Refraction And Accommodation
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Refraction and Accommodation of the eye
1) Normal refractive condition and accommodation
(1). Ocular refractive system The ocular refractive system is composed of the cornea,the aqueous humor,the lens and the vitreous.
Refractive system
1) Normal refractive condition and accommodation
People can see object clearly,for the light rays sent out by object pass through eye refractive system,form images on retinal macula.
Refractive condition is decided by the refractive power and length of ocular axis.
Reduced eye ： Including a node,a principle node(refracting surface) and two foci
Refractive power
Refractive power is diopter (D), if the focal length of a spherical lens is 1m, its refractive power is one diopter （ D ） . D =1/ f Corneal refractive system : 43.05D Lens refractive system ： 19.11D Total refractive power of globe ： 58.64D
（ 2 ） Emmetropia and accommodation
Emmetropia: The condition is the absence of refractive error. Or nonaccommodated paralle light rays refracted by ocular refractive system accurately focus on the retina which is called emmetropia.
（ 2 ） accommodation
Accommodation ： in order to see near object clearly,the eye increases lens curvature to strengthen ocular refractive power to make near object form clear image on the retina.
accommodation
Accommodation is the eye’s way of changing its focusing distance: the lens thickens, increasing its ability to focus at near. A young person’s ability to accommodate allows him or her to see clearly far away and up close. At about the age of 40, the lens becomes less flexible and accommodation is gradually lost, making close-range work increasingly difficult. This is known as presbyopia.
（ 2 ） accommodation
Accommodative scope ： the differce between far and near points . Far point: Under nonaccommodated condition,the farthest point the eye can see clearly is called far point. Near point:Under largest accommodation, the point the eye can see nearest distance is called near point.
（ 2 ） accommodation
Near reflect ： When both eyes gaze at a quite near object simultaneously, besides accommodation,and corresponding diminution of binocular pupils,there are contraction of both medial rectus muscles and adduction of both eyes( convergence).
2) Ametropia （ nonemmetropia ）
Ametropioa ： The refractive power and the length of the globe are not correlated so that parallel light rays refracted by ocular refractive system do not come to focus on the retina(fovea centralis). Including hyperopia,myopia and astigmatism.Or is the presence of refractive error.
（ 1 ） Myopia(nearsighted ) Under nonaccommodated condition,parallel light rays refracted by ocular refractive system come to focus in front of the retina Classification: ★ Mild ： below -3D ★ Moderate ： from-3D to -6D ★ High myopia ： more than -6D
The far point is in finited distance
（ 1 ） Myopia(nearsighted )
（ 1 ） Myopia(nearsighted )
Nearsightedness or myopia, occurs when light entering the eye focuses in front of the retina instead of directly on it. This is caused by a cornea that is steeper, or an eye that is longer, than a normal eye. Nearsighted people typically see well up close, but have difficulty seeing far away.
（ 1 ） Myopia(nearsighted )
This problem is often discovered in school-age children who report having trouble seeing the chalkboard.
（ 1 ） Myopia(nearsighted )
Detection Nearsightedness is detected with a vision test and refraction.
《 Etiology 》
‹1›.Axial myopia: Axial of the eye is quite long,but the refractive power is normal. *The eye longer than average *Most high myopia are axial
《 Etiology 》
‹2›.Refractive normal but increases.
myopia: The axis is the refractive power
Refractive myopia * The curvature of cornea is large *The curvature of lens increase * Spasm of accommodation
《 Clinical findings 》
‹1›.Visual acuity Near vision may be in normal, distance vision is bad. ‹2›. Asthenopia It is as a result of the disturbance of balance between accommodation and convergence, overusing of convergence, not or less use of accommodation.
《 Clinical findings 》
‹3›. Exotropia Due to contradiction between accommodation and convergence, with the time going, hypofunction of convergence is induced to bring about exotropia .
《 Clinical findings 》
‹4›.Fundus chang A grey crescent in temporal side of the disc may be occurred , choroidal atrophy, posterior sclera staphyloma, macular hemorrhage, lattice degeneration, retinal tear and retinal detachment may be seen. ‹5›.Vitreous liquefaction and opacity
《 Correction of myopia 》
In order to let the parallel light rays focus on the retina, it is necessary to place a concave lens in front of the eye. Besides wearing of glasses,there is contact lens which is worn onto the cornea .
