The eye and its diseases

The structure and functions of the visual system

The visual system is the most important sense organ sending information to the central nervous system about all the changes occurring in the surrounding environment. It works on the principle of the cybernetic system, that is to say, it has the eyeball at the exterior taking pictures, ways of sending the message and cortical centers for interpreting the image.

The eyeball is spherical and it is formed of three membranes:

The external membrane, called the sclera which is white pearl, fibrous and very resistant, non-extensible to the adult but very extensible to the child. It is also called the sclera shell because it maintains the shape of the eyeball. It is formed of elastic fibers, which also consist of some cells called fibrolite that are very resistant and opaque. Genetically, these cells have to be able to synthesize collagen and polizaharides in order to ensure the fiber resistance. In case the fiber resistance is lower than necessary than the eyeball changes its length or shape generating refraction troubles like: myopia, astigmatism, cheratocon. The physiological role of the sclerotic is to protect the other components of the eyeball.

A part of the sclerotic is transparent, the cornea, through which the light penetrates the eye. So, the cornea is like the window of a room. The main role of the cornea i 24224j919y s to allow the light rays to reach the retina, after being deviated. 

The middle membrane, named the uveal tract, has two parts: anterior and posterior. The anterior one has two elements: ciliary body and iris.

The ciliary body is composed of ciliary muscles and the ciliary processes.

These muscles are independent of human will and work by instinct. They have very thin connections with the transparent lens. They do have the most important function in the eyeball, to accommodate any image from any distance. This function is very useful to get a clear representation of all the objects, irrelevant of the distances at which they are placed. The muscles either contract or relax the lens in order to accommodate the image.    

The ciliary processes are very important for maintaining the normal pressure inside the eyeball. They also have the role of feeding the cornea and the lens, which are not vascularised.

The iris, is a membrane in front of the lens and it is colored in brown, blue or green differing from individual to individual. In the middle of the iris there is a hole called the pupil.

The pupil can diminish and enlarge its diameter according to the light in the surrounding environment, having automatically the role to control the quantity of light that penetrates inside the eye up to the retina.

The choroid, is compared with a vascular sponge, because it is mostly composed of vases of different sizes, having the role to feed the retina and the other parts of the eye. The choroid also contains a pigment that achieves the so-called dark room of the eye.

The third layer of the eye is the nervous type retina formed of 10 sub-layers in which exist 3 types of brain cells:

the first neuron: the cone and the rod;

the second neuron: the bipolar cell;

the third neuron: the ganglion cell.

At this level the transformation of the lightning radiation in electric energy is made, conveying the visual message to the brain. Cones and rods are the most important brain cells containing photosensitive substances, namely iodopsine and rodopsine, substances rich in an essential element, vitamin A. Cones are located in the center of the back of the eye, in the area called the optic spot.

There are in general up to 8.000.000 cells which are in charge with the perception of the form of the fixed elements (the sense of forms)  and of the monochromatic light, meaning the colors ( the chromatic sense).

Rods consist of aprox.60.000.000 elements and have the property to perceive the fading intensity of light, namely to allow the orientation in reduced light, in darkness ( the lightning sense).

The stimulant for these brain cells is the electromagnetic radiation, namely the white composed light, consisting of very fine particles called photons.
The photons penetrate the eye through the transparent cornea, lens and retina. Reaching the cones and rods, they generate a micro bombardment because they have weight and speed, separating the molecule from the photosensitive substance (iodopsine and rodopsine). There takes place a physic-chemical transformation, made through paramagnetic resonance and the electrons are thrown on external orbits, creating a potential difference. This potential difference is transmitted through the other brain cells (bipolar and ganglion cells), optical nerves and optical ways up to the brain where the image is formed  through a physical mechanism.

This electrical property of the retina's neurons allows the transformation of the light in electrical energy, which delivers the visual message to the brain.

The visual image, as it portrays all we see in the external world, is complete and is formed in an optical stage, a physiological stage and a physical stage.

The optical part of the eye, composed of the transparent cornea, positioned  in the anterior part of the eye, is able to guide the lightning radiation it receives. The lens behind it, that is all transparent, is positioned behind the cornea in the so-called anterior room of the eye, which from an optical point of view only has the role to guide the light rays.

The third element and very important is the transparent convex lens lens. This lens is connected through very fine fibers to the ciliary muscles. The lens is positioned behind the iris and has the possibility either to increase its refringenta allowing the short sight or to diminish its refringenta allowing the far sight, through the reflex contraction of the ciliary muscles. This property is the visual accommodation, which allows us to see clear from any distance we look at. Behind the lens and in the rest of the content of the eye is located the vitreous, a transparent, colorless mass of material based on  a very fine chemical collagen structure. The role of the vitreous is to guide the light rays toward the retina.

Annexes of the eyeball. In order to work properly, the eyeball, like a camera, has some annexes to help it.

The orbit has a pyramidal shaped cavity. At the top you can see the Optic hole through which the optic nerve goes to the brain. The orbit protects the eyeball against external aggressions. The orbit also contains the six muscles that control the eyeball movements.

Eyelids, that protect the eyeball against dust, smoke and external corps.

Tiers system is an annex useful for lubricating the cornea and the conjunctiva and also for nourishing and oxygenating the eyeball.

Conjunctiva is a very thin pinkish membrane on the posterior side of the eyelids. It is highly vascularised and it protects the eyeball against external aggressions. Four of the extrinsic muscles are horizontal and two are oblique. Organized in this way, the eyeball transmits the images to the brain, through the optic nerve.

Most frequent eye diseases

The eyes have the same qualities as a true optical apparatus. The retina plays the part of a sensitive photographic tile due to the fact that the image is formed on it as it does in a photographic camera. The image appears because the eye has a dioptrical system. The image created on the retina is smaller and reversed, compared to the object we are looking at. The eye's dioptres are made up of: the transparent cornea, the aqueous humor, the lens and the vitreous.

This assembly plays the part of a convergent lens with a focal distance of 23 mm, that is exactly the length of the anther-posterior axis of the eye. In a normal eye the posterior focus can be found on the retina in the macula, a small area that provides our most central, acute vision. The eye's refraction is given by the deviations produced by the eye's diopters when light rays cross them.

The normal state of an eye (emetropy) is produced due to the correlation of these diopters; the ray hits the centre of the retina on the macula and the image is normally formed. The eyes that suffer from certain defections (ametropy are those in which the rays don't meet on the retina but in front of or behind it.

The ametropies are of two types: the diopters affection when correlating and a genetic structure affection. The ametropies of correlation appear at the eyes in which the components of the dioptrical apparatus don't correlate like in a normal eye.

The ametropies of structure appear due to the fact that different components of the dioptrical apparatus have the affections. The stigmatic ametropies are hypermetropia and myopia, while the astigmatic ones are the astigmatism and its types. The medium, the distance from which we read, light, the size of letters can determine, in some cases, ametropies.

Ocular Aberrations

Spherical Aberrations: The peripheral rays that go inside the eye suffer a more pronounced deviation compared to the central rays and give birth to a particular focal surface. The spherical aberration is reduced due to the peripheral smoothing of the cornea and the presence of the iris diaphragm; the more the pupil is dilated the bigger the aberration; the less the pupil is dilated the smaller the diameter of sight.

Chromatic aberrations: The speed of light spreading varies according to the ray's length. Inside the eye the spreading isn't the same for all the colors. Therefore instead of just one focus, there is one for each color. In the normal eye there is a centre for red situated behind the retina, one for violet situated in front of it and another one for yellow and green situated on the retina. If the eye suffers from myopia, the centre for red will be closer to the retina; the eye suffering from myopia sees better the color red. In the case of hypermetropy, the centre for violet is closer to the retina and the eye sees better when the light is violet.

The eye's diopter has a medium value of 58 D out of which 43,05 D are given by the cornea and the rest by the lens. Among the two the lens is the only one that can change its power of refraction according to the object seen. This is how the value of the total refraction of the eye grows.

The ocular refraction is divided into:

-the dynamic refraction in which accommodation intervenes, changing the curve created by the lens

-the static refraction in which accommodation doesn't intervene, the eyes are on an "accommodation stand-by".

The static refraction

When the eye is on the "accommodation stand-by" the parallel light rays, after having been projected by the eye-diopter, meet on, in front of or behind the retina. When the centre is on the retina the eye is hemitropic, while, when the centre is not on the retina, the eye suffers from ametropy. Therefore, we can determine two types of ametropies: spherical and non-spherical.

The spherical ametropies: the parallel light rays are united in a point because the refraction is the same for all the eye-meridians. In this way, the pencil of light rays, which crosses the eye, takes the shape of a cone with its vertex either in front of or behind the retina.

