The Eye As An Optical Device

By | December 23, 2014

A primary function of the eye is to form a crisply focused image on the retina. The optical components through which light travels to reach the retina are the cornea, aqueous humor, lens, and vitreous body. For proper image formation these components must remain transparent and maintain precise relationships to one another. One of the exciting areas of ongoing research is attempting to define the mechanisms whereby these relationships are maintained during growth of the eye.

A point source of light located at the visual horizon emits rays that are convergent, divergent, and parallel relative to the eye. At a great distance only rays that are essentially parallel will enter the pupil of the dog. Divergent and convergent rays will pass peripheral to the pupillary aperture. If the eye is focused at infinity (is emmetropic), parallel rays will be refracted and imaged as a point on the dog’s retina. If the dog’s eye is ametropic (either myopic or hyperopic), the rays will not be properly imaged on the retina and will form a blurred circle. As an object is brought closer and closer to the dog’s eye, the percentage of divergent rays entering the pupil will increase, requiring an increase in refractive power of the eye to properly image the object on the retinal plane. This is accomplished in the dog by increasing the optical power of the lens (accommodation).

The eye can be considered to consist of several optical surfaces, the combined action of which accurately focus images on the retina. The refractive power of an optical component is determined by its refractive index, thickness, and surface curvature. Refraction is greater in structures with more highly curved surfaces (i.e., having a smaller radius of curvature). The refractive power of an optical component is influenced by the difference in refractive indices between it and the surrounding media; the greater the difference, the greater the refractive power. It is primarily for this reason that the cornea is the most powerful refractive component of the eye. The difference in refractive indices encountered by a light ray en route to the retina is greatest at the air-cornea interface (the refractive index of air is 1 and the refractive index of the cornea is 1.3745). Although the lens has the highest refractive index of any of the optical components (approximately 1.53), it is surrounded by media with similar refractive indices (aqueous refractive index is 1.3364; vitreous refractive index is 1.3359).

The lens is the only structure capable of changing its refractive power. The accepted paradigm for mammals is as follows: In the resting state, the lens is maintained in a relatively flattened configuration by the intrinsic elastic tension exerted on its equator by the ciliary body acting through the zonular fibers. To focus on near objects (accommodate), the ciliary muscles contract, counteracting this elastic force, allowing the lens to “round up” (decrease its radius of curvature) thereby increasing its refractive power. The lens rounds up because of the intrinsic elasticity of the lens capsule. It has not been proven, however, that this is the mechanism of accommodation in the dog. The raccoon has been shown to accommodate through linear translocation of the lens (lens moves forward). The anatomy of the ciliary muscle in the dog suggests that a similar mechanism for changing refractive state is possible.

The mean refractive state across a large number of dog breeds (N = 90) was −0.05 (±1.36) D but varies widely (range, −6.00 to +6.00 D). Breeds with a mean myopic (≤ −0.5 D) refractive state include Rottweiler, Collie, Miniature Schnauzer, and Toy Poodle. Similar to humans, myopia in the Labrador Retriever is caused by an elongated vitreous chamber. Approximately 8% to 15% of Labrador Retrievers are reported to have myopia (nearsightedness), which may have a significant genetic component. Breeds with a mean hyperopic (≥ +0.5 D) refractive state include Australian Shepherd, Alaskan Malamute, and Bouvier des Flandres. For all breeds the length of the vitreous chamber and the degree of myopia increases with age.

The accommodative range of the canine eye is approximately 2 to 3 diopters. Because a diopter is a term that expresses the refractive power of an optical system in reciprocal meters, this means that most dogs can accurately focus objects to within one half to one third of a meter of the eye. Myopic individuals cannot accurately focus objects in the distance.

A schematic eye for the dog has been published. A schematic eye is an internally consistent mathematical model that approximates the normal static optical behavior of the eye for a given species. The optics of an individual animal’s eye affect on the appearance of the fundus as viewed by direct and indirect ophthalmoscopy. Owing to the intrinsic optical properties of the eye, which are related to the axial length of the globe, the ophthalmoscopic features of a small eye (such as the rat) will be perceived as much more magnified than that of the dog. By direct ophthalmoscopy (using a standard direct ophthalmoscope) the canine fundus will appear approximately 17 times more magnified in lateral extent relative to its actual dimensions.

The value for axial magnification, which is related to the square of lateral magnification, is approximately 400 times greater for the dog’s eye. Axial magnification refers to the apparent displacement of an object along the optical pathway, either in front of or behind a reference plane. Similarly, the millimeter equivalent of a one diopter change in ophthalmoscopic focus is directly proportional to the axial length of the globe and is approximately 0.275 mm in the dog. In other words, a one diopter shift in ophthalmoscopic focus will shift the focal plane of the ophthalmoscope 0.275 mm anterior or posterior to the original plane of focus.