Intraocular light scatter, reflections, fluorescence and absorption: what we see in the slit lamp

The present work relates the turbidity of the eye to microscopic spatial fluctuations in its index of refraction. Such fluctuations are indicated in electron microscope photographs. By examining the superposition of phases of waves scattered from each point in the medium, we provide a mathematical demonstration of the Bragg reflection principle which we have recently used in the interpretation of experimental investigations: namely, that the scattering of light is produced only by those fluctuations whose fourier components have a wavelength equal to or larger than one half the wavelength of light in the medium. This consideration is applied first to the scattering of light from collagen fibers in the normal cornea. We demonstrate physically and quantitatively that a limited correlation in the position of near neighbor collagen fibers leads to corneal transparency. Next, the theory is extended to predict the turbidity of swollen, pathologic corneas, wherein the normal distribution of collagen fibers is disturbed by the presence of numerous lakes-regions where collagen is absent. A quantitative expression for the turbidity of the swollen cornea is given in terms of the size and density of such lakes. Finally, the theory is applied to the case of the cataractous lens. We assume that the cataracts are produced by aggregation of the normal lens proteins into an albuminoid fraction and provide a formula for the lens turbidity in terms of the molecular weight and index of refraction of the individual albuminoid macromolecules. We provide a crude estimate of the mean albuminoid molecular weight required for lens opacity.

The optical density of the human lens changes during life. Literature concerning both the spectral density function and the rate of such changes is reviewed. Analysis indicates that two components govern the spectral lens density function, with one increasing gradually during life. The average lens density increases linearly at 400 nm by 0.12 density unit per decade between the ages of 20 and 60 and by 0.40 density unit per decade above age 60. A tabulation of the two components of the average 32-yr old lens is given, as are equations to derive the average spectral lens density functions for observers aged 20-80.