《 Correction of myopia 》
Treatment
The treatment for nearsightedness depends on several factors such as the patient’s age, activities, and occupation. Vision can corrected with glasses, contacts, or surgery. Refractive procedures such as LASIK can be considered for adults when the prescription has remained stable for at least one year.
Treatment Refractive procedures
Clear Lens Replacement (CLR)
Photo Refractive Keratectomy (PRK)
Radial Keratotomy (RK)
LASIK (LAser in SItu Keratomileusis)
RK ( Radial Keratotomy ) is used to treat low to moderate levels of nearsightedness. It was first performed in 1973 by a Russian eye surgeon. Microscopic incisions are placed in radial pattern on the cornea, causing it to flatten.
RK
Radial keratotomy
Radial keratotomy: Eight to 16 radial incisions are made in peripheral cornea to reduce ocular refractive degree so that to get the aim of correction of myopia .
PRK( Photo Refractive Keratectomy ) is the original procedure that first used the Excimer laser to correct low to moderate amounts of nearsightedness.
The patient's data is entered into a computer that controls the laser. The laser sculpts the central optical area of the cornea as the surgeon carefully monitors the procedure through a microscope attached to the system. Tissue is removed or ablated until the cornea is shaped into the desired curvature.
LASIK (LAser in SItu Keratomileusis) is a procedure used to correct vision problems such as myopia nearsightedness), hyperopia (farsightedness), and astigmatism. A special instrument called a keratome is used to gently lift a flap of corneal tissue, then laser is used to reshape the cornea. The corneal flap is carefully repositioned.
This procedure allows light to focus more precisely on the retina, resulting in clearer vision. For many patients, LASIK means a new lifestyle - free from the restrictions of glasses.
LASIK
LASIK
LASIK
If you are at least 21 years old and your glasses haven't required a significant prescription change in one year, you may be a candidate for LASIK.
《 Prevention of juvenil myopia 》
‹1›.Culture good health habit to use eyereading distance is about 30-35cmnot to read lying in bedin dark place or in sunto avoid long-term readingworking or watching TV in near distance as possible. ‹2 ›. To examine visual acuity and eye regularly.
《 Prevention of juvenil myopia 》
‹3›. To strengthen constitution to pay attention to nourishment let eye and whole body develop normally.
（ 2 ） .Hyperopia
Hyperopia is the state in which the unaccommodated eye would focus the image behind the retina. Parallel light rays refracted by ocular refractive system focus behind the retina, not form clear image.
Hyperopia
Hyperopia
Hyperopia
Farsightedness or hyperopia, occurs when light entering the eye focuses behind the retina, instead of directly on it. This is caused by a cornea that is flatter, or an eye that is shorter, than a normal eye. Farsighted people usually have trouble seeing up close, but may also have difficulty seeing far away as well.
Hyperopia
Young people with mild to moderate hyperopia are often able to see clearly because their natural lens can adjust, or accommodate to increase the eye’s focusing ability. However, as the eye gradually loses the ability to accommodate (beginning at about 40 years of age), blurred vision from hyperopia often becomes more apparent.
Hyperopia
Signs and Symptoms Difficulty seeing up close Blurred distance vision (occurs with higher amounts of hyperopia) Eye fatigue when reading Eye strain (headaches, pulling sensation, burning)
Hyperopia Classification Mild: less than+ 3D, moderate: +5D or less than +5D High: more than +5D
《 Etiology 》
‹1›.Axial hyperopia is indicated that ocular axis is short but refractive power is normal. ‹2›.Refractive hyperopia is indicated that ocular axis is normal, but refractive power is weak.
《 Clinical findings 》
‹1›.Visual acuity Mild hyperopia may be compensated by accommodation, so far or near vision may be normal in mild hyperopia of juvenile,if the degree of hyperopia is large or accommodative power decreases with aging,both far and near vision decrease in different degrees,and near vision is much worse than far one.
《 Clinical findings 》
‹2›. Asthenopia Long-term near work, excessive accommodation often may induce asthenopia, its symptom is blurred vision, distending pain in superciliary arch. ‹3›. Esotropia Due to excessive accommodation leading to more convergence which is easy to induce esotropia .
《 Clinical findings 》
‹4›. Fundus examation more than moderate degree, the disc is smaller than normal with blurred margin ,a little elevated , quite similar to papillitis or edema may be seen.
