Visual acuity measured with clinical Maxwellian-view systems: effects of beam entry location

Maxwellian-view projection systems are used clinically to evaluate visual acuity in cataract patients because they allow the clinician to direct a beam of light around any localized opacity. However, theory predicts that, as the beam is displaced from the visual axis, acuity will suffer with those instruments using white light due to the effects of ocular chromatic aberration. Using three commercially available Maxwellian-view instruments, we examined the decline of visual acuity with beam decentration in the clear pupil of normal subjects. With all three instruments (Lotmar interferometer, Randwal interferometer, and the Potential Acuity Meter) acuity fell by a factor of 3 for beams placed near the edge of a dilated pupil. Control experiments with monochromatic light indicated that lateral chromatic aberration was the primary cause of acuity loss with the two interferometers, but other off-axis aberrations also contributed to the acuity loss with the Potential Acuity Meter. A detailed analysis of the beam in the pupil plane indicates that the beam's complex structure may be altered if the beam strikes the iris or an opacity in the ocular media. These findings indicate that potential acuity may be significantly underestimated with polychromatic Maxwellian-view systems.