R ep o rt XXIV Congress of the ESCRS , London , 2006 Measuring optical and visual quality clarifies the big picture

EXPERTS in the field of visual research discussed the relationship between optical quality and quality of vision during a symposium at the XXIV Congress of the ESCRS.They also examined the other nonoptical factors that can influence visual performance. Estimations of the optical quality of the eye can be achieved by metrics in both the spatial and frequency domain, but ultimately sensitive subjective testing is necessary to determine an individual’s visual function, said Raymond A Applegate OD, PhD, College of Optometry, University of Houston,Texas. Metrics of the spatial domain are based on the eye’s point spread function, which can be determined through wavefront sensing and double pass techniques. Comparison between the eye’s point spread function and that of a diffraction-limited system yields the Strehl ratio, he added. Measurements in the frequency domain are those that define the modulation transfer function (MTF) of an optical system. The MTF defines how much contrast, as a function of spatial frequency, is transferred by the optical system from the object to the image plane. It is also possible to plot the modulation transfer function in three dimensions for all possible orientations of the sinusoidal gratings.This forms a volume that can be compared to that of a diffraction-limited optical system. Dr Applegate noted that when LASIK was first introduced the volume under the MTF was reduced to less than that of a normal eye. It improved with the introduction of wavefront-guided LASIK. “Since wavefront came in it has become pretty common to get fairly normal MTF after surgery.The goal is to improve it still further.” Visual acuity has traditionally been measured with high contrast charts. Such testing correlates very poorly with the actual optical quality of the eye in terms of its optical transfer function. However, mesopic low contrast visual acuity does correlate very closely with the optical transfer function. In a study that compared metrics of optical quality with visual acuity results in a series of subjects with normal eyes, higher order aberrations could account for only four per cent of the variance in photopic high contrast acuity but could account for 37 per cent of the variance in low contrast mesopic acuity. “In a clinical environment the effect of minor variations in optical quality can be assessed with mesopic low contrast acuity, but not with photopic high contrast acuity,” Dr Applegate added. Dynamic aberration