Diopter ocular

Diopter ocular . It is the adaptation of the eye to distance (increased dioptric capacity of the lens and ocular convergence) and light ( miosis when there is a lot of light and mydriasis in the penumbra).

Summary

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• 1 The eye as a physical instrument
• 2 Transparent eye media
• 3 The lens (Biconvex lens)
• 4 Construction of the image
• 5 Six cardinal points of Gauss
  • 1 Main focuses
  • 2 Main points
  • 3 Walnut Points
• 6 light rays
• 7 Visual accommodation
  • 1 Extracapsular mechanism
  • 2 Moment of accommodation
  • 3 Mechanism of accommodation
• 8 Functional capacity of the accommodating mechanism
• 9 Tired sight
• 10 Amentropy
• 11 Refractive Effects
  • 1 Astigmatism
  • 2 Irregular Astigmatism
  • 3 Aberration of spherity
  • 4 Chromatic aberration
  • 5 Entoptic phenomena
  • 6 Flying flies
• 12 Sources

The eye as a physical instrument

The eye as a physical instrument. Lenses acts as the tears , cornea , aqueous humor , crystalline mood and vitreous humor ; the dark coating is represented by the choroid; the sensitive plaque, by the retina ; the Diaphragm , by the iris, and the shutter, by the eyelids.

When the light rays reach the Retina they have changed the direction they offered when they first touched the eyeball . It is known that a ray of light, when crossing a homogeneous medium, does so in a straight line; It propagates in the same direction by striking perpendicularly to the surface of any refracting substance or when the refractive indices of the ray’s source are equal and of the new substance that the ray must pass through. But unless these circumstances occur, the light rays, as they vary in their path, suffer a deviation; If the environment of origin is less refractive than the one they are now traveling, they approach the perpendicular drawn at the point of incidence, moving away from it otherwise.

Transparent eye media

The transparent media of the eye , cornea , aqueous humor , crystalline and vitreous humor . They represent a composite refractive lens, the focus of which is on the retina and therefore at a point corresponding to the posterior aspect of the vitreous. As a whole, these refractive means must present, like all lenses, a place located on the anteroposterior axis of the eye, where the two axes of the luminous cones that enter it intersect; it is the so-called optical center of the eye, which occupies a point located inside the lens.

To get an idea of ​​the path of light rays in the eyeball, it will be necessary to bear in mind, as in any optical system, the distances between the surfaces of refraction and reflection (anterior face of the Cornea . Anterior and posterior face of the lens, retina), refractive indices (aqueous humor, crystalline, vitreous body) and the rays of the three curvatures. The knowledge of these data, which constitute the optical constants of the eye, is mainly due to Helmholtz , Gullstrand and Listing .

The cornea, the aqueous humor and the vitreous body have the same index of refraction, allowing the lens, being surrounded by means of such refringence, to fully retain its own convergent action.

The medium consisting of tears , cornea and aqueous humor exerts a convergent erect on the light rays, which depends on both the common index of refraction and the radius of curvature of the cornea . Not all rays passing through the transparent cornea and the anterior chamber reach the retina. Most of them stumble in their course with the iris and are returned to the outside. Only those who fall into the central opening of this membrane are those who continue their journey through the interior of the eye to contribute to vision.

The lens (Biconvex lens)

The Cristalino is a convex lens , which can be imagined as two prisms joined at the base. Because it is more refractive than aqueous humor, it makes the rays coming from it converge even more when they reach it. And since the radius of curvature of its posterior face is less than that of the anterior face, it turns out that the refraction of the rays is more effective, with respect to their convergence, at the exit of the lens than at their entrance.

The refractive index of the vitreous body is lower than that of the lens, from which it appears that the convergence of the light rays that have passed through that lens increases even more in the new medium, since they tend to deviate from the normal one at the point of convergence. The march of the light rays in the vitreous body can be compared to the one they follow when they meet in a focus at the exit of a lens, crossing a medium of identical composition to that offered by the path before entering it. Indeed, as we have pointed out earlier, the aqueous humor and the vitreous body have the same index of refraction; therefore, the degree of convergence of the rays at the lens entrance will be the same as at the exit. I mean, just like a glass lensIt gathers the rays coming from the air in such a way that they form focus, this convergence being a consequence not only of the refraction of the rays upon their entry into the lens, but also of the identical process that takes place upon their exit, the lens produces a phenomenon similar, considered in its relations with the aqueous and vitreous humors .

