What role does chromatic aberration play in IOLs?
In vivo testing
A second test was devised to evaluate LCA clinically. This is a challenge, because there is no ophthalmic diagnostic device that measures chromatic aberration. Therefore, a chromatic refraction technique was developed to characterize the impact of LCA on pseudophakic vision.
In this prospective clinical pilot study, 23 patients were implanted with a Tecnis ZCB00 lens in one eye and an Acrysof SN60WF lens in the fellow eye. The SN60WF lens does have a chromophore, but bench top testing indicated that the chromophore does not affect LCA.
At 30 to 60 days after the second-eye surgery, chromatic refractions were performed. To do this, a white light manifest refraction was performed using a standard high-contrast Snellen chart to correct for any residual refractive error.
Then, using a larger optotype (e.g., the 20/50 line), the chart was covered with a 650-nm red filter and the sphere was readjusted. The same was done with a 450-nm blue filter, and all three numbers were recorded.
The difference between the red and blue refractions is the chromatic refractive difference. Note that the limitations of available color filters significantly reduced the overall brightness of the eye chart, although contrast was slightly enhanced by the red filter.
In vivo measurements with the ZCB00 lens resulted in a difference in refraction between the red and the blue color filter of 1.12 D in photopic conditions and 1.11 in mesopic conditions. With the SN60WF lens the chromatic refractive difference was 1.28 in photopic conditions and 1.30 in mesopic. The difference between the two materials under mesopic conditions was statistically significant (Figure 2).
It is very interesting that in vivo measurements correlate with optical modeling and bench testing. The lens material with higher LCA in bench testing indeed resulted in a greater chromatic refractive difference in the clinical study. This validates both the in vitro model as well as the in vivo testing technique.