Hyperopia
Detection and Diagnosis Hyperopia is detected with a vision test called a refraction. Young patients’ eyes are dilated for this test so they are unable to mask their farsightedness with accommodation. This is called a wet refraction.
Hyperopia
Treatment The treatment for hyperopia depends on several factors such as the patient’s age, activities, and occupation. Young patients may or may not require glasses or contact lenses, depending on their ability to compensate for their farsightedness with accommodation. Glasses or contact lenses are required for older patients.
Correction of hyperopia
Corrected by convex lens which makes parallel light rays become convergent ones,so that the focus falls on the retina . Contact lens may be used for correction too.
Correction of hyperopia
（ 3 ） . Astigmatism
Due to the difference of refractive power in every meridian of eye, so outside light rays can’t focus on the retina to form clear image.
（ 3 ） . Astigmatism
Astigmatism means that the cornea is oval like a football instead of spherical like a basketball. Most astigmatic corneas have two curves – a steeper curve and a flatter curve. This causes light to focus on more than one point in the eye, resulting in blurred vision at distance or near. Astigmatism often occurs along with nearsightedness or farsightedness.
《 Etiology 》
Most common cause is that the radii of curvature of cornea and lens are different in each meridian,generally, their difference between two main meridians is biggest. May be divided into two sorts: regular and irregular ones.
Astigmatism
‹1›. Regular astigmatism
Regular astigmatism indicates the difference of refractive degree of cornea on two meridian vertical each other, be corrected by cylindrical lens. Divided into 5 sorts
‹1›. Regular astigmatism
1›. Simple hyperopic asstigmatism 2›. Simple myopic astigmatism 3›.Compound hyperopic astigmatism 4›. Compound myopic astigmatism 5›. Mixed astigmatism
《 Clinical finding 》
‹1›Visual acuity Blurred both in seeing far or near. ‹2›Asthenopia, distending ophthalmalgia, head ache , tearing,nausea,vomiting. ‹3›fundus the disk is vertically elongated with blurred margin.
Astigmatism can be detected and measured with corneal topography , keratometry, vision testing and refraction.
《 Treatment 》
Astigmatism can be corrected with glasses (cylindrical lens ), contacts, or surgically. The most common surgeries used to correct astigmatism are astigmatic keratotomy (procedures that involve placing a microscopic incision on the eye) and LASIK. The objective of these procedures is to reshape the cornea so it becomes more spherical or uniformly curved.
（ 4 ） . Anisometropia
Anisometropia indicates the difference of binocular refractive power is more than 2D,if the difference is more than 2.5D,because the images on binocular retina are different in size which arouse difficult in fusion,destrying of binocular vision.
(5). Presbyopia
Presbyopia, also known as the “short arm syndrome,” is a term used to describe an eye in which the natural lens can no longer accommodate. With aging, accommodative function decreases gradually, at about 40-45 years of age,difficult takes place in near work or reading,this decrease of physiologic accommodation caused by aging is known as presbyopia.
《 Clinical finding 》
Most people first notice difficulty reading very fine print such as the phone book, a medicine bottle, or the stock market page. Print seems to have less contrast and the eyes become easily fatigued when reading a book or computer screen. The symptom of presbyopia is difficult to see near thing.
Early on, holding reading material further away helps for many patients. But eventually, reading correction in the form of reading glasses, bifocals, or contact lenses is needed for close work. However, nearsighted people can simply take their glasses off because they see best close-up.
Signs and Symptoms Difficulty seeing clearly for close work Print seems to have less contrast Brighter, more direct light required for reading Reading material must be held further away to see (for some) Fatigue and eyestrain when reading
Detection and Diagnosis Presbyopia is detected with vision testing and a refraction.
《 Treatmrnt 》
The treatment for presbyopia is very simple, but is entirely dependent on the individual’s age, lifestyle, occupation, and hobbies. If the patient has good distance vision and only has difficulty seeing up close, reading glasses are usually the easiest solution. Presbyopia needs convex lens to compensate the deficiency of accommodation,making its near point in normal scope.
For others, bifocals (glasses with reading and distance correction) or separate pairs of reading and distance glasses are necessary. Another option is monovision: adjusting one eye for distance vision, and the fellow eye for reading vision. This can be done with contact lenses or permanently with refractive surgery.
