Physiological optics. Ophthalmologic tools.

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Physiological optics. Ophthalmologic tools.
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• Eye is like a camera. The external object is seen
  like the camera takes the picture of any object.
  Light enters the eye through a small hole called
  the pupil and is focused on the retina, which is
  like a camera film. Eye also has a focusing lens,
  which focuses images from different distances on
  the retina. The colored ring of the eye, the
  iris, controls the amount of light entering the
  eye. It closes when light is bright and opens
  when light is dim.

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• Cornea
• Front part of the eye, transparent. Protects
  front of eye and bends light to form an image on
  the retina.
•   Pupil
• A black hole in the center of the iris. Allows
  light to enter into the eye.
•   Iris
• Pigmented (the color of the eyes). Its muscles
  contract and relax to alter the size of its
  central hole or pupil. Protects the
  photoreceptors in the retina from being damaged
  by too much light.
•   Retina
• The lining at the back of the eye containing two
  types of photoreceptor cells. It is a screen on
  which images are formed as a result of light
  being focused onto it by the cornea and lens.
•  Lens
• Transparent, flexible disc behind the iris
  attached by muscles. Brings the light entering
  through the pupil to a focus on the retina.
•  Macula
• The macula contains a high concentration of
  photoreceptor cells that convert light into nerve
  signals. Send visual signals to the brain.
•  Optic nerve
• Bundle of sensory neurons at back of eye. Carries
  signals from the photoreceptors of the retina to
  the brain.

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• The Eye as an Optical System
• The eye can be considered as an optical system
  with a positive power of about 58 D. It has two
  main refractive elements, the cornea and the
  lens. The cornea bulges in the front of the
  eyeball, and because its first surface is in
  contact with air, it bears most of the power of
  the eye (about 45 D). The eyeball has a mean
  length of 24 mm, and the image is formed at the
  interior of the back side, where the retina is
  found. The aqueous humor, which has a refractive
  index of 1.336, is located in the midst of the
  cornea and the lens. The volume behind the lens
  is filled with vitreous humor, with index 1.337.
  The power of the lens is not fixed, and it can
  expand surface curvature and power via the ciliar
  muscles surrounding it. This process is called
  accommodation, and serves to bring near objects
  into focus.

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• Ammetropies and Refractive Error
• Emmetropia is defined as the condition for which
  the relaxed eye (without accommodation) images a
  distant object onto the retina (fovea).
• We say that the eye suffers from refractive error
  when it fails to bring into focus the image of a
  distance object. The condition in which a
  refractive error occurs is called ammetropia.

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• Myopia The image of a distant object forms
  before the retina because the eye is too
  powerful, too large, or both. Refractive error is
  negative and the remote point is located in front
  of the eye. Near point is also located in front
  of the eye, at a smaller distance from it.
  Glasses or contact lenses with concave lenses
  will correct the eye's error and bring the images
  of far-off objects into sharp focus on the
  retina. Concave lenses curve inward, like the
  inside of a bowl.

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• Hyperopia The image of the distant object forms
  behind the retina as a result of any of the
  followingreasons 1) the eye does not attain
  enough power, 2) the eye is too small, or 3)
  combination of both. In case the refractive error
  is smaller than the amplitude of accommodation,
  the hyperopic eye may bring the images of
  distance objects into focus via accommodation.
  The remote point of the hyperopic eye is located
  behind (to the right of) the same. The near point
  can be located at the right of the remote, when
  amplitude of accommodation is smaller than the
  refractive error. Otherwise, it is located at a
  finite distance in front of the eye. When blurred
  vision occurs, an ophthalmologist may prescribe
  glasses or contact lenses, with convex lenses to
  reinforce focusing power. A convex lens is
  rounded outward, like the outer surface of a
  globe.

