Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

and attenuation of the various wavelengths by the ocular media.

In humans, radiation wavelengths of 300–2500 nm are transmitted through the cornea. Not all wavelengths, however, are transmitted through the cornea equally, as transmission is directly related to wavelength. In the rabbit, the cornea transmits 89–93% of the light at 370–500 nm, falling to 50% transmittance at 310 nm and a mere 2% at wavelengths below 290 nm.

      Additional attenuation of transmission occurs inside the eye. Even though light with wavelengths of up to 2500 nm passes the cornea, there is barely any transmission of wavelengths greater than 1950 nm through the AH, and in humans, the lens only transmits wavelengths between 390 and 1400 nm. A similar range of wavelengths is transmitted through the pig eye. The implication of these numbers is that the aqueous and lens act as color filters, preventing UV and IR light with very short and very long wavelengths (which has passed the cornea) from reaching the retina. The UV filtering by the lens is of particular importance, as UV light is a risk factor in a number of retinal diseases. Therefore, the current IOLs are coated with UV filters to restore this protection in pseudophakic patients. In this context, it is noteworthy that aphakic humans can detect UV radiation following lens extraction, because the lens serves as a filter, blocking out light of shorter wavelengths. In other words, human opsin is capable of absorbing UV light, but these wavelengths do not reach the retina of phakic subjects.

      Additional ocular structures, such as tear film and eyelids, also act as color filters, causing significant attenuation of short‐wavelength light. Thus, when cumulative transmittances are calculated for the successive components of the eye, a maximal transmittance rate in humans of 84% is obtained for light between 650 and 850 nm, while in rabbits the transmittance rate to light between 370 and 500 nm is 90%. Obviously, transmission will be further reduced by ocular opacities. Age is another factor affecting transmittance. Transmission of light at 480 nm through the human lens decreases by 72% from the age of 10 years to the age of 80 years, thus affecting color perception of the elderly.

      Ocular surfaces can also reflect back incoming light, depending on the angle of incidence. Light that strikes a surface at an oblique angle is reflected back; it is not transmitted into the new medium. Most of the reflection that takes place in the eye occurs as incoming light strikes the cornea because of the large difference in refraction indices between the cornea and air. Reflection that occurs at the cornea–air interface affects not only incoming light but also outgoing light.

      Light that is not transmitted and not reflected can be either scattered in the eye or absorbed by pigments. Foremost among these pigments are the photopigments of the photoreceptor outer segments, which absorb photons and thus initiate the visual process. Additional absorption processes in the eye may have clinical implications. Cyclophotocoagulation in glaucoma patients is based on the preferential absorbance of 810 and 1064 nm radiation of the diode and Nd:YAG lasers, respectively, by melanin‐containing tissues.

      Geometric Optics

      Refraction

      In vacuum, light travels at a constant speed (c) of approximately 3 × 10⁸ m/s. As it strikes denser media, light undergoes three changes: (i) its velocity is reduced; (ii) its wavelength shortens; and lastly (iii) it is bent (unless it struck the surface of the medium at a 90° angle).

      Vergence

An object that bends (or refracts) light is called a lens. When a single ray of light strikes a lens, the ray undergoes simple refraction, as depicted in Figure 2.9. Most objects or images, however, generate a pencil of light rays rather than a single ray. When a pencil of rays strikes a lens, they spread apart (i.e., diverge) or come together (i.e., converge). Convergence, or positive vergence, occurs when light strikes a convex lens (Figure 2.10a–c). Such a lens has a positive power, indicating that it forms a real image, which means that incoming rays from the object are converged and focused on the other side of the lens (see Figure 2.10a and b). On the other hand, divergence, or negative vergence, occurs when light strikes a concave lens (see Figure 2.10c). The negative power of the concave lens indicates that it forms a virtual or aerial image, which means that the diverging rays are traced, using imaginary extensions, backward to a “focused” virtual image “located” on the same side of the lens as the object (dashed, “imaginary” lines). The vergence power (i.e., amount of bending) of a lens is measured in units called diopters. One diopter (D) is the vergence power of a lens with a focal length (f) of 1 m when in air.

Figure 2.9 Refraction of light as it passes from one medium to another is governed by Snell's law, summarized in the formula below the diagram. The angle of refraction ( θ _r) is a function of the angle of incidence ( θ _i) and the refractive indices of the two media. In this representation, n _i < n _r; therefore, θ _i > θ _r.

      Visual Optics

      Refractive Structures of the Eye

       Precorneal Tear Film and Cornea

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