1) Normal refractive condition and accommodation
(1). Ocular refractive system The ocular refractive system is composed of the cornea,the aqueous humor,the lens and the vitreous.
Refractive system
1) Normal refractive condition and accommodation
People can see object clearly,for the light rays sent out by object pass through eye refractive system,form images on retinal macula.
Refractive condition is decided by the refractive power and length of ocular axis.
Reduced eye ： Including a node,a principle node(refracting surface) and two foci
Refractive power
Refractive power is diopter (D), if the focal length of a spherical lens is 1m, its refractive power is one diopter （ D ） . D =1/ f Corneal refractive system : 43.05D Lens refractive system ： 19.11D Total refractive power of globe ： 58.64D
（ 2 ） Emmetropia and accommodation
Emmetropia: The condition is the absence of refractive error. Or nonaccommodated paralle light rays refracted by ocular refractive system accurately focus on the retina which is called emmetropia.
（ 2 ） accommodation
Accommodation ： in order to see near object clearly,the eye increases lens curvature to strengthen ocular refractive power to make near object form clear image on the retina.
accommodation
Accommodation is the eye’s way of changing its focusing distance: the lens thickens, increasing its ability to focus at near. A young person’s ability to accommodate allows him or her to see clearly far away and up close. At about the age of 40, the lens becomes less flexible and accommodation is gradually lost, making close-range work increasingly difficult. This is known as presbyopia.
（ 2 ） accommodation
Accommodative scope ： the differce between far and near points . Far point: Under nonaccommodated condition,the farthest point the eye can see clearly is called far point. Near point:Under largest accommodation, the point the eye can see nearest distance is called near point.
（ 2 ） accommodation
Near reflect ： When both eyes gaze at a quite near object simultaneously, besides accommodation,and corresponding diminution of binocular pupils,there are contraction of both medial rectus muscles and adduction of both eyes( convergence).
2) Ametropia （ nonemmetropia ）
Ametropioa ： The refractive power and the length of the globe are not correlated so that parallel light rays refracted by ocular refractive system do not come to focus on the retina(fovea centralis). Including hyperopia,myopia and astigmatism.Or is the presence of refractive error.
（ 1 ） Myopia(nearsighted ) Under nonaccommodated condition,parallel light rays refracted by ocular refractive system come to focus in front of the retina Classification: ★ Mild ： below -3D ★ Moderate ： from-3D to -6D ★ High myopia ： more than -6D
The far point is in finited distance
（ 1 ） Myopia(nearsighted )
（ 1 ） Myopia(nearsighted )
Nearsightedness or myopia, occurs when light entering the eye focuses in front of the retina instead of directly on it. This is caused by a cornea that is steeper, or an eye that is longer, than a normal eye. Nearsighted people typically see well up close, but have difficulty seeing far away.
（ 1 ） Myopia(nearsighted )
This problem is often discovered in school-age children who report having trouble seeing the chalkboard.
（ 1 ） Myopia(nearsighted )
Detection Nearsightedness is detected with a vision test and refraction.
《 Etiology 》
‹1›.Axial myopia: Axial of the eye is quite long,but the refractive power is normal. *The eye longer than average *Most high myopia are axial
《 Etiology 》
‹2›.Refractive normal but increases.
myopia: The axis is the refractive power
Refractive myopia * The curvature of cornea is large *The curvature of lens increase * Spasm of accommodation
《 Clinical findings 》
‹1›.Visual acuity Near vision may be in normal, distance vision is bad. ‹2›. Asthenopia It is as a result of the disturbance of balance between accommodation and convergence, overusing of convergence, not or less use of accommodation.
《 Clinical findings 》
‹3›. Exotropia Due to contradiction between accommodation and convergence, with the time going, hypofunction of convergence is induced to bring about exotropia .
《 Clinical findings 》
‹4›.Fundus chang A grey crescent in temporal side of the disc may be occurred , choroidal atrophy, posterior sclera staphyloma, macular hemorrhage, lattice degeneration, retinal tear and retinal detachment may be seen. ‹5›.Vitreous liquefaction and opacity
《 Correction of myopia 》
In order to let the parallel light rays focus on the retina, it is necessary to place a concave lens in front of the eye. Besides wearing of glasses,there is contact lens which is worn onto the cornea .