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• Astigmatism The eye shows different powers at
  different meridian planes. We will assume that
  the meridian planes with maximum and minimum
  power are orthogonal (regular astigmatism). These
  are called the principal meridians. The power
  difference between them is termed the astigmatism
  of the eye. When characterizing astigmatism,
  aside from the value of the difference between
  powers, it is also necessary to provide the
  orientation of any of the principal meridians.
  The condition can be treated with cylindrical
  lens glasses or with hard contact lenses. A
  special "toric" soft lens is also available now.
  Cylindrical lenses are shaped like slices from a
  tube and compensate for the defects of the eye by
  bending the light rays inward.

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• Presbyopia The amplitude of accommodation
  reduces with age and along with it, the ability
  to focus near objects. When amplitude of
  accommodation reduces be- low 3 D (which usually
  happens between 40 and 50 years of age in
  Europe), comfortable reading at 33 cm is no
  longer possible, and ophthalmic compensation is
  necessary for activities using near vision. This
  is a common condition and simple convex lens
  reading glasses are used to correct it.

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LENSES

• Materials used in spectacle lens Spectacle
  lenses are made from three different sources of
  materials. In spectacle lens, there are many
  materials used. Natural media, quartz (or) rock
  crystal, semi-precious stones (i.e. Topaz, Ruby,
  etc) were widely used for making lenses.

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• Glass materials
• Now-a-days spectacle lenses are made from either
  plastic or a high quality glass material.
  Although many types of glass materials are used
  in optical industry, crown glass (1.523) material
  is extensively used for making single vision
  ophthalmic lenses. It is a soda-lime-silica
  material that contains about 70 silica,
• 12 calcium oxide and 15 sodium oxide and some
  other materials in smaller percentages like
  potassium, borax, arsenic etc. Flint glass,
  material (1.620) is used in the making of bifocal
  or achromatic lens. It contains 60, lead oxide,
  30 silica, 8 soda and potash and small
  percentage of arsenic.

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• Plastic materials
• Plastic lenses are generally made from two
  different materials. They are
• 1. Original plastic lens made of (PMMA)
  Polymethylmethocrylate
• 2. Modern hard resin lens from allyl diglycol
  carbonate (CR 39) which is harder and more
  resistant to scratches than other plastic lens
  materials.
• Plastic lenses are made from a very high quality
  material as glass. Plastic lenses are about half
  the weight of glass and are highly
  impact-resistant.

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Corrective lens

• Multi focals designs
• There are two basic types of multifocal lenses
  used.
• 1. In one-piece (or) solid type designs, the same
  material (glass or plastic) is used throughout
  the lens and changing the curvature of lens
  varies the power. (Fig.1)
• The Executive bifocal (glass or plastic) is a
  modern version of the original Benjamin Franklin
  bifocal which has two lenses in each eye of which
  the lower half is used for closer view and the
  upper half for distance.

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• 2. The fused multifocal lenses are made of two or
  more glass materials with different refractive
  indexes when the segment with higher indices is
  fused into the main lens the surfaces of fused
  lens have no change of curvature.
• (Fig. 2) Falling into a category between one
  piece and fused lenses are cement bifocals. Two
  lenses of the same type having the same index of
  refraction are attached together to form a lens
  with the special features of the one-piece lens.

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• Progressive addition lens
• Among the various types of multifocal designs
  progressive addition lens has become very popular
  now-a-days. Over 150 PAL designs have been
  introduced since 1984 with more than 70 PAL
  designs currently available in the market.
• The progressive addition lenses gradually
  increases in power as the line of sight comes
  downwards through the lens.

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• High power lenses
• In high-powered lenses a strong distortions would
  occur through the edges of the lens inherently.
  To avoid these distortions special lens have been
  designed to minimize the distortion and the
  weight of these lenses.
• The types most commonly used are
• (i) Lenticular lens
• (ii) Aspheric lens

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• Lenticular lens
• The lenticular lens may be described as a small
  in diameter or circular and mounted on a longer
  diameter, thin planocarrier which is edged to fit
  into the frame. The main disadvantage of
  lenticular lens is that it gives a bulls eye
  effect making it more conspicuous than the other
  lens.

