Double pass–technique limitations for evaluation of optical performance after diffractive IOL implantation

Recently, Moreno et al.1 proposed using a double-pass method (Optical Quality Analysis System [OQAS], Visiometrics) to evaluate the postoperative ocular optical quality in eyes with an apodized diffractive multifocal intraocular lens (IOL). Large changes in optical power due to different wavelengths, as well as the division of light into more than a single order, are intrinsic features of diffractive elements. Diffractive multifocal IOLs are designed to allow the visible light near the peak of the visual response curve to be split between the first order (near addition) and the zero order (no power addition). Hence, within the visible light spectrum, the split will be progressively biased toward the first order for shorter wavelengths (blue light) and toward the zero order for longer wavelengths (red light). With the OQAS, a nonvisible, near-infrared wavelength (780 nm) is used. Most of the light energy will be directed into the zero order (nondeviated) after diffraction by the diffractive multifocal IOL. A much lower percentage of the infrared light will be diffracted in the first order than with any other visible wavelength. Therefore, the effect of the diffraction of the visible wavelengths in the first and higher orders will be significantly underestimated. In addition, the first-pass aperture of the instrument has a fixed diameter of 2.0 mm, whereas the aperture on exit can be varied from 2.0 to 7.0 mm. Most current diffractive multifocal IOL optics feature the first diffractive step zone within a 2.0 mm central diameter. This would cause any incident visible light to be diffracted on the first pass, whereas it is assumed that this pass should be minimally affected by the optics of the subject’s eyes to provide realistic retinal image quality evaluation. These limitations of the double-pass method call into question its ability to provide an accurate representation of the optical quality of eyes implanted with diffractive multifocal IOLs.