The non-spherical ametropies: the parallel light rays don't meet in a point because the diopter's refraction isn't the same for all the meridians.

If we take two perpendicular meridians, one of maximum refrigency, the other of a minimum one, each of them projects its image in a different point and it results in two parallel lines situated in two different areas- Strum's lines.

In the non-spherical ametropies, if the focus is formed in front of the retina, the eye suffers from myopia, and if it is formed behind it, the eye suffers from hypermetropy. There are 3 cases:

the refrigent system of the eye is normal but its anther-posterior axis is too long or two short (axial ametropies); to each mm of length, plus or minus, a refraction flaw of approximately 3D corresponds;

the anther-posterior axis of the eyeball is normal but the curved rays of the diopters (cornea, lens) are abnormal (curve ametropies);

the eye is normally built, but the refraction indicative is too strong or too weak (indicative ametropies);

The ametropies may happen accidentally, produced by moving an element of the dioptrical system (for the normal eye). The discordance between the eye's refringent power and the length of the axis can be measured and corrected.

Dynamic refractions

The eye is able to see not only the objects situated at an infinite point (beyond 5 meters) but also those situated in between that object and infinity, due to the lens's ability to modify its refrigency. That is why the images of the seen objects can be formed on the retina, no matter at what distance they may be.

The accommodation is the eye's property to see clearly at a distance, so that the image projected on the retina can be clear when the object moves in between two extreme points, punctum remontum and the proximal point. When the object is closer the accommodation is stronger . The refrigency changes are caused by the change of the lens's curving under the action of the ciliary muscles. When the eye passes from long distance sight to short distance sight the macular circles start a reflex contraction of the ciliary muscle. That relaxes Zinn's zonula and the lens's curve gets bigger, especially at the level of its anterior face (the curving ray of its anterior face gets 4 mm smaller and the posterior face's gets 0.5 mm smaller). The curving of its anterior face is much more visible in the axial region because the crystal is thinner within the centre, which results into a conical deformation.

Outside the accommodation period, the zonula is held stretched by the radial fascicle of the ciliary muscle (Brucke's muscle) which produces the distal accommodation. The contraction of the ciliary muscle works on the choroid on which it is fixed, moving the ora serrata towards the sclera muscle (0, 05 mm for each accommodation dioptre).

The circular fibers make up a 'neurotic point'; the centre of the reflex arc is found within the Edinger Westphal nucleus. The influx, which borrows the way of the MOC nerve, goes through the little oblique ram, the motor root of the ciliary ganglion and through the short ciliary nerves up to the eye. The radial fascicle is innervated by the 'sympathy nerve'; the influx leaves from the hypothalamic centre of Carpus and Kreidle, follows the cervical chain up to the superior cervical ganglion then through the internal carotid, into Gasser's ganglion and through the ophthalmic nerve , the nose's nerve and the long cilia nerves, and reaches to the eye. The accommodation is a reflex act started by the macular circles, when the image isn't clear on the retina. The way is made up of sensitive fibers. There are two ways, a short one used by the inferior animals (optic nerve) and a long one (a gained reflex) where the influx goes to the occipital cortex, then to the Edinger-Wesrphal's nucleus through the optic-mezencefalic fascicle.

Bombing the lens results into a higher refraction power and a smaller focal distance.

The relaxation of the ciliary nerve and the contraction of the radial muscle produce the stretching of Zinn's zonula which results into the lens's flattening. This will take the eye into the accommodation state. The furthest point where the image is clearly made on the retina, without the accommodation's intervention, is called; 'punctum remotum'. At the hemitrope eye that point is at the infinite and the eye doesn't need accommodation for this distance; the eye suffering from myopia has this point between the eye and the infinite while the hypermetrope eye has this point beyond the infinite (the negative punctum remotum).

The closest point, that of maximum accommodation effort, is called the proximal point ; it is situated at variable distances, differing according to the age and refraction of each person. If a person is suffering from myopia the proximal point is closer to the eye than at a hemitrope or a hypermetrope eye.

The accommodation power is proportional with the convergent one; when the subject looks at an object situated at the infinite, the accommodation and the convergence are mule and the visual axes are parallel if the object gets closer, to be seen clearly, the eyes execute a convergent move for the image to be formed on both eyes; the closer the object to the eye, the higher the convergence and accommodation effort is.

When the eyes look at an object situated at a distance of 1 meter they accommodate of 1D and the convergence is of 1 metrical angle; if the object gets to a distance of 50 cm, they accommodate of 2D and their convergence is of 2 metrical angles; at 25 cm the accommodation is of 4D and their convergence is of 4 metrical angles.

The light sense

The light sense, the most elemental sight function, the faculty concerning the perception of light, to distinguish between the clear and the obscure, is determined through the weakest light capable of producing a light sensation; through the smallest difference of light that can be perceived.

The light sensitivity has 3 divisions: -scotopical, mezopical, and photopical. The sensitivity threshold is expressed through the smallest energy quantity able to start (to produce) a visual sensation. It represents 5-14 quarts.

The alteration of night sight in some cases has been studied for a long time. Bruckner (1961) says that adapting to darkness is sometimes altered when referring to correlation myopia. It is always altered when talking about high myopia and sometimes it is accompanied by alterations of day-sight. This problem has been presented by Jazlesicol (1950) in the reports of the French Ophthalmology Society. In 1966 Redisicol said that the alteration of a light sense is proportional to the myopic grade and the corioretinopathy existed. He observed that frequently the plenitude of the adaptation curve and the disappearance of it's a point.

The chromatic sense

Schabel (1874) remarqued that around the scotome, which corresponds to the myopic conus, there is a poor area of chromatic perception. The scotome would be first for green then for red and blue.

Wettendorfer (and Hess) observed that in cases of myopias over 6D there is a zone, concentrically to the fixation point, with a reduced sensitivity for red and white. The zone would correspond to the circled Scotome described by recent scientists.

The frequency of congenital acromatopsy associated to a high myopia would vary with the geographic environment because Hussels has seen it 5 times among the inhabitants of the Pingelop island; more frequency than in Mekil. Redi (1965) observed several alterations of the chromatic sense looking at 49 subjects who have been suffering from myopia for 8-58 years. (3-20D myopia)

Anxiety of refraction

It can be emphasized through skiascopy, oftalmetry or computerized refractometry. Therefore:-

-it can be supracorrected myopia, or an insufficient correction of hypermetropia;

-the inverse excesses can be the cause;

-the bad correction of an astigmatism; the bad tolerance of the optic correction;

-Ansiometropie;

-the astigmatic correction of a person over 30 years who had never carried cylindrical Lents it can't be supported;

-the bad position of the Lents in front of the eyes; the bad centrance; he bad distance between the eye and the glass;

-the glasses too tilted downwards or too convex.

When the Lents are changed, it must be taken care the resemblance of these and of the frames of glasses, with which the ill man changed his glasses.

Accommodation anxiety

The diminution of the extension of accommodation with the age, the Donders curve, valid also in our days, it assumes the appearance of normal presbition between 40-50 years. This manifests by a visual fatigue in the evening of after visual effort, headaches, and the tendency to outstrip objects or textiles for seeing them.

The visual fatigue manifests earlier at hypermetropia, later at myopia and varies with the distance at which one subject works and the precision or size of the working object.

The treatment is the prescription of glasses for near of course after the determination begins with the refraction, adding or decreasing one number of dioptries after the situation, taking into account that the normal figure at 60 years is of 3D and at 70 years is of 3,5D.

The Lents change at presbiopy, approximately once at two years It can be prescribed bifocal Lents, after case, with tablets, graduals.

Myopia

Myopia is the ametropy characterized by an excess of refrigence of the ocular diopter

,which determines the compounding of the images of objects situated at infinite, in a focus in front of the retina. The retina is therefore shamed of the elongation of the crossed rays in a focus diffusion circles, therefore an unclear image.

From the symptoms point of view, in the small myopia exists only one subtraction of the sight for distance, and for near the sight is good. In the big myopia the sight is diminished both for far and near.

The disease can be characterized as an ocular deviation through correlation absence between the ocular diopterthe length of the eye's axle, having as consequence the compounding of the image of the objects situated at the distance over 5m in front of the retina..