Image construction

Figure 1 .

The construction of the image, taking into account the exposed data, is a very laborious process. You start by building the image produced by the first refractive surface; This image is used as the object for the next refractive surface; the second image serves as an object for the third surface, and so on. The problem is greatly simplified by constructing the schematic eye, applying the Gauss theorem ( figure 1 ).

The eye, although not exactly, can be considered as a centered optical system, understanding by that one in which the centers of curvature are on the same line, which is the main axis. Assuming centralization and given the preceding data, it is easy to trace theoretically the deviation that light rays experience when crossing the various refractive media. For this purpose, the problem is made simpler, by reducing all the refractive means to three spherical diopters or to one, and adopting the six cardinal points of Gauss.

Six cardinal points of Gauss

Main focus

When the light rays that hit the refracting surface come from a remote point (practically infinite), they are parallel to the main axis, and after refraction they meet at a point located on that axis: this meeting point is called the main focus . As found behind the refracting surface, posterior principal focus, F . This focus is 24.1 mm from the anterior surface of the cornea (practically in the retina ).

A light rays come from inside the and come parallel to the main axis, after refracting they will meet in front of the cornea at a point located on the main axis: this point is called the previous main focus, F , and is 15’7 mm. In front of the vertex of the cornea. In short: the rays parallel to the main axis, coming from the outside, are refracted passing through the posterior focal point located on the retina; reciprocally, the rays parallel to the main axis incident on the other side of the system, are refracted or emerge passing through the previous main focal point.

Main points

They are also two, H and H , and represent the intersections of the main planes with the optical axis or main axis: a light object located in a main plane emits rays parallel to the main axis, which form a right-hand image of the same size in the another main plane. The first and second main points are very close to each other in the anterior chamber at 1.7 and 2.0 mm, respectively, behind the cornea . Just as F and F ‘ correspond to the main foci of a single lens, H and H’ correspond to the conjugated foci of the same lens.

Walnut spots

They are two points on the optical axis that have the following property: every incident ray that passes through a nodal point emerges from the system, following a parallel direction, passing through the other nodal point.

Finding both K and K ‘ corresponding to the optical center of a single lens, close to the posterior surface of the lens, at 7.0 and 7.3 mm. Behind the vertex of the cornea .

As the two main points are very close and the same happens with the nodal points, it is accepted that the first two are confused into a single one located in the intermediate position of both, the same happening with the second two. Hence the idea for “LISTING” was born to simplify the study of the geometric elements of the eye, reducing it to another equivalent but simpler system (reduced schematic eye). This has a unique surface of ideal refracting power, located in the anterior chamber at 1.35 mm. Ahead of the cornea and with a radius of 5.7 mm. The anterior nodal point or optical center of the reduced eye is 7.08 mm. The main point, at 2’3, and the posterior focal point, at 24’13 behind the anterior surface of the cornea. The anterior focal point is 15.7 mm. Ahead of the cornea. The distance from the nodal point to the retina, that is, the focal length of the eye, is 24’13 – 7’08 = 17’05 mm. The refractive power is therefore 1,000 / 17’05 = 58’65 diopters.

Light rays

Once the aforementioned measures are known, the path followed by the light rays can be traced and the image can be constructed on the retina , as illustrated in the aforementioned figure 1 . The retinal image, as seen in the diagram, is inverted, smaller than the object, and projected onto a curved surface. Point A is drawn on the retina at a and B at b ; therefore, the further away the object is, the smaller its image will be, since its size is determined by the visual angle ANB , formed by the AN and BN rays.gathered at the optical center N. The limit of retinal discrimination corresponds to a visual angle of 60 °; In other words, in order to be separately perceivable, two light points must form an angle of not less than 60 ° with the optical center.

If we see objects in their true position, it is probably because reinvestment is a brain function developed by associating visual sensations with those of touch. It is a psychological process outside the capacity of our analysis.

