It is worth emphasizing that in an optical test bench, what can be assessed is the formation and characteristics of the secondary out-of-focus images produced by the MIOLs, which gives rise to a background that shows halo appearance and blurs the focused image. The presence of such background is a necessary but not sufficient condition for the perception of the halo by a patient because even though Stevens' power law predicts a logarithmic behavior of the sensory magnitude or perception,
36 it is not possible to infer from it that the figures displayed at logarithmic scale are a direct representation of what a patient would see.
Our results (
Fig. 4) show that the images obtained in each focus plane of the MIOLs have a background or halo that originates blur and causes reduction of the image contrast. Moreover, these results allow for a comparison between the halo produced by MIOLs at their distance focus and the blur produced by a monofocal IOL, which is mainly due to aberrations and is considerably weaker (
Fig. 2). In the case of the MIOLs, the halo is primarily due to the intensity and size of the out-of-focus image (or images) corresponding to the other foci of the lens, which turns out to depend on both the add power
17,26 and how the MIOL distributes the energy among the foci.
19,37 For a given pupil size, the fraction of the energy that is out of focus (and thus the halo) also depends in a complex way on a variety of factors such as the energy expended in higher diffraction orders,
37 scattering produced by the diffractive steps,
38 and the residual level of aberrations, mainly SA, once the MIOL has compensated (totally or partially) the positive SA of the cornea.
20,33
As for the optical quality of the MIOLs, their different designs are intended to obtain the best possible images at their focal points; hence, the comparison of the optical quality of the lenses has been carried out at their best foci. Otherwise, and especially in the case of the near focus (we recall that all the studied MIOLs had different add powers), the use of a fixed plane at a particular vergence could benefit the IOL whose add power approached the selected vergence the most.
In the case of the distance focus, this study shows that the SV25T0 reaches the highest MTF score and thus the best optical imaging quality, which is in agreement with previous work.
39 This is consistent with the design of this lens: a low add power and an apodized diffractive profile to enhance the performance of its distance focus. For instance, for a 3-mm pupil, the SV25T0 MIOL has the smallest number of diffractive steps, a small remnant level of SA (once the lens has partially compensated for the SA of the cornea), and the lowest diffraction efficiency for the near focus (only 20%, whereas the ZKB00, AT LISA, and AT LISA tri have diffraction efficiencies of 48%, 35%, and 30%, respectively).
10,40 Consequently, the SV25T0 is the MIOL where the factors (diffraction efficiency of the near focus, scattering produced by the diffractive steps, and remnant SA) that determine the amount of out-of-focus energy (or halo) at the distance focus and contribute to degrade its optical quality have the smallest contribution. Conversely, the SV25T0 has the lowest optical quality at the near focus. Altogether, these results are consistent with recent clinical findings showing that patients implanted with the SV25T0 had excellent distance vision (comparable to that of patients with monofocal IOL) but their near vision was worse than that of patients with other types of MIOLs.
40
The other MIOLs (ZKB00, AT LISA, and AT LISA tri) show a more balanced performance in their foci, with their near focus outperforming the near focus of the SV25T0 as deduced from the MTF values for all pupil size conditions (
Fig. 6). Since it has been suggested that the imaging quality of the MIOLs (determined by the MTF) could have an influence on the visual quality of patients,
35,41 and also because these MIOLs have higher add powers, one could infer that they could be a good choice for patients with demanding near vision activities. However, according to our results (
Fig. 2), these lenses are also prone to induce visual disturbances associated with halos.
26 This drawback must be carefully considered with patients whose expectations after surgery were a low level of visual disturbance.
40
The case of the AT LISA tri deserves further explanation. The MTF values of this MIOL as a function of the pupil size (
Fig. 6) and the through-focus MTF (
Fig. 7) are in quite good agreement with earlier reported works.
22,39 The AT LISA tri has MTF scores for its distance and near foci similar to those of the bifocal AT LISA, but the trifocal MIOL is the only one that provides a real intermediate focus. This result suggests that the trifocal lens may provide better intermediate vision without compromising near and distance vision. Interestingly, while conventional defocus curves obtained in patients with bifocal IOLs show two peaks corresponding to distance and near vision with a significant drop in visual acuity at intermediate distance,
42,43 the first outcomes with the AT LISA tri show a comparative smaller reduction of intermediate visual acuity with no significant differences in the range from −2.0 to −1.0 D (at the spectacle plane).
13,44 However, we have also shown in
Figure 2 that the AT LISA tri would more likely produce the largest halo in the distance focus for a 4.5-mm pupil. These two findings may help to explain the results of recent clinical studies with this MIOL showing that intermediate vision of patients efficiently improved, but at the same time, 10% of patients reported perception of halos.
13
As additional comments, we recall that our results were obtained from on-axis analyses, that is, with the MIOLs aligned with the optical system. Earlier work
45,46 has shown that tilt and/or decentration of IOLs have an impact on their optical performance. In addition, the human eye naturally includes pupil decentration (with respect to the cornea and crystalline lens) as well as lens tilt. Another potential issue of the study concerns the SA of the artificial cornea. Although there is a general consensus about the need to use artificial cornea models with positive SA to properly test IOLs of aspheric design,
33 there is not yet agreement about the specific value of the corneal SA that should be used. For instance, Pieh and coworkers
45 used three corneas with SA (6-mm pupil) of +0.054, +0.172, and +0.416 μm, respectively, for in vitro testing of monofocal IOLs. The model eye cornea of Carson et al.
47 had a SA of +0.2 μm and was used to test the MIOLs SV25T0 and ATLISA tri. The artificial cornea used in our eye model was designed with a SA of +0.27 μm for a 6-mm pupil. Taking into account the SA of the MIOLs (SV25T0: −0.20 μm; ZKB00: −0.27 μm; AT LISA bi and AT LISA tri: −0.18 μm), the maximum remnant SA would be only +0.07 μm in the case of the SV25T0 and +0.09 μm for the AT LISA bifocal and trifocal. Since the maximum pupil diameter used in this work was 4.5 mm, one expects even smaller remnant values of SA,
48 and hence differences in the optical performance of the MIOLs associated with differences in the SA compensation can be neglected. Finally, the measurements of the MIOLs were performed using a single wavelength of 525 nm close to the maximum peak sensitivity of the human eye in photopic conditions. However, the MIOLs will commonly work under polychromatic light such as daylight. All these issues should be taken into account with respect to a closer approximation to patients implanted with this type of MIOL.