Halos and multifocal intraocular lenses: Origin and interpretation

Abstract

ObjectiveTo present the theoretical and experimental characterization of the halo in multifocal intraocular lenses (MIOL). MethodThe origin of the halo in a MIOL is the overlaying of 2 or more images. Using geometrical optics, it can be demonstrated that the diameter of each halo depends on the addition of the lens (ΔP), the base power (Pd), and the diameter of the IOL that contributes to the “non-focused” focus. In the image plane that corresponds to the distance focus, the halo diameter (δHd) is given by: δHd=dpn ΔP/Pd, where dpn is the diameter of the IOL that contributes to the near focus. Analogously, in the near image plane the halo diameter (δHn) is: δHn=dpd ΔP/Pd, where dpd is the diameter of the IOL that contributes to the distance focus. Patients perceive halos when they see bright objects over a relatively dark background. In vitro, the halo can be characterized by analyzing the intensity profile of the image of a pinhole that is focused by each of the foci of a MIOL. Results and conclusionsA comparison has been made between the halos induced by different MIOL of the same base power (20D) in an optical bench. As predicted by theory, the larger the addition of the MIOL, the larger the halo diameter. For large pupils and with MIOL with similar aspheric designs and addition (SN6AD3 vs. ZMA00), the apodized MIOL has a smaller halo diameter than a non-apodized one in distance vision, while in near vision the size is very similar, but the relative intensity is higher in the apodized MIOL. When comparing lenses with the same diffractive design, but with different spherical-aspheric base design (SN60D3 vs. SN6AD3), the halo in distance vision of the spherical MIOL is larger, while in near vision the spherical IOL induces a smaller halo, but with higher intensity due to the spherical aberration of the distance focus in the near image. In the case of a trifocal-diffractive IOL (AT LISA 839MP) the most noticeable characteristic is the double-halo formation due to the 2 non-focused powers.