Methods for the Measurement and Analysis of Light Scattered in the Human Eye

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Scattering of light is present in the human eye and ends up illuminating the retina away from the image of a source, although some of the light can be back-scattered through the pupil and this is particularly evident in patients with corneal scars or keratoconus. Scattered light is present in small amounts even in what one normally refers to as pure transparent materials. When the scattering centres are of equal or larger dimension than the wavelength of the incident beam, the angular distribution of the scattered beam tends to follow closely the direction of the incident beam. Scattered light in the eye has the effect of reducing visual acuity and contrast sensitivity and can also affect significantly the accuracy of psychophysical measurements of visual performance such as colour discrimination. When the level of scattered light is large, either as a result of changes in the structure of the dioptrics of the eye or the presence of intense sources of light, this results in significant impairment of vision, sometimes described as visibility glare (Vos, 1984; Vos and Bouman, 1959). Scatter in the human eye and in particular its angular dependence has been the subject of numerous studies (Holladay, 1927; Stiles and Crawford, 1937). In the case of small sources, the angular dependence of scattered light in the eye can be described adequately by a point spread function which decreases with the reciprocal of the square of the visual angle between the source and the point of interest on the retina (Stiles and Crawford, 1937). This relationship shows clearly that little or no Rayleigh scattering takes place in the eye since the very small particles involved in Rayleigh scattering would, in the extreme case, correspond to the ideal forward diffuser, with no angular dependence (Rayleigh, 1912). Measurement of overall scatter level and its angular dependence in the eye can provide valuable information on the number and the size of the particles involved. Monitoring changes in light scatter parameters may provide useful information on the underlying morphological changes in the dioptrics of the eye. Since the scattering of light is not uniform over the pupil and the formation of ocular opacities tends to start in the periphery of the lens, measuring the effect of pupil size on light scatter may provide an additional parameter worth investigating. Extended annular sources of light scatter are often required in order to increase the light flux level entering the eye. This is usually the case when the scatter source is of relatively low luminance and generated on visual display units. The use of extended sources makes it more difficult to extract accurately the angular dependence of light scatter in the eye since the scatter source can no longer be taken to have a single eccentricity. Errors in estimates of light scatter parameters can also be introduced when the light flux level entering the eye does not remain constant for different scatter source eccentricities. In this paper we describe how the various problems mentioned above can be overcome and the light scattered in the human eye measured using a stable display and appropriate computational methods.