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Optics and refraction for 5th year medical students
Mutaz Gharaibeh,MD

Physical optics
• The visible part of the EM spectrum, to which the retina is
sensitive, lies between the wavelengths 390nm & 760nm.
• Light should be correctly focused on the retina for the eye
to generate accurate visual information.

Light travels in a straight line. But when it reaches another
medium it will bend. This is called refraction.
This is how lenses work, and also how we see.

The focus must be adjustable to allow clear vision for both near & distant
objects (accommodation).
Refractive components of the eye or the focusing power is :
** 2/3 cornea and “air/tear” interface
** 1/3 lens
These two elements converge parallel rays because :-
1) Cornea has higher refractive index than air.
2) lens has higher refractive index than vitreous humour.
3) Cornea & lens are spherically convex in shape
1.

Diopter (D) : measurement of the refractive power of a lens, equal to the reciprocal
of the focal length in meters “ 1/meters” . Simply describes the power of the lens.

**** A negative lens is a diverging lens ,concave ,corrects for myopia
**** A positive lens is a converging lens ,convex ,corrects for hypermetropia.
Example: +1 D lens focuses light at 1m.
+2 D lens focuses light at 0.5m.

Definitions
• Presbyopia: Loss of accommodative ability of the lens
resulting in difficulties with near tasks.
• Astigmatism: the curvature of the cornea and/or lens is not
spherical and therefore causes image blur on the retina.
• Aniseikonia: a difference of image size between the 2 eyes as
perceived by the patient.
• Anisometropia: a refractive power difference between the 2
eyes (usually > 2D)
• Aphakia: ( Phakos=lens), aphakia is no lens.
• Pseudophakia: artificial lens in the eye.

Refractive errors
Emmetropia :- when parallel rays of light from a distant object are brought to
focus on the retina with the eye at rest “not accommodating “.

Ametropia :- when parallel rays of light are not brought to a focus on the retina in
an eye at rest. It’s simply “refractive error”. A change in refraction is needed to
achieve sharp vision.
Ametropia is divided into:
1) Myopia: Near sightedness
2) Hyperopia (Hypermetropia):
= Far sightedness.
3) Astigmatism :non spherical cornea.

Hypermetropia
Hypermetropic eye is too short for its converging ability.
The eye’s optical system is less powerful, and/or the eye is too short.
A lot of children are farsighted ,this makes sense as they have small weak eyes.
So it’s a stage in normal development of the eye (physiologic) , or due to any
cause that shortens the globe (pathological Hyperopia).
Special forms: Ectopia lentis, post op. aphakia.
Rays of light converge behind the retina, which causes blurring of vision for near
objects ± distant objects.
Hypermetropes must accommodate with distant gaze to bring image into retina.

Hypermetropia- why?
• The length of the eyeball is shorter than it should be.
• stage in normal development of the eyes—at birth
eyes are hypermetropic (2.5 to 3.0 Diopters)
• When persists in adulthood it represents an
imperfectly developed eye.
• Lens changes (cataract)

Hypermetropia -symptoms
• Eye-strain (ciliary muscle is straining to
maintain accommodation) – Asthenopia
• “watering” /“redness”.
• Have difficulty in seeing near objects ,with
excellent far vision ( this is a myth ) , as they
have difficulty seeing both near and far with
more difficulty seeing near objects
• Headaches in later part of the day.
• Complain of blurring of text.

Clinical features:
- Youth: usually don’t require glasses, they have sufficient accommodative ability to focus
the image on the retina, but may develop accommodative ET.
- 30s-40s: blurring of near vision due to decreased accommodation, may need
reading glasses.
- >50s: blurring of distance vision due to severely decreased accommodation. corrected
by adding a powerful lens, a converging (convex) lens.
Complications: Angle-closure glaucoma, particularly later in life.

Myopia
Globe too long relative to refractive mechanisms, or refractive mechanisms too
Strong ,strong big eye.
Usually presents in 1st or 2nd decades ,rarely begins after the age of 25,except in
pts with D.M or cataract.
Keratoconus is a pathological cause of myopia.
Blurring of distance vision, the near isn’t affected .
Complications ( though rare) : retinal tear or detachment, macular hole, and open
angle glaucoma.
Not prevented with refractive correction.
Management: corrected by a diverging lens “concave”, or refractive eye surgery.

DEGREE OF MYOPIA
Low myopia:
usually describes
myopia of −3.00
diopters or more
Medium myopia
usually describes
myopia between −3.00
and −6.00
diopeters Those with
moderate amounts of
myopia are more likely
to have
Pigmented dispersion
syndrome or pigmented
glaucoma
High myopia usually
describes myopia of
−6.00 or less
(toward -10.00).
People with high
myopia are more likely
to have
retinal deatachment and
primary open angle
glaucoma,
also more likely
to experience floaters,
shadow-like shapes
which appear singly or
in clusters in the field
of vision.

How myopes
usually see
. In high myopia even near vision
is affected e.g. reading

Astigmatism ”
‫بؤرية‬ ‫الال‬
:
Light rays aren’t refracted uniformly in all meridians due to non spherical shape of
cornea or lens, parallel rays passing through theses different planes are brought to
different points of focus.
The cause of astigmatism is unknown. It is usually present from birth, and often
occurs together with nearsightedness or farsightedness.
Astigmatism makes it difficult to see fine details, either close up or from a distance.
Corrected with a cylindrical lens or refractive eye surgery.

Another misnomer :
Astigmatism is not ‫انحراف‬

Accommodation
Near focusing of the eye.
Ciliary muscle contraction……..zonules relax……..lens more spherical .
Eyes converge, pupils constrict.