In some cases she accompanies from the start, or more often, in curse of her development, from the spoilt of all the ocular membranes. From the beginning, the definition includes two categories of myopia-one resulted only from a discrepancy of the ocular diopter with the ocular axle too long doesn't surpasses minus 6 diopter and it doesn't advances in which the prior posterior axle's elongation is accompanied of degenerative processes rough gradual, which installs at varied ages (it can touch minus 20, mines 30diopter -gradual myopia)

The first type of myopia it can also be called uncorrelated myopia, if we admit that the ocular refraction it depends of the correlation or uncorrelation of the different ocular diopter (cornea, prior camera, lens) with the prior posterior axle of the eye. The myopia is one biologic version of the eye, resulted only from the unmatched parameters which determines the ocular refraction, also known under the name of conformation myopia, healing myopia, scholar-appears at about the age of 6 years to 10 years and develops in the scholar period, having one gradual development until the age of 20years, when it becomes equable .She doesn't surpasses 6-7D and doesn't accompanies with ocular complications, she has weak evolving character and it will be from an hereditary nature .After some specialists, myopia owes to some inadequate work conditions(insufficient lightening, vicious position of the head, of the body, excessive meticulous works etc.),after other ones, myopia owes to some morbid childhood states, which leads at the weak of the body. Clinically these children don't see well at distance in change they see really well from near.

The second type is qualified as big, morbid t myopia. She is, often, family and hereditary .The absence of correlation between the ocular diopter and the prior posterior is more accentuated in most of cases than in simple myopia, but this isn't a general rule, because the characteristic morbid modifications, of the ocular membranes can be met also in the eye with moderate excess of refraction .This seeing made by Badke (1952) shows that degenerative myopic wounds aren't obligatory associated with a certain elongation of the eye. They can accompany also myopia of curvier or index with normal axle. This form of myopia, the morbid myopia, degenerative, represents, therefore one distinct morbid entity, often associated with elongated eye. It can be present mostly at birth is considered as being the secondary of one distortion of the sclera collagen which owes to a development anomaly of the secondary optic, which from the abnormal embryonic induction over the mesoderm from vicinity would determine secondary anomalies in the compounding of the sclerotic and may be of the choroids. Sometimes, myopia accompanies also muscular astenopya which consists in the weak of the convergent, that is the internal convergent muscles because the continuous necessity of the myopia to converge because, for clear seeing, he must take close very much the objects to the eye. By the weak of the convergent, it produces one diversion of one eye and the install of a divergent strabismus followed by the disappearing of binocular sight.

Along these years, taking into account different criterions there were made numerous classifications of myopia.

Donders proposed one clinic classification of myopia accepted of the majority of oculists. Identical to this, there are 3 kinds of myopia:

1) Stationary

2) Temporary gradual

3) Gradual

The myopic eye is an imperfect optic instrument.

The eye is the most speeded optic instrument .The myopic eye is an imperfect instrument, although ones claim to be more adjusted activity connected with modern civilization.

To understand the characteristics of myopic eye, we must briefly say the optic characteristics of hemitrope eye, which is considered normal and the most frequent met in every adult population. With a view to that, we shall present, following the description of Bennet and Francis, the parameters of the sketched eye, simplified of Gullstrand modified by Emsley frequent reproduced in the ocular optic treaties.

This optic model consists in an aria of Fe, which separates the air from the medium with a refraction index of 4/3(1,333) representing the cornea and the lens. The power of this refracting aria is of 60D and its curve given by the formula:

R= (4/3-1)X1000/Fe=5.5mm

The center of the sphere, from which is cut at 5.5mm from this refracting aria, makes of the unique knot point of the system. the prior focus of the eye F is of 1/60m , that is at 16,67 mm of Fe and the posterior F' one is at (4/3)x16,67mm that is 22,22mm of Fe.

Results that the reduce hemitrope eye has a length of 22,22mm which is the retina ray at this distance of the refracting aria.

A is the pole of the refracting aria; N is the knot point I is the prior focus F'is the posterior focus. In the hemitrope eye, the retina corresponds to F'.In the myopic eye the image of the same object situated at a bigger distance than 5m compounds before the retina, his diopter system having a focus distance smaller than the prior posterior axle's length. The correlation absence of 2 parameters determined for the ocular refraction achieves to:

-may be by a too strong ocular diopter;

- may be by a too long prior posterior of the eye.

In one eye with the ocular diopter stronger than 60D (the axle assumed as being normal ),the parallel radiant rays which penetrates are crossed before the retina.

The inadequate refraction of the ocular diopter in the myopia can be owed to the accenting of the cornea's curve (the move of his curve ray).

In any case speaking of curve myopia, or the raising of the refraction index of the mediums (the cornea, the lens) when we were talking about index myopia.

When the prior-posterior axle of the eye is bigger and the diopter is normal we were talking about axle myopia.

In any myopia form, the prior posterior axle of the eye is bigger than the focus distance of the ocular diopter .The second focus of the ocular diopter a the posterior focus is situated forward the retina M' and a removed object will give birth to a diffuse circle on the retina, result of the image compounded in the elongation of the parallel rays refracted crossed before the retina.

For the myopic eye to compound an clear image on the retina the object must be situated in PR, in the knot point of the eye so that the reflected rays by him to be divergent. When they touch the eye PR must lay at k distance, with is the conjugated distance of K1. The ocular refraction is the mutual of the K distance and it can be represented with K symbol.

K distance from point PR of its refracting aria, measures the ametropy degree which express, but, better the diopter. In emetropy, K=0, the proximal point is at infinite. Therefore K=1/K (meters) is the ametropy degree, being positive for hypermetropia and negative for myopia. That's why M and PR are conjugated points; the K ocular axle's length can be calculated in K or Fe terms.

The sight of a myopic eye can become clear if it's placed on S, in the frontal plane, one adequate concave Lents, for the image of an object situated at the infinite(that is at the distance of 5m) compounds in PR the knot Pont of the eye. The image therefore compounded, proceeds as a real object going to compound this image on the retina.

The wearing of proofreader lens necessary varies with the distance between the lens and the cornea vortex. In the calculus formula it is noted the conventional sign + or -of the lens.

It was determined by Fransceschetti and Gernet (1956) the distance between the cornea and the prior face of the lens by optic ways (the oculus Goldmann) and the thickness of the lens and the vitreous by echography. The examined material was spited in small myopia (uner7D) and in both age groups of over and fewer than 21. The established optic dates includes the myopia degree, in diopter the curve ray of the cornea, the ocular axle's length the entire refraction, the lens refraction, the crystal line's thickness. They show that the average of the curve ray of the myopic eye cornea (7,6-7,9mm) doesn't differs in the examined material, of that of hemitrope eyes(7,8mm at men and 7,68 mm at women).The ocular axle average(at adults with big myopia of 29,9mm at young mans of 27,7mm)is certain bigger that at hemitrope(23mm at men,22,7 at women. At the myopic adults the shortest axle was of 27mm and at myopic children of 25,5mm.

The entire ocular refraction is the result of varied combinations of different ocular parameters, which tacked separately their distribution curves symmetrical and by binominal type. Numerous works broached the variability of different parameters which conditions the ocular refraction.

Tron (1931) built, relying on 200 cases, variability curves watching: the curve ray of the posterior face of the lens, the crystal line's refraction the total refraction of the eye, the prior posterior axle's length. The founded curves including the one of the optic axle, corresponds with the binominal curve, if it's excluded the myopia of over 6D.He also founded out one growing of the axle combines by preference with a wicker diopter.

By statistic estimation, it established at children smaller than10 years. Wibaut (1932) -one correlation between the cornea and the entire refraction .This isn't met at adults.

Wibaut concludes that the cornea's refraction and the crystal line's one completes themselves both, because between the chrysalis's refraction and the axle's length exists one negative correlation and that the entire refraction doesn't correlate positively only with the ocular axle's length.

The myopia's of every degree are accompanied often by astigmatism. He is corneal and lens us. The big myopia can have also astigmatism owing to the posterior stabiles (astigmatisms funds) .The value of astigmatism in general lines, is proportional with the myopia's degree and rarely surpasses 3D.

Myopia is less frequent than Hypermetropia and consists only exceptional at birth but he represents a big importance by it's character, or often gradual, of spoiled ocular membranes and of the complication which produces it.

Hypermetropia

Hypermetropia is the ocular gothic deficit (emetropya) in which the focus compounds not on the retina but behind her. It can be a consequence of an insufficient diameter prior posterior of the ocular globe (the axle hypermatropia) or of one insufficiency or curve of the ocular diopter(curvier hypermetropia) or of one modification that is of the subtraction of the refraction index of hypermetropia, which brings to a refraction deficit.

The parallel rays installment which comes from the fixed object and penetrates the hypermetropic eye in rest state, will compound his focus behind the retina, which perceives one circular diffuse spot.

Constant is met in childhood, when the eye isn't completely developed. Hypermetropia stays a long time undiscovered. When she exist , it appears ocular fatigue signs with headacluis, after long reading, paint and ocular congestions after ocular efforts which makes this to suspect it. This ocular symptoms ours to the fact that the hypermetropy makes accommodation efforts and when he watches from distance and from near, trying by this to correct one part or the entire refraction defect. Hypermetropia has in most of cases sizes under +5 diapers, exceptional is +15 diapers. The oculist specialist medic will determine the hipermetropy degree objective and will indicate the necessary correction with glasses with convergent lens, saying of "plus" necessary in rapport with the age the ametropy degree for each case of different parameters which conditions the ocular refraction.

