Sight is one of the most valuable human senses. While the visual system is a relatively complex part of the brain, the process is driven by a humble optical element: the eye. It forms images on the retina, where light is absorbed by photoreceptors. With their help, electrical signals are transmitted to the visual cortex for further processing.
The main elements of the optical system of the eye: the cornea and the lens. They perceive light and project it onto the retina. It is worth noting that the device of the eye is much simpler than that of cameras with multiple lenses created in its likeness. Despite the fact that only two elements play the role of lenses in the eye, this does not impair the perception of information.
Light
The inherent nature of light also affects some characteristics of the optical system of the eye. For example, the retina is most sensitive in the central part for the perception of the visible spectrum, corresponding to the radiation spectrum of the Sun. Light can be seen as transverseelectromagnetic wave. Visible wavelengths from approximately blue (400 nm) to red (700 nm) make up only a small fraction of the electromagnetic spectrum.
It is interesting to note that the nature of the particle of light (photon) can also affect vision under certain conditions. The absorption of photons occurs in photoreceptors according to the rules of a random process. In particular, the intensity of light reaching each photoreceptor determines only the probability of absorbing a photon. This limits the ability to see at low brightness and adapt the eye to darkness.
Transparency
In artificial optical systems, transparent materials are used: glass or plastics with a refractive fixer. Similarly, the human eye must form large-scale, high-resolution images using living tissue. If the image projected onto the retina is too blurry, fuzzy, the visual system will not work properly. The reason for this may be eye and neuronal diseases.
Anatomy of the eye
The human eye can be described as a fluid-filled quasi-spherical structure. The optical system of the eye consists of three layers of tissues:
- external (sclera, cornea);
- internal (retina, ciliary body, iris);
- intermediate (choroid).
In adult humans, the eye is an approximate sphere 24 mm in diameter and consists of many cellular and non-cellular components derived from ectodermal and mesodermal germlinesources.
Externally, the eye is covered with a resistant and flexible tissue called the sclera, except for the front, where the transparent cornea allows light to enter the pupil. Two other layers under the sclera: choroid to provide nutrients and retina where light is absorbed by photoreceptors after image formation.
The eye is dynamic due to the action of six extrinsic muscles to capture and scan the visual environment. Light entering the eye is refracted by the cornea: a thin transparent layer, free from blood vessels, about 12 mm in diameter and about 0.55 mm thick in the central part. Water tear film on the cornea guarantees the best image quality.
The anterior chamber of the eye is filled with a liquid substance. The iris, two sets of muscles with a central hole whose size depends on contraction, acts like a diaphragm with a characteristic color depending on the amount and distribution of pigments.
The pupil is the hole in the center of the iris that regulates the amount of light entering the eye. Its size ranges from less than 2 mm in bright light to more than 8 mm in the dark. After the pupil perceives light, the crystalline lens combines with the cornea to form images on the retina. A crystalline lens can change its shape. It is surrounded by an elastic capsule and attached to the ciliary body by zonules. The action of the muscles in the ciliary body allows the lens to increase or decrease its power.
Retina and cornea
There is a central depression in the retina wherecontains the largest number of receptors. Its peripheral parts give less resolution, but are specialized in eye movement and object detection. The natural field of view is quite large compared to the artificial one and is 160×130°. The macula is located nearby and functions as a light filter, supposedly protecting the retina from degenerative diseases by screening out blue rays.
The cornea is a spherical section with an anterior curvature radius of 7.8 mm, a posterior curvature radius of 6.5 mm and an inhomogeneous refractive index of 1.37 due to the layered structure.
Eye size and focus
The average static eye has a total axial length of 24.2 mm and distant objects are focused exactly at the center of the retina. But deviations in the size of the eye can change the situation:
- myopia, when images are focused in front of the retina,
- farsightedness when it happens behind her.
The functions of the optical system of the eye are also violated in case of astigmatism - an incorrect curvature of the lens.
Image quality on the retina
Even when the optical system of the eye is perfectly focused, it does not produce a perfect image. Several factors influence this:
- diffraction of light in the pupil (blur);
- optical aberrations (the larger the pupil, the worse the visibility);
- scattering inside the eye.
Specific shapes of the lenses of the eye, variations in the refractive index and geometry features are disadvantages of the optical system of the eyecompared to artificial counterparts. The normal eye is at least six times lower quality and each creates an original bitmap depending on the aberrations present. So, for example, the perceived shape of stars will vary from person to person.
Peripheral vision
The central field of the retina gives the greatest spatial resolution, but the less vigilant peripheral part is also important. Thanks to peripheral vision, a person can navigate in the dark, distinguish between the motion factor, and not the moving object itself and its shape, and navigate in space. Peripheral vision is predominant in animals and birds. Moreover, some of them have a viewing angle of all 360 ° for a higher chance of survival. Visual illusions are calculated on the features of peripheral vision.
Result
The optical system of the human eye is simple and reliable and perfectly adapted to the perception of the surrounding world. Although the quality of the visible is lower than in advanced technical systems, it still meets the requirements of the body. The eyes have a number of compensatory mechanisms that leave some of the potential optical limitations negligible. For example, the large negative effect of chromatic defocusing is eliminated by appropriate color filters and bandpass spectral sensitivity.
In the last decade, the possibility of correcting eye aberrations using adaptiveoptics. This is currently technically possible in the laboratory with corrective devices such as intraocular lenses. Correction can restore the ability to see, but there is a nuance - the selectivity of photoreceptors. Even if sharp images are projected onto the retina, the smallest letter to be perceived will require multiple photoreceptors to interpret correctly. Images of letters smaller than the corresponding visual acuity will not be distinguished.
However, the main visual disturbances are weak aberrations: defocusing and astigmatism. These cases have been easily corrected by various technological developments since the thirteenth century, when cylindrical lenses were invented. Modern methods involve the use of contact and intraocular lenses or laser refractive surgery procedures to edit the structure of the patient's optical system.
The future of ophthalmology looks promising. Photonics and lighting technology will play a key role in it. The use of advanced optoelectronics would allow new prostheses to restore far-sighted eyes without removing living tissue, as is currently the case. New optical coherence tomography could provide full-scale real-time 3D visualization of the eye. Science does not stand still so that the optical system of the eye allows each of us to see the world in all its glory.
