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H. Ren, G. Yoon; Investigating the Effect of Negative Spherical Aberration on the Depth of Focus Using a Spatial Light Modulator. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2787.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the effect of negative spherical aberration on the depth of focus (DOF) in multifocal intraocular lens designs with a programmable liquid crystal spatial light modulator (SLM).
The SLM (Hamamatsu) was used to induce spherical aberration. An optical system equipped with the SLM and a computer projector was developed to measure the modulation transfer function objectively with and without negative spherical aberration. Images of letter E displayed on the computer projector were captured with a CCD camera through the optical system. Contrast of each image at different amounts of defocus (0 to +4D) was calculated. With the same system, subjective visual acuity in three subjects was also measured to evaluate DOF with and without negative spherical aberration. The eye’s pupil was dilated and accommodation was paralyzed with 1% Tropicamide. Defocus was subjectively corrected before the visual acuity measurements. All measurements were conducted for a 4 mm pupil under monochromatic light condition (wavelength = 632.8 nm). DOF was defined as a dioptric range of defocus where the measured visual acuity is better than 20/40.
Spherical aberration (-0.385 µm) that optimizes retinal image quality at 2 D defocus was chosen to increase DOF. With a letter E of 20/40 size, the contrast was decreased from 0.2 to 0.05 from 0 D to 1.25 D defocus when the spherical aberration was induced with the SLM, but, had higher than 0.05 contrast between 1.25 D and 2.6 D. However, with no spherical aberration, the contrast decreased from 0.81 to 0.02 from 0 D to 1.25 D, and was significantly lower than that with negative spherical aberration from 1.25D to 2.6D. A range of defocus for the higher contrast with spherical aberration was shifted toward distant vision as the amplitude of spherical aberration is decreased. Visual acuity measured at different amounts of defocus showed that for two subjects, the DOF was not significantly increased with negative spherical aberration used in the objective contrast measurements although one subject showed an increased DOF by approximately 1.0D. This subject has the smallest astigmatism while there was no significant difference in the higher order aberrations between three subjects.
Our preliminary study demonstrated that negative spherical aberration can increase the DOF when the eye’s aberration is relatively small. In addition a spatial light modulator can be a useful means to evaluate multifocal lens designs of ophthalmic lenses for presbyopia.
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