《 Correction of myopia 》
Treatment
The treatment for nearsightedness depends on several factors such as the patient’s age, activities, and occupation. Vision can corrected with glasses, contacts, or surgery. Refractive procedures such as LASIK can be considered for adults when the prescription has remained stable for at least one year.
Treatment Refractive procedures
Clear Lens Replacement (CLR)
Photo Refractive Keratectomy (PRK)
Radial Keratotomy (RK)
LASIK (LAser in SItu Keratomileusis)
RK ( Radial Keratotomy ) is used to treat low to moderate levels of nearsightedness. It was first performed in 1973 by a Russian eye surgeon. Microscopic incisions are placed in radial pattern on the cornea, causing it to flatten.
RK
Radial keratotomy
Radial keratotomy: Eight to 16 radial incisions are made in peripheral cornea to reduce ocular refractive degree so that to get the aim of correction of myopia .
PRK( Photo Refractive Keratectomy ) is the original procedure that first used the Excimer laser to correct low to moderate amounts of nearsightedness.
The patient's data is entered into a computer that controls the laser. The laser sculpts the central optical area of the cornea as the surgeon carefully monitors the procedure through a microscope attached to the system. Tissue is removed or ablated until the cornea is shaped into the desired curvature.
LASIK (LAser in SItu Keratomileusis) is a procedure used to correct vision problems such as myopia nearsightedness), hyperopia (farsightedness), and astigmatism. A special instrument called a keratome is used to gently lift a flap of corneal tissue, then laser is used to reshape the cornea. The corneal flap is carefully repositioned.
This procedure allows light to focus more precisely on the retina, resulting in clearer vision. For many patients, LASIK means a new lifestyle - free from the restrictions of glasses.
LASIK
LASIK
LASIK
If you are at least 21 years old and your glasses haven't required a significant prescription change in one year, you may be a candidate for LASIK.
《 Prevention of juvenil myopia 》
‹1›.Culture good health habit to use eyereading distance is about 30-35cmnot to read lying in bedin dark place or in sunto avoid long-term readingworking or watching TV in near distance as possible. ‹2 ›. To examine visual acuity and eye regularly.
《 Prevention of juvenil myopia 》
‹3›. To strengthen constitution to pay attention to nourishment let eye and whole body develop normally.
（ 2 ） .Hyperopia
Hyperopia is the state in which the unaccommodated eye would focus the image behind the retina. Parallel light rays refracted by ocular refractive system focus behind the retina, not form clear image.
Hyperopia
Hyperopia
Hyperopia
Farsightedness or hyperopia, occurs when light entering the eye focuses behind the retina, instead of directly on it. This is caused by a cornea that is flatter, or an eye that is shorter, than a normal eye. Farsighted people usually have trouble seeing up close, but may also have difficulty seeing far away as well.
Hyperopia
Young people with mild to moderate hyperopia are often able to see clearly because their natural lens can adjust, or accommodate to increase the eye’s focusing ability. However, as the eye gradually loses the ability to accommodate (beginning at about 40 years of age), blurred vision from hyperopia often becomes more apparent.
Hyperopia
Signs and Symptoms Difficulty seeing up close Blurred distance vision (occurs with higher amounts of hyperopia) Eye fatigue when reading Eye strain (headaches, pulling sensation, burning)
Hyperopia Classification Mild: less than+ 3D, moderate: +5D or less than +5D High: more than +5D
《 Etiology 》
‹1›.Axial hyperopia is indicated that ocular axis is short but refractive power is normal. ‹2›.Refractive hyperopia is indicated that ocular axis is normal, but refractive power is weak.
《 Clinical findings 》
‹1›.Visual acuity Mild hyperopia may be compensated by accommodation, so far or near vision may be normal in mild hyperopia of juvenile,if the degree of hyperopia is large or accommodative power decreases with aging,both far and near vision decrease in different degrees,and near vision is much worse than far one.
《 Clinical findings 》
‹2›. Asthenopia Long-term near work, excessive accommodation often may induce asthenopia, its symptom is blurred vision, distending pain in superciliary arch. ‹3›. Esotropia Due to excessive accommodation leading to more convergence which is easy to induce esotropia .
《 Clinical findings 》
‹4›. Fundus examation more than moderate degree, the disc is smaller than normal with blurred margin ,a little elevated , quite similar to papillitis or edema may be seen.