Visual accommodation

Figure 2 . Mechanism of accommodation.

Visual accommodation. If we try to focus a certain object with a photographic machine , we can resort, as usual, to move the bellows that carry the device.

But in the eyeball the lens does not move forward or backward and the sensitive surface, the retina, is fixed. However, the eye adapts to distances by a particular focus mechanism, which is known as accommodation.

Placed at a distance of six or more meters, its rays can be considered parallel, and they form their focus on the retina without the intervention of the aforementioned mechanism. But in the case of split rays from points located at a smaller distance, despite their divergence they also coincide in the retina thanks to the process of accommodation. In essence, this process consists of a bulging of the lens, the more accentuated the closer the object is, and therefore, the more divergent the rays are.

Obliquely observing the eyeball adapted for near vision will check the projection of the iris forward, pushed by the lens when bulging; on the other hand, such modification does not occur in the subjects in which the lens has been removed. With the help of a special device ( ophthalmometer ), it can be seen how the corneal image does not change in size as it accommodates, while the crystalline images, and especially the one that corresponds to the anterior face, decrease markedly when the latter is bulged.

Changes in radii of curvature in such circumstances, the following figures:

The lens has an elastic structure; it is enough to free it from the compressive influence exerted by the suspensory ligament so that it adopts a spherical shape. In the act of accommodation, the ciliary muscle contracts, pushing the choroids forward and, consequently, relaxing the suspensory ligament. This action decreases the tension of the lens capsule, allowing it, the effect of its elasticity, to increase convexity, especially on the anterior side ( Figure 2 ).

During accommodation there would be not only an increase in curvature in the central parts of the lens, but at the same time a decrease in curvature at the periphery, the zonula would behave in the opposite way. In short, the question of whether the zonula contracts in the near vision (accommodation) or in the distant vision.

Extracapsular mechanism

Regarding the extracapsular mechanism of accommodation, the analysis of the directions, in which the traction or relaxation of the zonular fibers work , allows them to be divided, from the point of view of their action, into two groups:

1. It is inserted, on the one hand, on the posterior aspect of the ciliary body, and generally ends on the anterior aspect of the lens.
2. The second group is made up of shorter fibers that, starting from the anterior face of the ciliary body, come to end at the equator and posterior face of the lens.

These two orders of fibers are arranged, as a whole, as intertwined, there being a certain antagonism between them. When the ciliary muscle contracts, it pushes the choroids forward, keeping the posterior insertions of the long zonular fibers in the same direction . As the muscle relaxes, the fibers are taut by the choroid at one end and by the elasticity of the capsule at the other.

Moment of accommodation

At the moment of accommodation, that is, when the gaze stops fixing at a distant point to do so at a nearby point, the constriction of the pupil and the convergence of the eyes occur simultaneously . Pupillary narrowing does not play an active role in the accommodation mechanism (eyes without irises or with iridian paralysis retain this mechanism), but it is useful for near vision by acting as a diaphragmand suppress the effects of sphericity aberration on the peripheral parts of the lens. At the same time that the pupil is narrowed, the free edge of the iris is directed forward and, consequently, the anterior chamber in its central part becomes shallower; but as it was indicated earlier, its volume does not vary, since at the same time it widens in the marginal parts.

Mechanism of accommodation

In order for the mechanism of accommodation to come into play, it is necessary to direct the gaze to a nearby object. It is a reflex whose starting point is the retina:

The afferent pathway. The optic nerve. The efferent path.

The common ocular motor, through which the impulses provoking the contraction of the ciliary muscle arrive ; and thus, the electrical excitation of the isthmus of the brain in the posterior part of the third ventricle, that is, in the area of ​​the nuclei of the mo c ., determines a bulging of the lens. The sympathetic plays the role of inhibitor in this function; under its excitement, relaxation of the ciliary muscle occurs and, consequently, crushing of the lens.

The nerve endings of the third pair, like all parasympathetic ones, are paralyzed by atropine, a drug that, therefore, renders the eye impossible for near vision; on the other hand, pilocarpine and eserine , due to their exciting effect, prevent distant vision. In animals with a common visual field, accommodation is performed simultaneously in both eyes .

