How The Eye Works

2 The healthy eye Light rays enter the eye through the clear cornea, pupil and lens. These light rays are focused directly onto the retina. The retina converts light rays into impulses, sent through the optic nerve to your brain, where they are recognized as images. 70% of the eye's focusing power comes from the cornea and 30% from the lens.

3 Refractive errors: myopia Myopia (nearsightedness) occurs when the eyeball is slightly longer than usual from front to back. Causes light rays to focus at a point in front of the retina, rather than directly on its surface.

4 Refractive errors: myopia Close objects will look clear, but distant objects will appear blurred (hence, “nearsightedness”).

5 Refractive errors: hyperopia Hyperopia (farsightedness) This vision problem occurs when light rays entering the eye focus behind the retina, rather than directly on it. The eyeball of a farsighted person is shorter than normal.

6 Refractive errors: hyperopia In adults (but not necessarily children) distant objects will look clear, but close objects will appear blurred (hence, “farsightedness”). Hyperopia, or farsightedness

7 Refractive errors: astigmatism In astigmatism, the cornea is curved unevenly—shaped more like a football than a basketball. Causes light rays to focus on two points in the back of your eye, rather than on just one. Distant and close objects may appear blurry. Astigmatism occurs when light passes through uneven cornea

8 Refractive errors: presbyopia Presbyopia is a normal condition in which your eyes gradually lose the ability to focus things up close. Begins at around age 40.

9 Correcting refractive errors Eyeglasses are the most common methods of correcting refractive errors; they refocus light rays directly on the retina. Contact lenses: Acting like eyeglasses, contact lenses float on the tear film that coats the cornea—they refocus light rays on the retina.

10 Correcting refractive errors Refractive surgery: A variety of surgical procedures that permanently alter the eye such that light rays are refocused on the retina to improve vision. The most common refractive surgical procedures are:  Laser In Situ Keratomileusis (LASIK)  Epithelial Laser In Situ Keratomileusis (Epi-LASIK)  Photorefractive Keratectomy (PRK)  Laser Epithelial Keratomileusis (LASEK)  Conductive Keratoplasty (CK)  Astigmatic Keratotomy (AK)  Radial Keratotomy (RK)  Phakic Intraocular Lenses (IOLs)  Accommodative IOLs and multifocal IOLs  Refractive Lens Exchange (RLE)  Intrastromal Corneal Ring Segments (INTACS)

Seeing Color Rods and Cones The retina contains two types of photoreceptors, rods and cones. The rods are more numerous, some 120 million, and are more sensitive than the cones. However, they are not sensitive to color. The 6 to 7 million cones provide the eye's color sensitivity and they are much more concentrated in the central yellow spot known as the macula. In the center of that region is the " fovea centralis ", a 0.3 mm diameter rod-free area with very thin, densely packed cones.retinafovea centralis 11

cones The experimental evidence suggests that among the cones there are three different types of color reception. Response curves for the three types of cones have been determined. Since the perception of color depends on the firing of these three types of nerve cells, it follows that visible color can be mapped in terms of three numbers called tristimulus values. Color perception has been successfully modeled in terms of tristimulus values and mapped on the CIE chromaticity diagram.Response curvestristimulus valuesColor perceptionCIE chromaticity diagram 12

rods The rods are the most numerous of the photoreceptors, some 120 million, and are the more sensitive than the cones. However, they are not sensitive to color. They are responsible for our dark- adapted, or scotopic, vision. The rods are incredibly efficient photoreceptors. More than one thousand times as sensitive as the cones, they can reportedly be triggered by individual photons under optimal conditions. The optimum dark-adapted vision is obtained only after a considerable period of darkness, say 30 minutes or longer, because the rod adaption process is much slower than that of the cones.rodsphotoreceptorsconesscotopicphotons 13

rods The rod sensitivity is shifted toward shorter wavelengths compared to daylight vision, accounting for the growing apparent brightness of green leaves in twilight. While the visual acuity or visual resolution is much better with the cones, the rods are better motion sensors. Since the rods predominate in the peripheral vision, that peripheral vision is more light sensitive, enabling you to see dimmer objects in your peripheral vision. If you see a dim star in your peripheral vision, it may disappear when you look at it directly since you are then moving the image onto the cone-rich fovea region which is less light sensitive. You can detect motion better with your peripheral vision, since it is primarily rod vision.fovea region 14

15 Common eye disease In addition to refractive errors, many kinds of eye disease can affect your sight; vision changes are not always evident right away.