Tron (1931) built, relying on 200 cases, variability curves watching: the curve ray of the prior face of the lens the thickness of lens, the curve ray of the posterior face of the lens, the crystallin's refraction, the total refraction of the eye, the prior-posterior axle length. The founded curves, including the one of the optic axle, correspond with the binominal curve, if it's excluded the myopias of over 6D. He also found out that growing of the axle combines, by preference with a wicker diaper.

By statistics estimation, it established at children smaller than 10 years Wibaut (1932) - one correlation between the cornea and the entire refraction. This isn't met at adults.

Wibaut concludes that the cornea's refraction and the crystallin's one couplets themselves both, because between the crystallin's refraction and the axle's length exists one negative correlation and that the entire refraction doesn't correlate positively only with the ocular axle's length.

The myopia's of every degree are accompanied often by astigmatism. It is cornea and lens. The big myopia's can have also astigmatism owing the posterior part (astigmatisms fungi). The value of astigmatism, in general lines, is proportional with the miggsrri's and rarely surpasses 3D.

Myopia is less frequent than hipermrtoipia the only exceptional is at birth, but that represents a big importance by its character, or often gradual, of spoiled ocular membranes and of the complication which are produced.

Astigmatism

As we have seen before, in hemitropic and spherical ametropias, the bright fascicle that crosses the ocular diopter, looks like a cone, whose pick is situated on the retina of the E eye, in front of it in the M eye and behind it in the H eye; in astigmatism, it does not form a cone, but it forms a conoid, in which didn't exist a punctiform focus but two focal straight lines existed; the refracted rays in the astigmatic eye does not converge only into one point, but they are supporting during the crossing of the ocular dioptre on two perpendicular lines(focal straight lines), which are situated in two distinct planes; between the two straight lines(focal interval) the bright fascicles build a conical ellipsoid, whose section successively becomes an ellipse; this ensemble forms Sturm Conoid.

The previous focal straight line is in the middle plane which has the biggest curve; it is also the perpendicular straight line on the plane which has the biggest curve. The main focus of the vertical meridian is a horizontal and focal straight line and the main focus of the horizontal focus is a vertical and focal straight line.

The accommodation removes the two focal lines away from the retina, but it does not move them one away from the other.

There are two types of astigmatism: regular and irregular.

- in irregular astigmatism there is a difference of refraction not only between the different meridians, but also between the different points of the same meridian; it is rarely found and it is produced in most of the cases by irregularities

of the cornea surface, by origin cicatrisation or changes of the lens lens(the incipient cataract). A.V. is very low and it can be corrected only by surgery or by contact lens.

The regular astigmatism, after the refraction of the main meridians, is divided into simple astigmatism, composed astigmatism and mixed astigmatism.

The simple astigmatism is that in which one of the main meridians is E and the other one is M or H; in this case the image of a point is a straight line, the parallel line of the astigmatic meridian; one of the focal lines of the conoid coincides with the retina, the other one being in front of it M and behind it H.

The composed astigmatism is the one in which both main meridians are short-sighted or hypermetropics, but of different grade. The image of a point is an ellipse, whose big axle is parallel with the most astigmatic meridian; in this case, the two focal lines are placed or in front of the retina (M composed astigmatism), or behind it (H composed astigmatism).

The mixed astigmatism is that in which one of the main meridians M is, and the other one is H, the retina being placed in the focal interval; the image of a point is in this case, a circle ellipse.

As symptomatology, an astigmatic person can not see well at distance but he also can not see well near him.

He sees deformed and he often confounds objects. A little astigmatism can be compensated by an astigmatic accommodation, which determines an unequal change of the refraction of the lens lens in its distinct meridians, but the sick person disorders at a long watching.

The astigmatism occurs when the cornea (through which the light comes to the lens) is easy deformed, producing a deformed image on the retina or double eyesight.

Presbiopy appears after the age of 45 and it is determined by a loss of the resilience of the lens capsule that does not permit the change of the refraction necessary at reading. Normally, for reading from 35 cm far away it is needed a growing of the lens refringency with +4 dioptre (adapted amplitude).

After 45 years old this adapted amplitude declines with about +0.5 dioptre every three years, so at the age of 70 it completely vanishes. Glasses for reading and working at something situated at a short distance must be worn and changed at about 3 years. The doctor will establish the wearing of the glasses regarding the eye refraction and the patient's age.

The strabismus (the functionally deviated eye)

The strabismus is defined by the two visual axles which are not aimed at the same object without existing diplopic, or as a result of the scanty sight of an eye, or by the neutralization of one of the images. Therefore, that is an aesthetic and binocular sight disorder.

The clinical types. We can individualize a certain number of clinical types; the exam permits the joining of this case in one of them.

The strabismus is:

-Convergent or divergent

-Pure horizontal or with a vertical component

-Intermittent or permanent

-Unilateral or alternating

The anamnesis pursues the explanation of the following elements:

1 The moment of the strabismus beginning:

-From the first days of life-the consequence of a real muscle or nervous malformation, this can be isolated or related to some other malformations. The treatment in this case is more complicated;

-At the age of 3 or 4 years old, the most frequent, intermittent at the beginning, then permanent.

2 The modality of strabismus beginning:

-Sudden or progressing:

During an infectious and catching disease

After a period of convulsions

Spontaneous

The brutal starting during a cold or after a convulsion must suggest us the presence of an initial eye movement paralysis.

3 The antecedents: the spreading is usually of a dominant type, because of the refraction vice, with a low grade of penetrability, from the simple heterotopy to the manifest strabismus.

The penetrability is stronger in the divergent.

The cataract

The opacity of the lens lens is produced by a catabolism deficit of the inter lens glucose, that determines the protein synthesis perturbation of the lens that is the permanent regeneration of the fibers which have the beginning point in the equatorial axe

A prosthetic distortion happens, that is responsible for the increase of the osmotic pressure and the water ratio. Other hypotheses were promoted, for example: the diminution of the K+ percentage ascorbic acid, the disappearance of the glutathione etc.

The progressive loss of sight is accompanied by fog, veil, photophobia, monocular diplopic, sometimes ocular hypo tonicity, by senile intumescences. =The cataract etiology.

The age related cataract

The most common form at patients over 65 years old is sometimes bilateral, lop-sided.

There are two types of cataract:

-cortical (the most frequent)

-nuclear (central, at old people who are often short sighted).

The evolution is variable:

-fast(especially at the hinder under capsular forms)

-the volume dimensions of the lens, which becomes white: the mature cataract with the diminution of the lens volume, with the nucleus floating in the capsular bag; that is the hyper mature cataract or morganian cataract (it happens especially at the cortical cataracts)

Complications:

The senile intrumescenta: the volume growing of the lens, with the nucleus floating in the capsular bag is the hyper mature or morganian cataract ( it happens especially at the cortical cataracts).

The senile: the growing of the lens volume, which determines the acute glaucoma (the proteins of the lens crossing through the adulterated capsule), making a secondary glaucoma with an open angle by orbicular obstruction or a facoantigenical uveite.

Haze complications stops after the extraction of the lens.

E cataracts of the grown- people that are not related to the age.

The cataracts of the old people that are not related to the age.

1. The unilateral cataracts

The traumatic cataract made by:

n    bruise(determines hinder under capsular cataract in "fern leaves"

n    The perforation of the lens capsule, which, as soon as is opened allows the cortex to imbue itself with water humor and then to fill.

n    The physics factors: electrocution, heat, ionizing radiation(X, B, Y rays, neutrons, infrared or ultra violets.

a.    The consecutive cataract of an eye disease can appear during:

-a strong short-sightedness

-glaucoma (after ant glaucoma surgeries)

-severe herodiciclity

-degenerative disease of the uvea

*Fuchs heterochromya associates unilateral hinder under capsular cataract, hyperemia of the iris endothelium precipitates.

*pigment glaucoma

*the diseases of the hinder segment

*the retina discoloring uncured (chalky appearance)

*pigment retinopathies

*intraocular tumor

The bilateral cataracts

The daybed

The real diabetic cataract is:

-rare at the young patients (15-20years old)

With a fast evolution, reversible at the beginning, under the daybed rigid equilibrium

Under capsular previous and posterior (at the most has the strict appearance of the daybed)

-under capsular previous and posterior (at the most has the strict appearance of a snow storm)

Hypo parathyroid

The cataract is under capsular previous and posterior, with intra lens crystals.