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• Aspheric lens
• An Aspheric lens is particularly designed to
  eliminate the pincushion distortion in the
  (aphakia) high plus lenses.

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• Fresnel lenses
• Fresnel lenses are sheets of Polyvinyl chloride,
  and it was designed by Augustine Fresnel. The
  Fresnel lenses are used for various purposes. It
  is used in ships and lighthouses as a
  light-condensing lens. As Fresnel lens is thin
  and weightless it would make an ideal cataract
  lens but it shows a pattern of fine concentric
  circles, which gives poor cosmetic appearance to
  the wearers.

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• Safety lens
• The risk of damage to the eye from broken glasses
  is minimized by the use of safety glass. It is
  however advisable to use it for those who are
  engaged in industrial works and sports.

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• Plastic hard-resin lenses
• These are safety lenses with no additional
  treatment, because they will take abuse much
  greater than that required to shatter a standard
  glass lens. A shattered hard-resin lens does not
  have the sharp splinters typical for broken
  glass. Hard resin lenses are superior to hardened
  glass for welding for if not metal may splatter
  on the lens.
• Another type of safety lens is the laminated lens
  in which a sheet of plastic is sandwiched between
  two pieces of glasses. If the lens is shattered,
  the glass particles adhere to the plastic.

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• Polycarbonate lens
• First introduced in plano safety goggles in
  industry, polycarbonate lenses are one the most
  impact-resistant lenses now available in the
  market. In this regard they out perform plastic
  and glass heat-treated or chemically treated and
  thus easy to scratches.
• Polycarbonate is now being moulded into
  ophthalmic Rx lenses that are coated to
  substantially reduce their tendency to scratch.

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CONTACT LENSES
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• A contact lens (also known simply as a contact)
  is a corrective, cosmetic, or therapeutic lens
  usually placed on the cornea of the eye. Leonardo
  da Vinci is credited with describing and
  sketching the first ideas for contact lenses in
  1508, but it was more than 300 years later before
  contact lenses were actually fabricated and worn
  on the eye. Rigid ones were produced and marketed
  first. Modern soft contact lenses were invented
  by the Czech chemist Otto Wichterle and his
  assistant Drahoslav Lím, who also invented the
  first gel used for their production.

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• Corrective contact lenses
• A corrective contact lens is designed to improve
  vision. For many people, there is a mismatch
  between the refractive power of the eye and the
  length of the eye, leading to a refraction error.
  A contact lens neutralizes this mismatch and
  allows for correct focusing of light onto the
  retina. Conditions correctable with contact
  lenses include myopia (near or short
  sightedness), hypermetropia (far or long
  sightedness), astigmatism and presbyopia.

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• Cosmetic contact lenses
• A woman wearing a cosmetic type of contact
  lenses the enlarged section of the image shows
  the grain produced during the manufacturing
  process. As the lines of printed dots are curved,
  these lenses were manufactured by printing onto a
  flat sheet and then shaping the sheet.

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• Therapeutic contact lenses
• Soft lenses are often used in the treatment and
  management of non-refractive disorders of the
  eye. A bandage contact lens protects an injured
  or diseased cornea from the constant rubbing of
  blinking eyelids thereby allowing it to heal.
  They are used in the treatment of conditions
  including bullous keratopathy, dry eyes, corneal
  ulcers and erosion, keratitis, corneal edema,
  descemetocele, corneal ectasis, Mooren's ulcer,
  anterior corneal dystrophy, and neurotrophic
  keratoconjunctivitis. Contact lenses that deliver
  drugs to the eye have also been developed.

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Types of Contact Lenses

• Daily-wear soft lenses (Yearly)
• The most popular type of lenses.
• Made of soft, flexible plastics that allow oxygen
  to pass through to the eyes.
• Short period of adaptation.
• More comfortable and more difficult to dislodge
  than RGP lenses.
• Available in bifocals and colors.
• Ideal for active and sportive lifestyles.
• Lens Care Products are very simple to use.
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Types of Contact Lenses

• Daily disposable soft lenses
• No lens Care products are required.
• Clean, fresh and sterile lenses replaced every
  day.
• Ideal for active lifestyles.
• Monthly disposable soft lenses
• Clean, fresh and sterile lenses when replaced
  every month.
• Available in most prescriptions.
• Useful as spare lenses.