16 Common eye disease Age-related macular degeneration (AMD) Many contributing factors, including the natural aging process. Early stage AMD may be hardly noticeable; usually a bilateral disease with one eye more affected than the other. Symptoms: blurry vision; dark or empty areas in central vision; straight lines may look wavy. No specific treatment for “dry” form of AMD; laser, photodynamic therapy (PDT), or anti-VEGF drugs may treat the “wet” form of AMD. With AMD, dark areas may appear in your central vision

17 Common eye disease Glaucoma A leading cause of loss of vision and blindness in the United States, especially for older people. Disease of the optic nerve; commonly due to increased intraocular pressure (IOP). When optic nerve fibers are damaged, blind spots develop. Blind spots or loss of vision usually go undetected until the optic nerve is significantly damaged. Treatment (all aimed at lowering intraocular pressure): eyedrops, laser surgery or conventional surgery to shunt fluid from the eye may be required, depending on the type of glaucoma and its stage. Early detection and treatment are keys to preventing vision loss from glaucoma. Normal vision Vision as it might be affected by glaucoma

18 Common eye disease Diabetic retinopathy Diabetes Mellitus is the inability of the body to use and store sugar properly, resulting in high blood sugar levels. Results in changes in veins, arteries and capillaries in the body, including the eyes. Damage occurs to the fragile blood vessels of the retina. Symptoms: blurred, decreased vision, loss of vision. With diabetic retinopathy, cholesterol or other fat deposits from blood, called hard exudates, may leak into the retina

19 Common eye disease Diabetic retinopathy Treatment: Usually laser surgery; occasionally conventional surgery. You can significantly lower your risk of vision loss by maintaining strict control of your blood sugar level and frequent visits to your ophthalmologist.

20 Common eye disease Cataract Age-related cataract is the most common form. The eye’s normally clear lens becomes cloudy, preventing light from focusing sharply on the retina. Symptoms: blurry vision; glare or light sensitivity; poor night vision; difficulty driving at night; yellowing or fading of colors; increased light required to read comfortably. Yellowing of colors is a symptom of cataract

21 Common eye disease Cataract Treatment: surgery removes the cloudy lens and replaces it with an artificial intraocular lens implant (IOL). If cataract symptoms are not adversely affecting your daily activities, you may not need surgery. (Simply have eyeglass prescription changed as needed.)

22 Preserve good vision with regular visits to an ophthalmologist or other medical professional Infants and young children should visit an ophthalmologist or other medical professional at the following intervals:  Newborn to 3 months  3 to 6 months  6 months to one year  3 to 3 1/2 years  5 years Other medical professionals include pediatricians, family physicians, nurse practitioners or physician assistants.

23 Preserve good vision with regular visits to an ophthalmologist or other medical professional Age-appropriate eye and vision evaluations should be performed in the newborn period and at all subsequent health supervision visits, since different childhood eye problems may be detected at each. The screening process includes a history in order to assess risk factors as well as an examination. Vision testing should be performed for a child at the earliest age that is practical, and it is recommended for all children starting at 3.

24 Preserve good vision with regular visits to an ophthalmologist Visit your ophthalmologist at the following intervals: Age years: At least once during this period  Those with risk factors for glaucoma (people of African descent or those who have a family history of glaucoma) should be seen every 3-5 years. Age years: At least twice during this period  Those with risk factors for glaucoma (people of African descent or those who have a family history of glaucoma) should be seen every 2-4 years. Age years: Every 2-4 years. Age 65 years or older: Every 1-2 years. People with diabetes should visit their ophthalmologist once a year.

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