The human eye is an optical device that is highly sensitive to changes in lighting. An important characteristic of a human optical instrument is the resolving power of the eye. Dots are perceived differently when hit by sensitive receptors.
What is the resolution of the eye
The human eye is a complex organ. The eyeball has the shape of a ball with a length of 24–25 mm and contains a light-refracting and light-perceiving apparatus.
The resolution of the human eye is the distance between two objects or lines seen separately. You can evaluate the resolution in minutes or millimeters, most often the number of lines that are visible separately in the interval of 1 mm is revealed. The reason for the change in the resolution of the eye is the anatomical size of the receptors and their connections.
Resolution of the human eye depends on factors:
- Nerves process the signal received by the retina.
- Optical - corneal irregularities, out of focus, iris diffraction, light scattering and disturbanceseyes.
The contrast of objects affects the resolution. The difference can be seen in daylight and at night. During the day, the effect of diffraction is increased by constriction of the pupil, and the deviation of the cornea from the correct shape does not affect the image. At night, the pupil dilates and becomes part of the peripheral zone of the cornea. The quality of vision is reduced when the cornea is damaged, which occurs due to the scattering of light on the photosensitive areas of the eye.
Determination of resolution
To identify the formula for the resolution of the eye, it should be understood that the resolution is the reciprocal of the smallest angle between directions by 2 points, at which different images are obtained.
The diffraction of light at the entrance pupil looks like a light circle in the center. The first diffraction minimum is at a certain angle from the center. To determine the resolving power of the eye, it is necessary to know the diameter of the pupil and the wavelength of the light. The pupil diameter is many times the wavelength.
More than 84% of the line of light passing through the pupil enters the Airy circle. The maximum indicator will be 1.74%, the remaining maximums show shares from the first. Thus, the diffraction pattern is considered to consist of a central bright spot with an angular radius. This spot projects an image onto the retina. This is how diffraction is formed.
Viewing angle
It has been established that the influence of the angle of view on the resolving power of the eye is great. In spacethere are 2 points that pass through the refractive medium of the eye and connect on the retina. The rays after refraction form an angle called the angle of view.
The angle of view will depend on the size of the object and its distance from the eye. The same object, but at a different distance, will be displayed at a different angle. The closer the object, the greater the angle of refraction. This explains that the closer the object, the more detailed a person can consider it. At the same time, it is known that the human eye distinguishes 2 points if they are displayed at an angle of at least 1 min. The light beam must fall in such a way on the 2 nearest nerve receptors so that at least one nerve element remains between them. Therefore, normal vision depends on the resolving power of the eye. After refraction, the angle of view remains 1 min.
Refraction
One of the characteristics of the organ of vision is the refraction of the eye, which determines the sharpness and clarity of the resulting image. The axis of the eye, the sides of the lens and the cornea affect refraction. These parameters will determine whether the rays converge on the retina or not. In medical practice, refraction is measured physically and clinically.
The physical method calculates from the lens to the cornea, not taking into account the features of the eye. In this case, it does not take into account what characterizes the resolution of the eye, and refraction is measured in diopters. The diopter corresponds to the distance through which the refracted rays converge at one point.
For the averagerefractions of the eye take an indicator of 60 diopters. But the calculation is not effective for determining visual acuity. Despite sufficient refractive power, a person may not see a clear image due to the structure of the eye.
If it is broken, then the rays may not hit the retina at the optimal focal length. In medicine, they use the calculation of the relationship between the refraction of the eye and the location of the retina.
Varieties of refraction
Depending on where the main focus is, in front or behind the retina, the following types of refraction are distinguished: emmetropia and ametropia.
Emmetropia is the normal refraction of the eye. The refracted rays converge in the retina. Without tension, a person sees objects removed at a distance of several meters. Only 40% of people do not have visual pathologies. Changes occur after 40 years. With normal refraction of the eye, a person can read without fatigue, which is due to the focus on the retina.
With disproportionate refraction - ametropia, the main focus does not coincide with the retina, but is located in front or behind. This is how farsightedness or nearsightedness is distinguished. In a near-sighted person, the farthest point is located nearby, the cause of incorrect refraction is hidden in an increase in the eyeball. Therefore, such people have difficulty seeing distant objects.
Far-sightedness occurs with weak refraction. Parallel rays converge behind the retina, and the image is seen by a person as blurry. The eyeball has a flattened shape and clearly displays distant objects. The disease most often develops after 40 years, the lens loses its elasticity and cannot change its curvature.
Color sensitivity of the eye
The human eye is sensitive to different parts of the spectrum. The relative luminous efficiency in the spectral circle is equal to the ratio of the sensitivity of the eye to light with a wavelength of 555 nm.
The eye only sees 40% of solar radiation. The human eye is highly adaptive. The brighter the light, the smaller the pupil becomes. A pupil with a diameter of 2–3 mm becomes optimal for high sensitivity.
During the day, the eye has a greater sensitivity to the yellow part of the spectrum, and at night - to blue-green. For this reason, night vision becomes worse, and the susceptibility of colors decreases.
Deficiency of the optical system of the eye
The eye, as an optical device, is not without flaws. The smallest linear distance between two points at which images merge is called the linear resolution period of the eye. Violation of the structure of the lens and cornea leads to the development of astigmatism.
Optical power in the vertical plane is not equal to the power in the horizontal. As a rule, one is slightly larger than the second. In this case, the eye can be nearsighted vertically, and farsighted horizontally. If the difference in these lines is 0.5 diopters or less, then it is not corrected with glasses and is referred to as physiological. With a greater deviation, treatment is prescribed.
Misalignment of the optical system of the eye
The resolution of the eye depends on the structure of the optical system of the organ of vision. The optical axis is taken as a straight line passing through the center. The visual axis is a straight line that runs between the nodal point of the eye and the foveola.
At the same time, the central fossa is not located on a straight line, but is located below, closer to the temporal part. The optical axis crosses the retina without touching the central fovea and the optic disc. A normal eye creates an angle between the optical and visual axes from 4 to 8o. The angle becomes larger with farsightedness, less or negative with myopia.
The center of the cornea rarely coincides with the optical center, respectively, the eye system is considered non-centered. Any deviation prevents the rays from converging on the retina and reduces the resolving power of the eye. The range of eye disorders is wide and may differ from person to person.