Why do we need accommodation in near vision???

This ability decreases with age esp. after 40.
Presbyopia : normal aging process, when
near images can’t be focused on the retina
due to reduced accommodative ability.
The focus is behind the retina as in
hyperopia.
If initially emmetropic: person begins to hold
reading material farther away and distance
vision is unaffected.
If initially hyperopic : presbyopia occurs
earlier.
Corrected with a convex lens for reading
(bifocal).

Refractive error
correction
Contact lenses
Spectacles
Refractive surgery

Contact lenses
1. Soft, hydrophilic  better tolerated physically but, less permeable
to oxygen.
2. Rigid gas permeable
Advantages of rigid gas-permeable:
o Their greater oxygen-permeability reduces the risk of
corneal damage from hypoxia
o Their rigidity allows easier cleaning and offers less risk of
infection
o Their rigidity permits an effective correction of astigmatism
o Proteinaceous debris is less likely to adhere to the lens and
cause an allergic conjunctivitis

Contact Lenses
• Hard contact lenses are mainly used in the treatment
of keratoconus.
• Soft contact lenses for:
1) Optical correction
2) Bandage contact lenses for the relief of pain
like in epithelial defects or bollous keratopathy.

Complications Of Contact Lenses
 Superficial punctate keratitis is the most common
complication. It is staining at 3 and 9 o’clock and this is
associated with incomplete blinking and drying in rigid
lens syndrome.
 The tight lens syndrome, characterized by indentation
and staining in the conjunctival epithelium in a ring
around the cornea.
 Acute hypoxia  Epithelial micro cysts, necrosis and
endothelial blebs.
 Chronic Hypoxia  Vascularization and lipid
deposition.

Refraction
Refraction is a process your eye care professional uses to measure your refractive error, or
vision problem to determine how much power is needed to bring your eyes to normal,
perfectly focused vision. Refraction consists of two parts, objective and subjective.
Objective Refraction:
refraction obtained without receiving any feedback from the patient, obtained by
using different instruments:
1) retinoscope or
2) auto-refractor .

Automated refractor
Computerized instrument that
shines light into your eye.
The light travels through the
front part of your eye to the back
part of your eye, then back again.
The information bounces back to
the instrument, giving an
objective measurement of your
refractive error.
Auto-refractors are quick and
easy to use, and require no
feedback from you.

Retinoscopy
The doctor will project a streak of
light into your pupil.
A series of lenses are flashed in front
of your eye.
By looking through the retinoscope,
the doctor can study the light reflex
of the pupil.
Based on the movement and
orientation of this retinal reflection,
the refractive state of your eye is
measured.

Subjective refraction
A subjective refraction requires responses from the patient. The doctor may use a phoropter
to measure your subjective refractive error to determine your eyeglass prescription.
Typically, you will sit behind the phoropter and look at an eye chart. The doctor will change
lenses and other settings while asking you for feedback on which settings give you the best
vision.
Sometimes eye doctors prefer to obtain a cycloplegic refraction , especially when trying to
obtain an accurate refraction in young children who may skew refraction measurements by
adjusting their eyes.
PMT = Post mydriatic test

Spectacles
eyeglass prescription

The parameters specified on spectacle prescriptions vary, but typically include the power to
which each lens should be made in order to correct blurred vision due to refractive errors,
including myopia, hyperopia, astigmatism, and presbyopia.

Refractive surgery
 Corneal surgery
 PRK - photorefractive keratectomy
 LASEK - laser epithelial keratomileusis
 EPI-LASIK - epithelial LASIK
 LASIK - laser in-situ keratomileusis
 Lens surgery

What is laser ?
Acronym "light amplification by stimulated emission of radiation" not
only visible light, hence infrared laser, ultraviolet laser, X-ray laser, and so
on .

Excimer laser : subtype of laser
Excited dimers, is a cool laser beam, because it causes little thermal damage
to adjacent tissues.
UV laser that works by photo ablative decomposition of the corneal stroma
laser energy destroys intermolecular bonds in cornea to alter its shape.
In myopia it flattens the cornea while in hypermetropia it steepens it. in
astigmatism it flattens the steepest meridian .

PRK
• PRK stands for PhotoRefractive Keratotomy.
• It this case, we don’t do any cuts in the cornea like LASIK,
instead we remove the corneal epithelium, and the Excimer
laser is applied to the cornea which is followed by application
of a contact lens to the eye until the epithelium heals
completely, then the contact lens is removed.
• PRK is usually done when the corneal thickness is not
enough to do LASIK.
• The procedure is usually painful post operatively up to 2 – 3
days until the epithelium heals.

PRK
Laser is applied to
the corneal surface
after removing the
epithelium , the
cornea is resurfaced
later.

LASIK
laser assisted in situ keratomileusis
* corneal flap (in situ)
* reshape the cornea (keratomileusis)

Complications of LASIK
 Operative Complications:
 Flap complications:
1) Free flap
2) Incomplete flap
3) Button hole of the flap
4) Irregular cut
5) Epithelial defects
6) Corneal perforation

 Laser Complications:
1) De-centered ablation
 Post Operative Complications:
1) Overcorrection
2) Under correction
3) Infection
4) Epithelium in-growth under the flap

 DLK  Diffuse Lamellar Keratitis

Complications Of PRK
 Overcorrection
 Under correction
 Corneal haze
 Infection

So if your sister is willing to do refractive
surgery , what to tell her ??

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optics.Dr.Mutaz.ppt

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