Multifocal Intraocular Lenses: Basic Principles

Multifocal intraocular lenses (MIOLs) can be made using refractive and diffractive surfaces or combinations of both. Depending on the optical design, MIOLs can be called bifocal or trifocal lenses. Bifocal IOLs provide foci for far and near distances, and trifocal lenses add a focus for intermediate distances. Furthermore, the proper use of some aberrations or the use of the pinhole effect can increase the extended depth of focus (EDOF IOLs) to provide better intermediate and near distance vision. Compared with the monofocal IOLs, MIOLs will present some drawbacks, such as halos, glare, etc., that can degrade the quality of vision (reduced contrast sensitivity or reduced retinal illumination). These disadvantages are mainly produced by the intensity and size of the images corresponding to the other foci produced by the lens. In addition, the centering and positioning of these lenses is a critical point and even more when aberrations are used in their designs. Light transmission and light distribution in MIOLs are related but not equivalent concepts. Light transmission is the amount of light that goes through the MIOL and depends on the transmission coefficient, and it is not expected to produce differences between diffractive and refractive MIOLs. However, the distribution of emerging light in each focus will depend on the refractive or diffractive nature of the MIOL. In diffractive MIOLs, the illuminated area will equally determine the width of the foci, whereas the energy of each focus will depend on the design. This behavior is different from refractive MIOLs where the width and energy were different between foci. Anyway, in diffractive MIOLs, there is always a loss of light that is destined to other non-desired foci.