A theoretical model to predict contrast sensitivity with bifocal intraocular lenses.

Bifocal intraocular lenses (BIOLs) cause a reduction in image contrast. However, the clinically measurable reduction is much lower than that predicted by experimental results. We therefore compared monofocal IOLs, the True Vista BIOL, and the diffractive BIOL. We measured the modulation transfer function (MTF) of all IOLs in a model eye. Clinically, we determined the contrast sensitivity (CS). For a comparison of the results, the MTF was converted into the theoretical CS of the eye using the age-corrected values of retinal CS. The calculated CS of the BIOLs was similar to the values measured clinically. However, the calculated CS of monofocal IOLs was considerably higher than the clinical values. This difference should be due to the MTF of the normal eye; the MTF of monofocal IOLs was considerably higher than that of the normal eye, whereas the MTF of the BIOLs was lower. Thus, the MTF of the normal eye becomes the limiting factor of this system. The proposed model takes into account the retinal CS and the MTF of the normal eye. Using the MTF of an IOL, the clinical CS can be calculated, and the clinical performance of new IOL designs may be predicted before clinical trials commence.