The method of characterizing the energy distribution at the focal planes of a DMIOL has been reported in detail elsewhere
22,23 and is illustrated in
Figure 1. The figure shows the VIS image of the pinhole object captured at the far focus of the ZKB00 DMIOL. Although the image in
Figure 1 is shown in logarithmic scale of intensity, for the sake of visualization,
15,30 the original intensity values in the linear scale provided by the digital 8-bit CCD camera were used in the calculations described below.
Briefly, the image in a given focal plane consisted of the core sharp image of the pinhole object surrounded by a blurred halo-shaped background. This background is principally formed from an out-of-focus image produced by the other focus of the MIOL but may have additional contributions from a variety of factors such as the energy expended in higher diffraction orders,
13 scattering produced by the diffractive steps of the lens,
31 and residual level of higher order aberrations, mainly related to insufficient correction of the corneal SA.
22,23 Because the gray level of a pixel of the image is proportional to the energy impinging on that pixel, it is possible to compute the energy of the light that reaches a particular region of the image by integrating the gray level of all pixels belonging to that region. In this way, the energy of the total image (
Etotal), which consists of the core plus the background regions, and the energy just in the focused pinhole region (
Ecore), were computed separately. The energy in the background (
Ebackg) could be calculated from the difference, (
Etotal) − (
Ecore). Finally, the normalized energy (
NEcore), which corresponds to the percentage of energy correctly focused in the core region, and (
NEbackg), the percentage of the energy spread in the background, were obtained as the ratios:
and
The procedure outlined above was also followed in the case of NIR images.
As for MTF curves, they were obtained as described previously,
15,32 using the corresponding VIS and NIR images of the four slit pattern and calculating the Fourier transform of the line spread function. In our setup, the spatial frequency of 50 cycles/mm approximately corresponded to an angular spatial frequency of 30 cycles per degree. The criterion for determining the best planes of focus was to choose those that maximized the area under the MTF curves. The MTFs for both VIS and NIR illumination for all the IOLs were obtained for pupil diameters of 3.0 and 4.5mm.