Optical system of the eye
Last reviewed: 23.04.2024
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The human eye is a complex optical system that consists of the cornea, the anterior chamber moisture, the lens and the vitreous. The refractive force of the eye depends on the radius of curvature of the anterior surface of the cornea, the anterior and posterior surfaces of the lens, the distances between them and the refractive indices of the cornea, the lens, watery moisture and vitreous humor. The optical force of the posterior surface of the cornea is not taken into account, since the refractive indices of the corneal tissue and the anterior chamber moisture are the same (it is known that refraction of the rays is possible only on the boundary of media with different refractive indices).
We can conventionally assume that the refracting surfaces of the eye are spherical and their optical axes coincide, that is, the eye is a centered system. In reality, however, there are many errors in the optical system of the eye. Thus, the cornea is spherical only in the central zone, the refractive index of the outer layers of the lens is less than that of the inner ones, the degree of refraction of the rays in two mutually perpendicular planes is not the same. In addition, the optical characteristics in different eyes vary significantly, and it is not easy to pinpoint them. All this makes it difficult to calculate the optical constants of the eye.
To assess the refractive power of any optical system, use the conventional unit - dioptre (abbreviated - dptr). The power of the lens with the main focal length of 1 m is accepted for 1dpi. Diopter (D) is the reciprocal of the focal length (F):
D = 1 / F
Consequently, the lens with a focal length of 0.5 m has a refractive force of 2.0 D, 2 m is 0.5 D, and so on. The refractive power of the convex (collecting) lenses is denoted by the plus sign, concave (scattering) by the sign " minus ", and the lenses themselves are called positive and negative, respectively.
There is a simple technique by which one can distinguish a positive lens from a negative lens. To do this, the lens should be placed a few centimeters from the eye and move it, for example, in the horizontal direction. When viewing an object through a positive lens, its image will blend in the direction opposite to the motion of the lens, and through the negative lens, on the contrary, in the same direction.
For calculations related to the optical system of the eye, simplified schemes of this system are proposed, based on the average values of the optical constants obtained when measuring a large number of eyes.
The most successful is the schematically reduced eye, proposed by VK Verbitsky in 1928. Its main characteristics: the main plane touches the apex of the cornea; the radius of curvature of the last 6.82 mm; the length of the anterior-posterior axis is 23.4 mm; the radius of curvature of the retina is 10.2 mm; the refractive index of the intraocular medium is 1.4; the total refractive power is 58.82 D.
Like other optical systems, the eye is characterized by various aberrations (from Latin aberratio - deviation) - defects in the optical system of the eye, leading to a decrease in the quality of the image of the object on the retina. Due to spherical aberration, the rays emanating from the point source of light are not collected at the point, but in some zone on the optical axis of the eye. As a result, a circle of light scattering is formed on the retina. The depth of this zone for the "normal" human eye ranges from 0.5 to 1.0 Dpt.
As a result of chromatic aberration, the rays of the short-wave part of the spectrum (blue-green) intersect in the eye at a smaller distance from the cornea than the rays of the long-wave part of the spectrum (red). The interval between the foci of these rays in the eye can reach 1.0 Dpt.
Virtually all eyes have one more aberration, due to the lack of an ideal sphericity of the refractive surfaces of the cornea and the lens. Asphericity of the cornea, for example, can be eliminated by using a hypothetical plate that, when applied to the cornea, turns the eye into an ideal spherical system. The absence of sphericity leads to an uneven distribution of light on the retina: the luminous point forms a complex image on the retina, on which the areas of maximum illumination can be allocated. In recent years, the influence of this aberration on the maximum visual acuity is actively studied, even in "normal" eyes with the aim of correcting it and achieving the so-called super-vision (for example, using a laser).
[Formation of the optical system of the eye
The consideration of the organ of vision of various animals in the ecological aspect testifies to the adaptive nature of refraction, that is, the formation of the eye as an optical system that provides the animal with optimal visual orientation in accordance with the characteristics of its vital activity and habitat. Apparently, it is not accidental, but historically and ecologically conditioned, the fact that a person is marked mainly by refraction, close to emmetropy, which best provides a clear vision of both far and near objects in accordance with the variety of his activities.
The regular approximation of refraction to emmetropy observed in the majority of adults finds expression in a high inverse correlation between the anatomical and optical components of the eye: in the process of its growth, there is a tendency to combine a more significant refractive power of the optical apparatus with a shorter anteroposterior axis and, on the contrary, with a longer axis. Therefore, the growth of the eye is a regulated process. The growth of the eye should be understood not just a simple increase in its size, but a directed formation of the eyeball as a complex optical system under the influence of environmental conditions and a hereditary factor with its species and individual characteristics.
Of the two components - anatomical and optical, the combination of which determines the refraction of the eye, anatomical (in particular, the size of the anteroposterior axis) is much more "mobile". Through it, mainly, and / regulating the influence of the body on the formation of refraction of the eye.
It has been established that in the newborn eye, as a rule, they have a weak refraction. As children develop, the refraction increases: the degree of hypermetropia decreases, the weak hypermetropia passes into emmetropia and even into myopia, and the emmetropic eyes become short-sighted in some cases.
In the first 3 goals of the child's life, the eye grows intensively, as does the refraction of the cornea and the length of the anteroposterior axis, which reaches 22 mm by 5-7 years, that is, about 95% of the adult's eye size. The growth of the eyeball lasts up to 14-15 years. By this age, the length of the eye axis approaches 23 mm, and the refractive power of the cornea is 43.0 Dpt.
As the eye grows, the variability of its clinical refraction decreases: it slowly intensifies, that is, it shifts towards emmetropia.
In the first years of a child's life, hyperopia is the predominant type of refraction. As the age increases, the prevalence of hyperopia decreases, and emmetropic refraction and nearsightedness increases. The incidence of nearsightedness is particularly noticeable, starting from 11 to 14 years, reaching about 30% at the age of 19-25. The share of farsightedness and emmetropia at this age is about 30 and 40%, respectively.
Although the quantitative indicators of the prevalence of certain types of eye refraction in children, given by different authors, vary significantly, the above general pattern of changes in eye refraction with age increases.
Currently, attempts are being made to establish the average age of eye refraction in children and use this indicator to solve practical problems. However, as analysis of statistical data shows, differences in the magnitude of refraction in children of the same age are so significant that such norms can only be conditional.