How The Eye Works

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How The Eye Works
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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.

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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.

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Refractive errors myopia

• Close objects will look clear, but distant
  objects will appear blurred (hence,
  nearsightedness).

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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.

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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
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Refractive errors astigmatism

• In astigmatism, the cornea is curved
  unevenlyshaped 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
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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.

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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
  corneathey refocus light rays on the retina.

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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)

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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.

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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.
• http//hyperphysics.phy-astr.gsu.edu/hbase/vision/
  rodcone.html

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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.

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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.
• http//hyperphysics.phy-astr.gsu.edu/hbase/vision/
  rodcone.html

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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.

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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
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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
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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
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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.

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Common eye disease

• Cataract
• Age-related cataract is the most common form.
• The eyes 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
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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.)

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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.

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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.

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Preserve good vision with regular visits to an
ophthalmologist

• Visit your ophthalmologist at the following
  intervals
• Age 20-29 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 30-39 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 40-64 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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Bionic Eye

• http//news.nationalgeographic.com/news/2008/04/08
  0422-bionic-video-ap.html
• http//news.nationalgeographic.com/news/2008/05/08
  0507-bionic-video-ap.html

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Lasik Surgery

• http//www.mayoclinic.com/health/lasik-eye-surgery
  /MM00607