, the Zernike terms 1, 4, 5 and 7, representing tilt, defocus, astigmatism and trefoil/coma increased. Spherical aberration (12) remained virtually unchanged (all other terms had zero values). The tear lens’s contribution depended on the design and fit with some combinations compensating for the RGP aberrations, e.g. for flat fitting BSA, coma of the tear lens was -0.3µm at 2mm decentration while the total coma value was +0.3µm. Conclusion: The tear lens contributes significantly to the optical aberrations of the RGP wearing eye when the lens is decentred. Depending on fitting and asphericities, the tear lens may accentuate or reduce the net aberration.