Functional capacity of the accommodating mechanism

Functional capacity of the accommodating mechanism. The normal or emmetropic eye indifferently appreciates objects located far away, without accommodating itself, such as those located in its proximity, accommodating itself. The extent of accommodation is the distance between infinity ( remote point ) and about 10 cm. in front of the eye ( Next Point ). The amplitude of accommodation is the difference between the refractive power of the eye at rest and when it is accommodated to the maximum. It is expressed in diopters, which represent the convex lens that would need to be placed in front of the eye to replace the accommodating mechanism in near-point vision. The diopter lens ( D ) is the one with a meter of focal length; the lens of a refractive power of2 D , will have a focal length of 1/2 m. that of 3 D, 1/3 of m.

Tired sight

In domestic animals, observations agree that the power of accommodation of the eye to distances is much more restricted than in man . In this it decreases appreciably with age, speaking of presbyopia or tired eyesight when the next point is farther away than 25 6 30 centimeters, which occurs from the age of forty, approximately. The cause of this defect, which can be corrected by means of a suitable convex lens, is that the lens gradually hardens from the central to the peripheral layers, thus losing its elasticity.

Amentropy

Figure 3 . Eméntrope eye, hiperméntrope eye and myopic eye .

The emmetropic eye forms the images on the retina . But if the anteroposterior axis of the eyeball is too long, the image falls in front of it, which is only reached by the diffusion circles; the eye is then said to be myopic . On the contrary, if the aforementioned diameter is too short, the rays come together behind the sensitive lamina, an alteration that constitutes hyperopia ( Figure 3 ). Such refractive defects are avoided with the use of lenses of different quality and power; In the case of myopia, divergent concave lenses are usable, and in hyperopia, they should be divergent convex.

Most of the individuals that present the aforementioned alterations also suffer from astigmatism, whose term means that the rays that come out of a point of an object are not found again once their refraction has been verified in the diopter. The astigmatic eye can be considered as the superposition in it of a large number of eyes of different refraction, from that of maximum myopia to maximum hyperopia as well, and oriented differently around the anteroposterior axis.

Refraction effects

The effects of refraction have an experimental basis. Along with the concepts of incident ray, normal and angle of incidence, it is necessary to consider now the refracted ray and the angle of refraction or angle that forms the normal and the refracted ray.

Astigmatism

Astigmatism is, in synthesis, due to the different refraction of the eye in the different meridians. Regular astigmatism is called that form in which, being the refraction equal in a whole meridian, there is a difference in the degree of refraction in each meridian. This form is very frequent in man , and has also been observed in cats and horses .

Irregular astigmatism

Irregular astigmatism is the variety in which there is not only a difference of refraction in the different meridians, but also in different parts of the same meridian.

Spherical aberration

Spherical aberration, which is that in the lens, as in any biconvex lens , the rays that pass through the marginal portion are refracted more than those that do so through the central part, and consequently, form their focus ahead of them. Such defect is partially compensated in the eye because the center of the lens is denser and therefore more refractory rays than the peripheral portion, and also because peripheral rays are largely eliminated by contracting the iris and acting as a diaphragm .

Chromatic aberration

Chromatic aberration designates a defect typical of all lenses and equally of the lens, by which each sector acts as a prism, breaking down white light into the colors of the spectrum. Since the rays from the violet end are refracted more than those from the red end, they form a closer focus, and it would appear that retinal images would be surrounded by red and violet halos . But this is not the case; firstly, because the medium refrangibility rays falling on the retina are the brightest, and the excitement caused by them reduces, by contrast, the sensitivity of the immediate parts, and secondly, because the visual apparatus is barely sensitive to the extreme rays of the spectrum .

Entoptic phenomena

The entoptic phenomena are those impressions generated in the ocular apparatus, but that the consciousness refers to the outside. Entoptic impressions recognize opaque or semi-opaque bodies that intercept the light rays that have to impress the retina , projecting the shadow onto it.

Flying flies

The so-called flying flies are impressions of this nature, due to small turbidity of the vitreous humor . Other turbidity is caused by mucous concretions or sebaceous droplets attached to the corneal surface ; sometimes dark rays are perceived, attributed to the radiated structure of the lens.