There are two common uses of eye models- the first in which the retina of the eye is being viewed by an external optical system such as an ophthalmoscope or a fundus camera so the retina is the object surface, and the other in which the eye is looking out through an optical system such as a spectacle lens or a visual instrument and so the retina is the image surface.

Models that we have found useful in a wide variety of applications are included here as files Eye_Retinal Image.zmx and Eye_Retinal Object.zmx. These files are included in the zip file which you can download from the last page of this article. Although these models have the same optical system they have considerable differences in the data editors, as described below. The session files are also included.

The Eye_Retinal Image model, above.

The Eye_Retinal Object model, above.

Also included is a model of an eye accommodated to 250mm (four dioptres of accommodation referred to the cornea), which is sometimes useful.  The file is Eye_Accommodated.zmx. On accommodation the lens poles move forward into the anterior chamber and backwards into the posterior chamber so the axial length of the lens increases, the diameter of the lens decreases slightly, and the surfaces change shape. Most accommodation occurs by an increase in curvature and forward movement of the anterior surface of the lens.

The Eye_Accommodated model, above

The values of the various parameters in these models have been taken from a large number of references, and I have not listed the sources here. The parameter values have generally been rounded off for simplicity when this has been found to not have a significant effect. (For example, the axial length is 24.0mm, the retinal radius is 11.0mm and the anterior lens surface is spherical with a radius of 6.0mm.) The models do closely represent an average of measurements on real eyes, with the exception of the use of a homogeneous crystalline lens. The actual gradient index of a real lens is replaced in these models by a small change in the conic factor of the posterior surface. This surface has been measured in real eyes to be more or less hyperboloidal, and the model eye shows that this is a critical factor in off-axis aberration control. The model eye posterior lens surface has been flattened slightly less than actually occurs to compensate for the lower refractive index towards the equator and this is partly offset by choosing the refractive index of the homogeneous lens close to the real eye maximum core index, which slightly increases flattening of the surfaces.

This homogeneous lens has the advantage of greatly reducing the time for optimisation and for NSC ray tracing and is adequate for most purposes. However in some cases, such as where the optical system of the crystalline lens itself is being explored, it is essential to use a gradient index model. The Knowledge Base article How to Model the Human Eye in ZEMAX describes how to do this.