Hyponym diseases, gonadic insuffiency

Trisomia 21

Wilson Disease

Ynderamatotics characters

These characters can appear during some coetaneous diseases:

-the atrophic dermatitis (previous under capsular in escutcheon), sometimes related to keratectomy.

-Werner disease; Associates calorific state prevailing at face (bird aspect of the head ) , at arms and legs, bilateral cataract with fast evolution, alopecia, osteoporosis and multiple endocrine contacts.

n    The Rothmund-thomson syndrome: recessive autonomic disease, associated with teleangiectasical erythematic of the face, of the exposed areas, photosensitivity of superficial body growth, bilateral cataract and development disorders.

n   

The iatrogenic characters

n    local corticotherapy

n    general corticotherapy

The cataract is usually bilateral, symmetrical, sub capsular hinder, with bright particles of various colors in the middle of opacity A.V. from near is precocious adulterated. Some other iatrogens etiologies can be: tobacco, the miotics anti cholinesterase's in long using, the largatic, synthesis antipalucides, avitaminosis C hemodialisis, dinitrophenol, and despherasus.

Extolphamia (bulging eyes)-ocular bulbs bulged. This disease is shown by bulging eyes bulbs. It is determined by a accumulation of tissue behind the ocular bulb, which can be in its turn caused by the secretion in excess of thyroidal hormone, by a tumor or an inflammation of the tissue situated behind eye.

Discromatopsy

Discromatopsy is the incapacity of making a distinction among certain colors, is especially red and green. This disease can not be treated. It is hereditary and affects 8% of men and 0.4% of women. It is seldom a problem and a lot of people suffering of this can perfectly manage in every day life.

The normal sight at man is related by the simultaneous focus of the eyes on an object. This gives the stereoscopic sight or three-dimensional and permits to men to estimate the distance and the shape of an object.

Nistagmus

Nistagmus is caused by a brain accident which affects the cerebellum or by the inflammation of the internal ear. Even just a rotation that can produce this dizziness can cause this disease. In many cases is caused by the deterioration of the retina that receive images and send visual impulses to the brain. The eye moves for stopping the image for not falling on the deteriorated part of the surface that receives bright fascicles. The treatment is possible taking in consideration the cause that produces it. There is no treatment in cases of brain or retina deterioration. If the deterioration is caused supplying problems a vitamin must be administered.

The Glaucoma

The glaucoma is the growing of the water amount that is accumulated between the cornea and the iris that exerts a big pressure and a constant pain. The sight can become dim, the disease producing the blindness if is not treated. In many cases a laser incision on the cornea, for diminish the pressure in the prior room of the eye.

Anisometropya

Anisometropia is a refraction difference between the two eyes of the same person; an eye can be ametrops and the other one hemitropic or both ametrops but of different grade (short-sighted or hypermetrics' or one short-sighted and the other one hypermetric. Clinically anizometropia manifests often by a visual discomfort sensation, the inequality A.V., and adaptation these others (spasm), eye-moving balance one-eye and two-eyes sight these others.

The Daltonism

The Daltonism is a sight deficiency which affects the ability of distinguish the colors is especially red and green. It appears especially at man. It is a disorder caused by the eye nerve which colors images on the retina. The first report about disease was made by the English chemist John Dalton who was also affected. The impossibility to distinguish the colors is named achromatopy. If the images are seen in shades of grey then the disease is called monochromatic. This disease is very rare and affects the same women and man.

The most common disease is still Daltonism. Few people are affected from this disease 1% women and 7%man. Usually the disease is hereditary and can appear same particular cases for example after a difficult disease. The disease does not affect the rest of the sight. Those who have problems with the distinction of the colors learn to associate the colors with some variations of the light. But many people do not realize they suffer from this disease.

To discover this disease where made many test to distinguish the colors. People who ones not suffered from this disease will se number 57 and those who suffered from Daltonism will se number 35.

The physiology of sight

In the human eye, the lens plays the role of the camera lens with a variable focal distance. It brings the rays, coming from an object, into focus on the retina at the back of the eye.
The refraction of light when it passes through the lens follows the same rule as when it passes through a prism. Consequently, when it is a colored image, its projection is not entirely on the retina. The violet and blue rays get focused in front of the retina and the red rays get focused behind the retina. This process explains the phenomenon known as “ chromatic deviation”. That’s why the eye has difficulties to distingue the colored details.
Retina has two layers of photosensitive cells: one with cone-cells and the other with stick-cells. The cone cells are color sensitive. There are about 6 million of cones in the middle area of the retina. The rods are by far more sensitive than the cones, but only to the light intensity and not to the colors. Stick cells are about 120 million. There are three different groups of cones responsible with the recognition of various colors of the spectrum: blue-sensitive, green-sensitive and red-sensitive cones.
When pass through lens the rays corresponding to various colors get focused in different points depending on their wavelength.
In the eyeball the lens plays the role of lens. The green-sensitive cones receive the rays in the same plan with retina. The blue-sensitive cones are having the lowest sensitivity.
By combining the three fundamental colors we may obtain the entire range of colors (i.e. red + green = yellow, red + blue = purple, green + blue = indigo).
When you analyze carefully a picture from a magazine you may notice that besides black there are other three colors: red, yellow and blue. Except yellow, all the others are fundamental colors.

The evolution of the tiers layer under the contact lens

This evolution depends on the type of the lens.

a) The rigid contact lens
The tiers layer is modified after wearing the rigid contact lens. The tiers layer is stable as long as the surface of the lens is smooth and regulated. After a time, mineral deposits may appear at the contact with the lens. These deposits turn the tiers layer in drops. Additionally, the tiers layer gets dry at the contact with the lens because of excess water.
b) The soft contact lens
Usually, under the soft contact lens the tiers layer is stable but sometimes after a long period of time, protein deposits may appear. These are very water-repellant and so the layer may get interrupted and the lens gets dry at the surface triggering the decrease of visual accuracy. 

The dioptrically power

The difference of power between the airy lens and the contact lens is about the difference of punctum remote -the proof reader lens (mm) by a side and punctum remote measure -the cornea for the contact lens on other side.

DL-the airy lens power (D)

DC-the contact lens power (D)

d-the distance between the eye and the airy lens (12mm-0.012 m)

For a short- sighted Dc is much bigger then the DL and for a hypermetropie the situation is backwards. The difference between the contact lens power and the airy lens power is growing if the ametropia is bigger and the distance is larger.

A particular case is the rigid contact lens where the power of the lens makes the shed film appear. When there is not a parallelism between the radius of cubature of the contact lens and the radius of cubature that is flatter (measured with kerometer) the mechanism effect is producing a 90% compensation of the corneal astigmatism, eventually through the interposition of a tears meniscus.

For the final calculus of the rigid contact lens can be also add the dioptrically effect. This one is convex if the radius of cubature of the posterior face of the contact lens is much sharper then the radius of cubature of the corneal and is concave if the radius of cubature of the posterior of the contact lens is much flatter then the radius of cubature.

The calculus is also using refraction index showing a 0.2 mm variation and also is creating and concave effect which have the value of 1D.This two elements (the distance between the airy lens and the eye and also the tears effect) is underlining the importance of approving a contact lens with radius of cubature of the posterior face really close of the radius of cubature calculate with a really close power of the refraction vice which is to be corrected.

For the supple contact lens the shed film effect is negligible.

The brighter efficacy

The brighter high tide which is penetrates the eye is created because of the correction type. In this case it exists a lower waste of light when it is crossing the contact lens through the airy lens.

The lost because of the reflex ion have the values:

-The eye without lens-2, 09%

-The eye without contact lens -2, 67%

-The eye without airy lens -4%

The luminosity is directly proportional with the pupil's diameter. Reducing the distance between the eye and the shortsighted is growing and is lowering at the hypermetrop. This growing of the clarity can explain some effect of the emolliating at the adapting debut.

Contact lenses classification

There are many types of contact lenses existing in our days, which are raising complex problems. This is why tried contact lenses classification was considering the next criterions:

1. after their role:

a. Optical

b. Esthetical

C therapeutically

d. Tectonically

e. Optical -muscular- cristagmus

2. after their position on the anterior pole

a. Corneal-a smaller diameter then the cornea's (8,2-10,7 mm)

-hard contact lenses which are permeable with oxygen or not

b.corneo- sclera: diameter=12.5-13.5mm)

-they lean on the limb, they can be hard or soft

c.sclerale: diameter=14-25mm

-they are covering completely the cornea

-they lean on the sclera

3. After the ay they are made

a. hard(rigid)- contact lenses not permeable for gases

-contact lenses from PMMA

-contact lenses from CAB

-contact lenses from pure acrilated silicone

-contact lenses from butilstiren

-contact lenses from florocopolimeres

-contact lenses from polysulfanecapolimers

b. soft(supple)-from HEMA

-again HEMA

c." piggy back"

4. after the optical characteristics

a. spherical

b. torics spheres cylindrical simple or biotic

c.mono focal

d. bifocals with simultaneous or alternative view

e. bifocals

5. after the way of using

A.discontinus wearing (daily)-the contact lenses are wearing only for 8-12 hours. They can be caps on the eye when sleeping.