Hyperopia
Detection and Diagnosis Hyperopia is detected with a vision test called a refraction. Young patients’ eyes are dilated for this test so they are unable to mask their farsightedness with accommodation. This is called a wet refraction.
Hyperopia
Treatment The treatment for hyperopia depends on several factors such as the patient’s age, activities, and occupation. Young patients may or may not require glasses or contact lenses, depending on their ability to compensate for their farsightedness with accommodation. Glasses or contact lenses are required for older patients.
Correction of hyperopia
Corrected by convex lens which makes parallel light rays become convergent ones,so that the focus falls on the retina . Contact lens may be used for correction too.
Correction of hyperopia
（ 3 ） . Astigmatism
Due to the difference of refractive power in every meridian of eye, so outside light rays can’t focus on the retina to form clear image.
（ 3 ） . Astigmatism
Astigmatism means that the cornea is oval like a football instead of spherical like a basketball. Most astigmatic corneas have two curves – a steeper curve and a flatter curve. This causes light to focus on more than one point in the eye, resulting in blurred vision at distance or near. Astigmatism often occurs along with nearsightedness or farsightedness.
《 Etiology 》
Most common cause is that the radii of curvature of cornea and lens are different in each meridian,generally, their difference between two main meridians is biggest. May be divided into two sorts: regular and irregular ones.
Astigmatism
‹1›. Regular astigmatism
Regular astigmatism indicates the difference of refractive degree of cornea on two meridian vertical each other, be corrected by cylindrical lens. Divided into 5 sorts
‹1›. Regular astigmatism
1›. Simple hyperopic asstigmatism 2›. Simple myopic astigmatism 3›.Compound hyperopic astigmatism 4›. Compound myopic astigmatism 5›. Mixed astigmatism
《 Clinical finding 》
‹1›Visual acuity Blurred both in seeing far or near. ‹2›Asthenopia, distending ophthalmalgia, head ache , tearing,nausea,vomiting. ‹3›fundus the disk is vertically elongated with blurred margin.
Astigmatism can be detected and measured with corneal topography , keratometry, vision testing and refraction.
《 Treatment 》
Astigmatism can be corrected with glasses (cylindrical lens ), contacts, or surgically. The most common surgeries used to correct astigmatism are astigmatic keratotomy (procedures that involve placing a microscopic incision on the eye) and LASIK. The objective of these procedures is to reshape the cornea so it becomes more spherical or uniformly curved.
（ 4 ） . Anisometropia
Anisometropia indicates the difference of binocular refractive power is more than 2D,if the difference is more than 2.5D,because the images on binocular retina are different in size which arouse difficult in fusion,destrying of binocular vision.
(5). Presbyopia
Presbyopia, also known as the “short arm syndrome,” is a term used to describe an eye in which the natural lens can no longer accommodate. With aging, accommodative function decreases gradually, at about 40-45 years of age,difficult takes place in near work or reading,this decrease of physiologic accommodation caused by aging is known as presbyopia.
《 Clinical finding 》
Most people first notice difficulty reading very fine print such as the phone book, a medicine bottle, or the stock market page. Print seems to have less contrast and the eyes become easily fatigued when reading a book or computer screen. The symptom of presbyopia is difficult to see near thing.
Early on, holding reading material further away helps for many patients. But eventually, reading correction in the form of reading glasses, bifocals, or contact lenses is needed for close work. However, nearsighted people can simply take their glasses off because they see best close-up.
Signs and Symptoms Difficulty seeing clearly for close work Print seems to have less contrast Brighter, more direct light required for reading Reading material must be held further away to see (for some) Fatigue and eyestrain when reading
Detection and Diagnosis Presbyopia is detected with vision testing and a refraction.
《 Treatmrnt 》
The treatment for presbyopia is very simple, but is entirely dependent on the individual’s age, lifestyle, occupation, and hobbies. If the patient has good distance vision and only has difficulty seeing up close, reading glasses are usually the easiest solution. Presbyopia needs convex lens to compensate the deficiency of accommodation,making its near point in normal scope.
For others, bifocals (glasses with reading and distance correction) or separate pairs of reading and distance glasses are necessary. Another option is monovision: adjusting one eye for distance vision, and the fellow eye for reading vision. This can be done with contact lenses or permanently with refractive surgery.
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