Adaptive Eye Model with a Gradient Index Lens
We develop a new wide-field schematic eye model, which provides a more realistic description of the optical system of the eye in relation to its anatomical structure. The wide-field model incorporates a gradient-index (GRIN) lens with a gradual increase of refractive index towards the centre of the lens. This feature allows us to fulfil properties of two well-known schematic eye models, namely Navarro’s model for off-axis aberrations and Thibos’s chromatic model on-axis model (Indiana eye). These two schematic models are based on extensive experimental data, which makes the derived wide-field eye model also consistent with that data. We propose a new method to construct a GRIN lens with its iso-indicial contours following the optical surfaces of given radius of curvature and asphericity. The refractive index distribution in the GRIN lens is defined by two or one single equation with six index coefficients. We use a parametric representation for these coefficients, that is, each coefficient is a function of few adjustable physical parameters (such as radii, asphericity of the lens surfaces and refractive index range) by means of which we could match spherical aberration and longitudinal chromatic aberration to experimental data.
The efficiency of this method has been demonstrated with three variants related to different age groups (20, 30 and 40 year old). For each age group we perform optimisation of the wide-field models using reverse ray-tracing duplicating the aberrations of Indiana eye. The resulting wide-field eye model is consistent with available data on ocular and intra-ocular spherical aberration and also comparable with off-axis aberrations of the Navarro’s model. Our wide-field model with a GRIN lens can be used as a starting system for the eye inverse problem, i.e. the reconstructing the optical structure of the eye from off-axis wavefront measurements. The parametric representation of the GRIN lens enables us to construct subject-specific eye model using only few physical variables that can be easily adjusted to take into account inter-subject variability and ageing effects.
The future implementation of the wide-field eye model includes the study of ocular aberrations and their origin in the eye, as well as changes in these aberrations with accommodation and age. In particular, variability of field aberration across the field is of great interest for optical design of imaging instruments (funds cameras, ophthalmoscopes and systems with adaptive optics). Using subject-specific wide-field eye model we might be able to find the optical conjugates of two deformable mirrors for optimal field correction with adaptive optics. The dual-conjugated adaptive optical systems have a potential to enlarge usable field of view for retinal imaging. Therefore, anatomically more accurate age-depended optical models of the eye could ultimately help an optical designer to improve wide-field retinal imaging or a surgeon to make a better choice of the required procedure for vision enhancement.