Image Contrast Depending On Object Distance For Bi- And Trifocal Iol.

Trifocal IOLs are a common alternative to bifocal IOLs in the selection of intraocular multifocal implants. The third focal point is usually placed between the near and far focal point and is supposed to enable better vision at the computer distance of 67.5 cm (defocusing -1.5 D) in contrast to bifocal lenses. Defocusing curves confirm this. The aim of these mathematical calculations is to question to wich extent  the superposition of 3 focal points reduces the image contrast at different object distances compared to a bifocal lens.

Medical University of Vienna

Using a software (Octave), image contrasts for multifocal lenses are calculated using the Michelson contrast equation. Different assumptions for the near addition and light distribution of bifocal and trifocal lenses are used for object distance between 6 m to 25 cm. As a result, image contrast values of a bifocal lens with a near addition of 4.0 D relative to the lens plane and with a light distribution of 70%:30% compared to a trifocal lens with a near addition of 2.0 and 4.0 D and a light distribution of 63%:12%:25% will be shown.

The bifocal lens shows better image contrast, especially in near distance

The image contrast depends on the light intensity of the focal point used, but also on the distance and intensity of the focal point in front of or possibly behind it. The classic diffractive bifocal lens with a distant focus and a clearly separated near focal point enables the best image contrast with intraocular multifocal lenses.