Transcript Document 7230939

Physiological optics.
Ophthalmologic tools.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Presbyopia: The amplitude of accommodation reduces
with age and along with it, the ability to focus near
objects. When amplitude of accommodation reduces below 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.
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, semiprecious stones (i.e. Topaz, Ruby, etc) were widely used
for making lenses.
• 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-limesilica 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.
• 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.
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.
• 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.
• Progressive addition lens
• Among the various types of multifocal designs
progressive addition lens has become very popular nowa-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.
• 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
• 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 bull’s
eye effect making it more
conspicuous than the other lens.
• Aspheric lens
• An Aspheric lens is particularly designed to eliminate the
“pincushion” distortion in the (aphakia) high plus lenses.
• 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.
• 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.
• 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.
• 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.
CONTACT LENSES
• 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.
• 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.
• 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.
• 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.
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.
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.
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.
OPHTHALMOLOGICAL DIAGNOSTIC
EQUIPMENT
Torch light
Description 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.
Ophthalmoscope
Description 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.
Retinoscope
Description 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.
• 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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