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Types of Contact Lenses

• Gas Permeable (GP)
• Made of harder plastic materials that do not
  contain water.
• Made of slightly flexible plastics, not as
  flexible as soft contacts, but they allow more
  oxygen to pass through to the eyes than do soft
  lenses.
• Comfortable for most people after a short period
  of adaptation
• Sharper vision than with soft contact lenses
• Causes less infection than soft lenses.
• Relatively long life (3-4) years
• Available in bifocals or multifocals
• Daily-wear and extended-wear designs available
• Disadvantage
• They may slip off the center of the eye more
  easily
• Once you don't wear these lenses for about week,
  it needs an adaptation period before they're
  comfortable again.

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Types of Contact Lenses

• Ortho-K
• A vision correction therapy for patients with
  moderate myopia and low amounts of astigmatism.
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• It involves wearing a contact lens retainer while
  you sleep to gently and painlessly corrects the
  surface of your eye.
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• This process is reversible and non-surgical.
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• Daytime free of contact lenses and spectacles.
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• Ideal for sports, swimming and for dusty or dirty
  environments.

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• OPHTHALMOLOGICAL DIAGNOSTIC EQUIPMENT

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Torch lightDescription and purpose A torch
light is the first instrument an ophthalmologist
uses to examine the eye of a patient. A good
torch light should give a circular patch of light
of nearly uniform brightness.
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OphthalmoscopeDescription and Purpose
Ophthalmoscopes are of two kinds direct and
indirect. Direct Ophthalmoscope which is usually
referred to as ophthalmoscope, and sometimes
briefly as the scope, is a very handy
instrument for the examination of the retina
around the fundus. Light from a bulb is reflected
at right angles and projected as a spot through
the iris of the patient to illuminate the retina.
This reflection is achieved using a front
silvered mirror or partially silvered mirror or a
total reflecting prism in different scopes. The
illuminated retina is seen directly by the doctor
through the iris of the patient. A disc with
lenses of different powers is provided in the
instrument and a lens of required power can be
brought in the line of sight to correct any
refractive error of the patient or of the doctor
himself if he does not look through his
spectacles. The doctor looks just above the front
silvered mirror and the reflecting prism or
through the partially silvered reflector
mentioned earlier. An image magnified nearly
fifteen times is seen.
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RetinoscopeDescription and purpose Different
types of retinoscope are used. One of them, the
streak retinoscope described here, is the most
common instrument. It is used in the objective
evaluation of the power of the spectacles needed
to correct the refractive error of patients and
also in determining the axis and cylindrical
power needed for patients with astigmatism. As in
direct ophthalmoscope (chapter-5) light from a
bulb is reflected at right angles and is
projected on to the eye of the patient. The light
is either a rectangular patch (known as plane
mirror mode) or a streak of light of variable
width (concave mirror mode). The instrument is
generally used in the cubicle of
refractionist/optometrist which is usually dark.
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• Indirect Ophthalmoscope
• Description and Purpose The modern Indirect
  Ophthalmoscope functions as the eye piece of a
  stereomicroscope for which a hand held high
  positive aspheric lens (17D, 20D or 30D) serves
  as the objective. When viewed properly, a
  magnified image of the retina is seen. Some of
  the advantages of the instrument as compared to
  the direct ophthalmoscope are (1) stereoscopic
  view (2) greater field of view (3) increased
  illumination and (4) reduced distortion. An
  additional advantage is that the doctor is at a
  distance from the patient. However, the final
  image seen is inverted and the magnification is
  much lesser than in a direct ophthalmoscope.

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• THANK YOU FOR ATTENTION!