Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 111

As mentioned, light is successively refracted by the various ocular structures as it passes through the eye on its way to the retina. Table 2.13 lists the refractive indices and powers of various ocular surfaces in humans. The most anterior optical surface of the eye is the PTF. By strict definition, it could be argued that the tear film is the most refractive layer of the eye. This is due to the large difference in refractive indices as light passes from air, which has a refractive index of almost 1, into the tear film, which has a refractive index of 1.337.

Figure 2.10 Refraction of light through various lenses. (a) A spherical convex lens with a power of 10 D focuses parallel light rays at a distance of 0.1 m. (b) A flatter, less spherical convex lens with a power of 5 D focuses parallel rays at a distance of 0.2 m. (c) Parallel rays passing through a concave spherical lens diverge. A virtual image is formed by tracing back (dashed lines) the diverging rays.

The cornea is the next tissue through which incoming light passes. The human corneal stroma has a refractive index of 1.376. Because this value is slightly higher than the refractive index of the tear film, passage of light from the tear film into the anterior layers of the cornea results in an additional 5 D of refractive power. However, these 5 D are “lost” when light passes from the posterior cornea into the AH, which has a refractive index nearly identical to that of the tears. When combined, the PTF and the cornea of humans contribute a net refractive power of 43 D.

Another factor affecting the refractive power of the cornea, besides the refractive index, is its curvature. Because the cornea converges light, it acts as a convex lens. As stated earlier, the refractive power of such a lens depends to a large extent on its curvature radius. Therefore, in large eyes, which are characterized by flat corneas, the refractive power of the cornea is reduced. Conversely, in small eyes with spherical corneas, its power is increased.

Table 2.13 Refraction constants in the human eye.

Structure	Refractive index	Refractive power (D)	Reference
Tears	1.336	43.0a	Montes‐Mico et al. (2004)
Cornea	1.376	42.3a	Duke‐Elder (1970); Naeser et al. (2016)
Anterior surface	1.401	48.2	Patel et al. (1995)
Posterior surface	1.373	−5.9	Patel et al. (1995)
Lens	1.41	21.9	Duke‐Elder (1970); Chang et al. (2017)
Anterior surface		8.4	Millodot (1982)
Posterior surface		14.0	Millodot (1982)
Vitreous/aqueous	1.336		Duke‐Elder (1970)
Retina	1.363		Millodot (1982)

^a The refractive power of the cornea and tears is not additive. Rather, that of the former arises from the latter, and from its interface with air. The net power of the tears and the anterior and posterior cornea is 43 D.