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This paper investigates the design of an ophthalmic rigid Contact Lens (CL) that has a Q-type aspheric surface to reduce the tolerance error budget and produce higher visual acuity. CLs for myopia correction are typically made with spherical surfaces; when such CLs are worn in darkness, the visual acuity degrades severely due to the spherical aberration generated by pupil dilation. To maximize visual acuity, an aspheric CL is essential because it can correct the spherical aberration. In this study, a CL is designed based on a schematic eye model for evaluating the Modulation Transfer Function (MTF) on the retina, and the result shows that the visual acuity produced by an aspheric CL is superior to that produced by a spherical CL. The traditional power-series polynomial is commonly used for aspheric surface design, but the aspheric slope cannot be controlled during optimization. The large slope departure from a best-fit sphere leads to high tolerance sensitivity and an inflection point; however, the CL does not permit such results. To solve this problem, the Q-type polynomial with slope constraint proposed by Forbes is applied to the aspheric CL in this study. The Q-type aspheric polynomial can not only enhance the optimization efficiency due to its orthogonal characteristics, but also reduce error budget for maximal manufacturing yield. The simulation results demonstrated that the spherical aberration is successfully reduced by the Q-type aspheric CL for better visual acuity in darkness.
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