B.prolonged wearing-lenses for 3-6 days continuous (including nights) with one half day break a week.

C. permanent wearing-contact lenses are wearing permanently for more weeks and months being in the "disposable system".

The advantages of the contact lenses correction comparing the airy lenses

The contact lenses correction is eliminating all the problems he acquired when the airy lenses are wearing. Because of this fact the correction with the contact lenses is creating indisputable advantages

1. Constant correction in all the views directions

2. The visual field is normal

3. Is not creating problems of special localization

4. Reducing the man faction, obtaining an anizeiconia of 8-10 %(in it's supported limits)

5. Negligible prismatic effect, if this contact lens is well focus

6. It's eliminating the optical aberrations of the airy correction, aromatically, spherical, the"comet" phenomena, "Jack in the box' phenomenal destroying the distortion phenomenal

7. It offers a superior binocular view or it make's possible the binocular view in case of aneometropy.

8. It creates an esthetical aspect, eliminating the wearing glasses handicap.

9. When wearing are very commode

With all this advantages the wearing of contact lenses has also these advantages.

1. Ocular and lentricular complications

2. The period in which they are wearing is determined by the human tolerance also by the type of contact lenses

3. Needs a perfect hygiene

4. Contact lenses life is short (1-2 years)

5. Expensive keeping

6. It needs many doctor visits

The contact lenses indications

1. Optical purpose indication

These groups contain patients that can get a satisfying view with the airy lenses or with an alternative of this lens

a. Statically optical indication

-short sightedness is very suitable for correction with contact lenses. This is eliminating the disease caused by the airy lenses with dioptry and higher magnification, improving the visual acuity.

-Hypermetropia over the 6-8 D

-regular astigmatism

-irregular astigmatism with keratinous can be corrected with hard contact lenses after the correction plan with airy lenses failed and before the corneal transplant become vital.

-the nistagmus associated with the refraction vice

b. Dynamic optical indication

- The prezbiopia

c. Associated optical indications

Universal afakia with a good sight at the congener eye. The contact lenses use makes possible the fusion and the reappearing of the binocular view, trying to beat the problems of the airy lenses. In the bilateral afakus, especially in children can be used the contact lenses for permanent wearing; this lenses made the visual field end the visual acuity better.

Anizometropia more then 2D

Optical rehabilitation after keratoplastics

Therapeutically indication

The soft contact lenses could serve like solutions in treating some corneal affection. The contact lenses are a focal so these lenses are not discomforting the view so they can be used as a short period treatment or longer period in the next affections or diseases.

A. Corneal irregularities

A contact lens could replace a irregular superficial surface of the cornea through a flatter and regular surface. So, the visual acquits will be better if the irregularities are not severe.

b. DISFUNCTION EPITELIALE DE CICATRIZARE

A contact lens is also good for a quick heeding of the epithelial diseases and also for protecting the new corneal skin by the movement of the eyelid and to permit the development of the contacts between the hemidesmozomi and Brown membrane.

C.EROZIUNI CORNEENE RECIDIVANTE

If they are associated with a corneal disease the long period wearing of the contact lens is necessary. If it's about the corneal Erosions posttraumatic, the lens wearing can be interrupted after few weeks

D.KERATOPIA BULOASA

In this case, the contact lens could act antialgical protecting the nerves because the trauma created by the movement of the eyelids. Also the contact lens flatters the BULA EPITELIALA and which makes from her EDEM EPITELIAL DIFUZ and the additional installing of hyper tonic solution (5%) could be helpful in reducing the Edam and for improving the visual acuteness.

E. KERATITA FILAMENTOASA

Can be treated with soft contact lens combine with artificial tears. The contact lenses indication in this case is based on the fact that the lens is reducing the lost of the existent tears. The main problem that could appear is the cornea infection, drying up the contact lens and the mucus warehouse on the lens. This treatment is good in case the patient could be frequently under control and to prevent complications.

f. PLAGI CORNEENE PERFORANTE

Plague corneae's small perforated or neperforante, which are not in for a surgical intervention, could be covered with soft contact lenses like a temporary solution, to give enough time for healing.

g. PTOZA PALPEBRALA

Contact lenses can be used at the patients with ptoza palpebrala but without the Bell phenomenon in which the chirurgical correction is contra indicated in this case, the contact lenses providing the risk of kertopia in sleep.

h. Pleopthic therapy

The contact lenses can be used like occlusion in amblyopic treatment, at children's can not tolerate the conventional occlusion.

i) Vehicle for drugs
By combining a contact lens with a drug and its contact with the cornea make possible to use it as vehicular.
j) Protection of cornea
By wearing the lens on eyes with trichinas, cornea is being protected.
k) Prevention of simblefaron
In the case of serious chemical injuries, the wearing of the lens in the acute phase of prevents the formation of simblefaron.
l) Recovering after keratoplastie and photokeratectomie
Contact lens could be used, for therapeutic purposes, after the transplant of cornea or refractive surgery.



3. Esthetic, cosmetic and psychological indications
a) Circumstances indications
- to the persons for whom the glass wearing is an esthetic handicap and from the whish to become independent
- to the persons that want to change the color of the iris;
b) Necessity indications
- persons who suffered an eyeball injury and whish to hide the unaesthetic view;
- aniridie and policorie or colobom irian.


4. Professional indications
- to those whom visual accuracy has to be very good
- as opposite to the glasses the contact lens don’t steam and do not need to be cleaned, allowing the free movement

Counter indications for wearing the contact lens
These are usually linked to the following conditions:
1. Lack of motivation represents an important obstacle in the prescription of contact lens.
2. The inability to ensure the hygienic maintenance and a regular visit to the physician; this is the reason why it is not recommended for kids under 14 years old and teenagers with psychological disease;
3. Inability to handle the lens: persons with Parkinson, attrite and old.
4. People afraid to put and take out the lens from the eye
5. Contaminated atmosphere (volatile acids and bases)
6. Cornea diseases
7. Chronic or acute inflammation of cornea
8. Conjunctiva diseases, chronic or acute inflammations
9. Pathology, of eyelash or tiers system
10. Allergy against the normal type of chemicals contained in the maintenance solutions
11. Epilepsy

There are also situations when the probability to wear contact lens is diminished:
         
        
) the chronic or allergic respirator affection (asthma) ;wearing contact lens can produce hipper tears and photophobia

b) The dry eye syndrome

c) The granular lids

d) Eye surgery or ocular muscle which can determine the development of cornea new blood sanguineous vessel or the endothelial affection;

e) Some cases of strabismus;

f) Burden (the initiation of wearing contact lens in burden can be a problem)but also in some cases are no problems with wearing contact lens;

g) Taking some pills: ant conception or antihistaminic pills;

Types of lens used for improving eyesight

Lens used for eye correction are meant to correct some refraction distortions of the eye .The lens are made of glass or plastic material. From the construction view, lens can be

-Spherical

-Plane

-a spherical

The lenses of the glasses have propriety to diverge the light two times: at angle of incidence and at angle of emergence.

The curve rays of the lens surface or generally the distances are considered positive if they are in the same way like the light rays.

STAS 10150-75 is referring to the lens for the symmetrical axe glasses, for the astigmatic glasses and for prism glasses, which are made of colorless glass.

In accordance with the admitted deficiency, lenses can be arranged in three classes.

1. The lenses of the symmetrical axis glasses.

This kind of lenses is made of two spherical surfaces or one spherical surface and one plane surface. The spherical lenses have two centers curves and the line that unites these centers it is named optical axis. The extremity of the lens is a cylinder surface and its axe represents the geometrical axe of the lens.

If these two: the geometrical axe and the optic axe coincide then the lens is central.

