Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 108

Human vision is limited to a wavelength range of 380–780 nm. This limitation is a result of two factors: the first is the absorption spectrum of the opsin component of the visual photopigment, and the second limiting factor is the transmission, reflection, 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.