Light scattering in the human eye is a complex phenomenon originating from the interaction of light with various types of local variations in the refractive index of the optical media of the eye. The purpose of this study was to develop a model of wavefront perturbation that describes straylight (light scattering) and aberration measurements in a large population of healthy eyes.
Although scattering occurs in all surfaces and media of the eye, the model assumes a single surface -located at the pupil plane- that introduces a random phase perturbation. The phase is generated by a modified midpoint displacement algorithm that generates a fractal surface with weighted amplitude across scales. The amount of straylight is controlled by a single parameter, the overall amplitude of the phase. The predictions of the model were evaluated with measurements of wavefront aberrations, straylight and biometric data in 284 healthy eyes of 284 subjects (105 male, 179 female, Age 45.5±15.9 years, range 18-84). We compared the amount of straylight and its correlation with high order aberrations and axial length.
The smallest features modelled in the phase fractal surface had dimensions of approximately 0.25 microns. This allowed calculation of straylight up to an angle of 15 degrees. The logarithm of the straylight parameter log(s) was correlated to the phase amplitude (h, in microns) with the following formula: log(s)=1.8log(h)+1.2. An exponential relationship describes the induced high order aberrations (RMS) with the induced straylight. A similar positive correlation between Straylight and high order aberrations was found in the experimental data. Figures show an example of phase map generated by the modified random midpoint displacement algorithm and the corresponding Point Spread Function compared with the CIE glare function.
Modelling straylight with a single phase surface at the pupil plane can partially explain the positive correlation between straylight parameter and axial length. This approach is computationally efficient, physically relevant and provides insight to the spatial characteristics of scattering structures in the optical media of the eye.