The spherical lenses can be divided in two categories:

-Convergent lens

-Divergent lens

The convergent lens

This type of lens has the propriety to bring together all the lights rays, which are parallel with the optical axe, in a spot named the focus reflection. The lens deviate the light rays towards the optical axe.

a) Biconvex lens for glasses are the slackest from the forming of the image point of view .Not even the light rays, which are parallel with the optical axe, and not even the ray, which falls oblique on the surface can create a correct image. This lens is made up to 6 dioptres out 0.25 in 0.25 dioptres in growing order and after that value out 0.5 in 0.5 dioptres. The diameter of this lens can alter between 48 and 72 mm. The biconvex lens can create a perfect image only right around in this optical center, on a small surface .At the end of the lens appears some geometrical defects and also chromatically distortions.

b) The plane convex lens for glasses is bound by a spherical surface and by a plane surface .The light rays are entering in the spherical surface and are exiting from the plane surface.

c) The periscope lens for glasses is lens formed of two surfaces: one is a convex surface and the other is a concave surface in which the entire value of the dioptres power, that contains a surface with a small curve, is 1.25 dioptres. This type of lens is better than the other ones.

d) The menisci lens for glasses that have a convex and a concave surface. The inner area has a curve ray of 87.17 mm. that is equivalent to 6 dioptres. From the quality point of the image these lenses are better than the periscope lens. They give a clear image even if the light rays falls oblique on the lens surface .The image obtained from a point-object it is almost a point .There ii realized because all the dots that forms the inner surface of lens is at the same distance from the cornea. The lenses have property to spread the read beams of the light which falls on their area. This type of lenses in the center are more slim than at the borders and the objects behold by them are moving on the same direction.
a) Biconcave lenses for glasses have both concave areas. They have bad quality.
b) Plane concave lenses for glasses have a plane area and a concave one. Oblique beams light pass by the area of the plane and are deviated only by the concave one.
c) Periscope lenses for glasses are the spherical lenses at which the absolute value of the dioptre power. His area has a curvature of 1, 25 DPT. Their values are conducted by the modification of the curve beam of her spherical area. If this beam sinks then the dioptrically value of the lens, grows.
d) Meniscus – concave lenses for glasses are identical with the periscope lenses.
2. Spherical lenses (astigmatic
This lenses are translucent corpus limited by cylindrical, torrical or ellipsoidal areas. In medical optics is used a spherical or torical lenses. A spherical lenses are also named astigmatic, this means them not form a punctual or stigmatically image; them form an oblong image.
· Cylindrical lenses. A plane and a cylindrical area limit them. Them can classify as follows
· convergent- cylindrical lenses
· divergent- cylindrical lenses. These have a cylindrical area inside.
Each plan, which is perpendicular on the axis, forms the focus in a point. This points are situated on a line and form the cylindrical lenses focus axis or the principal axis. Neutral and principal axis are perpendicular. The refraction power is changing gradually from the principal axis where is maximum to the neutral axis where is zero. The lenses with both axes cylindrical are name bi cylindrical lenses. At this type of lenses the two neutral axes are parallels. The cylindrical axis distorts the image.

b) The torical lenses. These are translucent mediums limited by torical areas. Torical area means an area which has in two perpendicular sections, two different beams. The torical area is obtained by rotate a circle arc around an axis which doesn’ t pass through the center of the circle arc and the obtained corp. is a tore.
The outside area of toricar lens is a torical area and the inside one is a plane area. At the concave torical lens the outside area is a plane one and the inside area is a torical one. Torical lenses haven’t neutral axis's, them have two principal axis's in two different directions with different refraction values.

c) The position of the astigmatic axis's and centering system.
Astigmatic axis's have two perpendicular axis's. For example: an astigmatic lens which has the maximum value +40dpt. and the minimum one 0,00dpt. Between the two axis's are smaller values +4dpt, bat bigger than 0,00dpt.
This values sinks gradually to neutral axis.
In general, astigmatism globe isn’t in horizontal or vertical direction; it is under un angle under 360 degrees or 360 degrees.
For correct the astigmatism are used 3 different systems
· Yena system
· Tabo system
· International system
Yena system has two semicircles divided in 2 perpendicular lines. The vertical axis is noted with 0 degrees and the horizontals one with 90 degrees.
Tabo and International systems have 2 semicircles with non- comes between 0 degrees
and 180 degrees.

3. Incorporated lens.
These are translucent mediums limited by a spherical and a spherical axis.
Them can be classify as follows
a) Sphere- cylindrical lens.
They have a spherical axis and a cylindrical one. Those lenses can have both axis concave or convex or a concave axis and the other one, convex. The neutral axis of this incorporated lens on the neutral axis of the lens has the spherical value and on the principal axis the algebraic sum from the spherical and cylindrical value. The difference between the 2 of the values is the power of the cylinder. The smaller power coincides with the spherical power of the lens. The cylinder axis is noted with 3 points.
At the incorporated lenses the globe with the cylinder are convergent or divergent astigmatic lenses.
b) Sphere-torical incorporated lenses.
These are the following values: on a direction they take the spherical value plus the minimum of the torical and on the perpendicular direction they take the spherical value plus the maximum of her torical area.
Spherical-torical lenses can be exteriors or interiors, after her torical area place which can be to the object plan or to the image-plan. Those lenses are astigmatic convergent or divergent. The torical area has antalkali of ± 6 dpt. The cylindrical power of the spherical toricar lenses consists in the difference of the value between the 2 perpendicular axis's. )Incorporated lenses with cross cylinder.
These lenses have differents power on direction of 2 principal axises.

4.Bifocus lenses.
These are translucenrs mediums limited by 3 areas. Them have 2 focus the upper part of the lens is used for seeing at distance and the other part for close approach.
The separation part between the 2 parts of lens can be visible or invisible and its name is skirt.The bifocus lenses with visible line have the following drawbacks:
· At the passing part from the eyesight distance to the close one, appears a bound image
· The skirt disturbs the eye;
· On the skirt can be dust.
The bifocus lenses with invisible line are manufactured of differents chunks of glass. The glass for distance eyesight is thick. At these lenses the skirt is invisible. These bifocus lenses have differents parts for close eyesight. The  circle one(kriptar) is well-known. Them can also be quadrangle or palladium.

5.Trifocus lenses.
Trifocus lenses are translucents mediums which are limited bz 4 until 6 areas with 3 focus.
At the trifocus lenses the parts with differents dyoptrics values can be put in differents ways. These parts can have differents geometric forms: semicircle, square, quadrangle. Barring these, are also known and used other methods. From lenses with more focus are known the lenses with variables powers(multifocus). These multifocus lenses were manufactured at the biginning of XXth century. The characteristic of these lenses is that the passage from o dyoptric to other is continuous. The dyoptrics values  are raising in the direction of the exterior part of lens.
The multifocus lenses aren΄t rife dispite of their advantages. At these lenses isn΄t the problem of a bound image.

6.Prismatics lenses.
Prismatics lenses are translucents mediums limited by 2 planes areas. The bulk border pf the prismatic lens forms the base of the lens.
Any ray of light which pass through the prismatic lens is diverted to the base. The opposite is the apex of the pzramid.
The principal section of a prismatic lens is that section which have a perpendicular plan on the refraction border and pass through the optic center.
The apex is the intersection between the principal section and the refraction border which is the most thin part of the principal section.

The chromatic seeing and some chromatic mistakes

The color sensation of the chromatic scale sensation, which results from the stimulation of the retina with the help of successive wavelength of the visual angle and the color, puerperal, form the chromatic scale.

The chromatic and a chromatic scale

The objects that are reflected in our eyes all the visible tight hays give us a white color sensation. The dark tint is given by the lack of light .For us to see dark colors we must have a retina. In the blind spot part, you cannot see black, you can't see anything. So it is not impossible that the black tint maybe a sensation given by some activities of the retina. In chromatic scale can be formed many different colors -some observes can signal out 160 different tints. Generally, we give some special names only those that represents different sensations. The wavelengths of normal colors are:

723-647 red, orange 647-585, yellow 585-575, green 575-492, blue 492-455, gray 455-424, violet 424-397.

To one's heart's content color

The expression "one's heart's content" represents the amount of colors or the absence of tempering for the sensation of white. Pale or full of colors tint represents the non-technical name for colors which are not tempered. However the monochromatic light don't produce a color completely free with out the sensation of being white, because the monochromatic rays give birth to some retina processes which are making the white color, as her own special color.

There are some laws in which the color seeing is very well explain. Some of them are more in the psychological domain.

1. Mixture or combined colors-when two or three wavelengths falls on the same surface of the retina, the sensation which results it is much different from the one which is provide by individual wavelengths.

2. Elementary colors -the experiences with combine colors shows that from the specter can be choose three wavelengths, one from the extremities of the red color, an other one from the extremities of the blue color, and one from the middle where the combination is different because of the proportion, generating a sensation of white, of a color hue with an intermediary tint or a purple out of specter (obtained during the mixture of two extremities of a spectrum). These three wavelengths are designated as elementary colors.

3. Complementary colors- for each color exist a complement, which combined with the main color, can create white. Because the spectrum colors are different it will be need different intensity tints. The colors that are closer in the spectrum series than the complementary colors give after there are combined an intermediary color.

4. After image - after a person stops looking at a color she will continue seeing that color for a short period of time (positive) or maybe she can see a complementary color (negative). This is a retinal phenomenon.

