Measurement of the scattering characteristics of the eye in relation to pupil size

Abstract

When the amount of scattered light in the eye is large, either as a result of changes in the structure of the eye or the presence of intense sources of light, this results in significant impairment of vision, sometimes described as disability glare (Vos and Bouman, 1959; Vos, 1984). The effects of scattered light on image resolution and sharpness can be quite significant, with massive loss of sensitivity particularly in the high frequency range (Hess and Woo, 1978; Koch and Lie, 1990). A gradual increase in scattered light with age makes it possible for the visual system to adapt to a lower image contrast and hence to tolerate as acceptable a poor level of image quality (IJspeert et al, 1990). Scattered light does not cause an immediate decrease in Snellen Acuity (which is usually measured with high-contrast letters). This apparently normal performance explains why increased scatter in often undetected in routine eye examinations. The effects of scattered light on visual performance become more apparent in patients with opacities of the ocular media following refractive surgery, in cases of corneal oedema, keratoconus, cataracts and various forms of corneal dystrophy (Koch, 1989; Elliot et al, 1989). The ability to measure reliably small changes in the level and angular distribution of scattered light in the eye has been of interest clinically in monitoring the effects of drugs on media opacities or the recovery of corneal transparency following photorefractive keratectomy (Lohmann et al, 1991). In spite of what are potentially useful clinical applications for the measurement of the scatter function of the eye, the monitoring of small changes in light scatter parameters has not been very successful in a clinical context, the level of variability being very high (Elliot and Bullimore, 1993). One aim of the present study is to investigate the extent to which measured variability can be attributed to instrumentation and measurement error or to genuine changes in the pattern of scattered light in the same eye. In addition, the scattering of light may not always be uniform over the pupil, as a result of non-uniform distribution of ocular opacities and scattering centres. Measuring the effect of pupil size on light scatter is therefore important and may account for some of the observed variability. Unless the size of the pupil is known at the time of measurement, and its effects accounted for, small changes in light scatter parameters (i.e., the k and n values below) cannot be separated from the effects of pupil size, and hence they cannot be attributed easily to other factors under investigation. Another aim of this study was therefore to investigate the effect of pupil size on the scatter function of the eye.