5. Contrasting colors-if a piece of blue paper it is situated on a yellow paper, there colors will be more accented because of the contrast between then.

Theories about the color sight

There were released many theories about the color sight. Neither one of those theories explain the real problem. Young and Helmholtz made the oldest and the simplest theory.

Stated by Thomas Young in 1801 and modified after a short period a time by Helmholtz, this theory issue the fact that there are three fundamentals colors: red, green, violet .In correspondence with these colors there are three classes of cones which contains three photochemical substances. The decomposition of these substances stimulates different nervous fibers and some impulses are conduced to different types of nervous cells in the visual cortex. Because of these facts the theory presumes that there are some nervous fibers, which correspond to some photochemical substances like red, green, violet.

When these three types of cones are pushed in equal mode results a sensation of white .The other colors, including yellow, are composed with the combination of some stimuli of those three receivers but in different proportions. It is also presumed that each photochemical substance acts in a certain degree of the visible rays from the spectrum, but if the wavelengths are very powerful at the red extremity than the spectrum acts most powerful on the red substance.

Helmholtz theory is in concordance with the nervous energy theory, because each photochemical substance serves for the stimulation of a nervous fiber, and the quality of the nervous sensation is in function of the ending fiber in the brain.

Color receiver- in the past the number of hypothetical color receivers and their sensibility for the wavelength were inferred specially by some technical experience made from the combination of some colors. Granit succeeds, with the help of some microelectronics, to register impulse from the ganglion cells isolated from a mammal retina. In this way was determined the minimal quantity of wavelength, proper for unloading the ganglion cells, namely the visibility curve for a retina. All the unites, which were studied, presented some kind of sensibility regarding the large range of the wavelength. The sensibility curve obtained at the eye, which became adapted at the dark, was in concordance with the absorption curve of purple

Visually .In the adaptation conditions at the light was obtained a similar curve with the visually photo curve. Polyah showed that the rods end the cones converge through bipolar cells in the same ganglion cell.

The displace from the scotopice curve to the photopical one means, probably, that the rods stops working at the intensity of the cones. Grant named the answer:" answer scolopic and photopic dominator". The constant

Answer is probably achromatic

Towards these dominant answers, some parts of the eye adapted to the light respond to a small scale of wavelength. There is the modulator answer; he can represent the activity of some individual cones. The visual curves tend to differ one from another but they can be integrated in three groups: red-yellow (500-700my), green (520-540) , and blue(450-570 my). The visual curve establish on other curves, which are in a proper concordance with the human eye. This proves that the three receiver's theory by Helmholtz can be real in the statistic way- the joining of the cones in three groups, when the receivers are small, but not sensible identical. Apparently, many ganglion cells let off many receivers; each one is sensible at a small gamma, and these together can offer the ganglion cells an identical sensible curve with the one of the human eye. These kinds of units can create the sensation of white. Other ganglions cells connected to the isolated receiver can serve for the deferential of colors.

Clinic correlation. Some anomalies of the color sight

The discovery of "excitation colors"(1794) it is due to the English physicist John Dalton, who couldn't also see the colors.

The color classification of color excitation was made on Young-Helmholtz theory of those three special receivers. The color excitation can be set about the modification of one of these receivers.

The color sight deficiency is no longer described in excitation terms for red, green, violet. Instead of these classification van Kries gave other categories: protanopia, deuteranopia and tritanopia implicating a simple deficiency of the first (protons), of the second (deuterons) or of the third (trios) receiver.

The suffixes "anomaly" and "anomie" stress the difference between the short color sight and the excitation for colors:

1Trichromatici -normal chromatic sight

-Protanoanomalie

-Deuteranomalia

2 Dichromatic: -protanopie

-Deuteranomalia

-Tritanopie

3 Monochromatic

This classification, like the related theory of Young-Helmholtz, characterize in a previous way the objectives phenomenon's of the mixture of colors at the persons with blindness, for colors and is not used to describe the spectrum's aspect. Dichromatic (colors orientation)

When different colors of light propagate at different speeds in a medium, the index of refraction is dependent of the wave of light. This phenomenon is known under the name of dispersion.

A well-known example is the glass prism, which spread an incidental ray of white light in a rainbow of colors. The photo lens doesn't refract all the constituted colors of the incidental light in equal angles, that's why is necessary of a special effort to create the proof -reader lens which unit all the colors in the same focus. The chromatic aberrations (A.C) are those deviations from the perfect forming of the image, which lead to dispersion.

In the case in which The Siegel aberrations are mono-chromatic, these happen even in the case of the light of one color, even if in general these aberrations of chromatic are making there appear in the case of polio-chromatic light.

Like discrimination between the two types of chromatic aberrations, the Chromatic Aberration Longitudinal (LCA) consists in the lens's inability to transmit different colors in the same plan of focalization. The main characteristic is that the rays of light of different colors are laid on the optic ax too but they are on a longitudinal direction.

This strange behavior is elucidated in the next picture for a distant source of light.

In this scheme, just the green light is in the optic ax's center. The green and red lights have a, so called," circle of confusion" in the movie's plan and that's why they're not included correctly in the picture

The transmitted rays under an oblique incident drive at a Transversal Chromatic Aberration, known under the name of lateral color. It refers at a lateral setting of The Plans of Focalization of different colors doesn't coincide with the rays of light. In the absence LCA all the colors are centered in the same plan, but the picture's quality depends of the light's intensity.

This behavior is illustrated in the next picture.

The forming TCA involves a big intensity of light, in the same time in which the forming LCA in lens doesn't require a variable focal of light.

This thing looks intuitive but in the case of lens of correction for LCA, the main lens doesn't have to coincide necessary for all the colors. So long that the focal of light is determinate by the distance from the real main plan to the picture's plan, the focal of light it could depend of the light's intensity even when they're in the same plan.

The previous pictures represent two simplified cases for that in practice the lateral and longitudinal components are coexisting.

A polio-chromatic subject fills a certain volume in the imagination's space, which is completed by a multitude of monochromatic pictures of different size and positions.

The chromatic aberrations manifest in angles big opened of older models of lens and in the case of new lens too. The aberrations doesn't register in the in the case of a uniform intensity of color's glow; these manifesting them self just a long of the granites which separate the darkness parts from the one with a bigger bright of the pictures.

These been told, the description of the perceptible effects of CA are presented also in Literature .It says that TCA is a graver then LCA because the last give birth to some 'threads of color' when the other reduce the picture's clarity. Oberkochren has a different opinion about LCA naming the graver aberration of chromatic's view.

Defects of the chromatic view

The most known from these one is the confusing of the red and green colors which is caused by the missing of the cellules with cones in the retina.

The spectrum

For the science man the light is one of the electromagnetic wave's forms. In the electromagnetic spectrum contained between the radio's waves and gamma rays occupies a small place. The radio's waves can touch the gamma raise's length only with subunits of the centimeter. Measured in nanometer, the visible length is placed on the length of wave contained between 400-700 nanometers.

In the chromatic picture the yellow places the biggest length of wave, orange a red colors (559-700 nanometers), and the medium length is placed by view and to the shortest is corresponded to blue and violet. The white light is the result of this color's mixture.

She is able to decompose like at a rainbow just when she's passed into a prism.

It is surprising that the retina reaction and at the colors which pass from ultraviolet, so placed out of the normal spectrum. This thing it is not obvious because the lens flirts the ultraviolet rays before these one got on the retina. Just sick people whose lens was took off chirurgical (because of cataract) is replaced with lens of glass or plastic material, see objects lighted by ultraviolet rays which stay invisible for the see people with lens.

The spectrum with a spectacular picture

The colored objects are seen colored with the pigment's help. These substances flirts selective certain lengths of wave, some are kept, others are liberated. The result of this flirt is explained by the fact that the ray of light which penetrate in the eyes doesn't contain in equal measures different zones of lengths of wave and to regulate this disequilibria the eye separate in a certain color. This process takes place in cellules with cones, which answer into electric impulses at the receptivity of the first color that is corresponded. The same impulses prevent also the complementary color's perception .So; the red color excites the cellules with cones sensible at red, whose signals are appreciated by the brain like being red, but under the influence of these signals the sensibility at view. The blue signals prevent the red and green signals (which together form yellow, the complementary color of blue).

All that damage the cellules discompose with cones drive to achromatopia. This hereditary disease touches more the men.

If they are missing cellules with cones sensible at red, the red light, which penetrates in retina, doesn't form the red color's perception, but it doesn't prevent the green color's perception.

The troubles of chromatic views appear the retina contains all the types of cellules with cones but their proportion it isn't proper

This manifestation is called tricromazie, into the equilibrium's deregulate appear inexactly perception of color because the eye sensibility falls in a certain zone of the spectrum.