1. REFRACTIVE ERRORS
SPS Y2-2012

4. WHAT ARE WE GOING TO DO TODAY?
• A little bit of optics
• Refractive media of the eye
• How the eye interacts with light
• Refractive Errors

5. OPTICS 101
“REFRACTION”
Emergent ray
Refracted ray
Incident ray

6. OPTICS 101: REFRACTION

7. OPTICS 101: ACCEPT IT!
• Light rays coming from infinity (e.g. sun, stars,
galaxies) are parallel
• All other light rays that originate from an object
(TV, bulb, book) are divergent
• Us ophthalmologists consider a distance of 6
meters or more as infinity

8. OPTICS 101: LENSES
CONVEX LENS
3M
Parallel light raysrays (incident)
Divergent light (incident) Converging light rays (emergent)
3M
MORE CONVERGING POWER
AMOUNT OF CONVERGENCE DEPENDENT ON NATURE OF INCIDENT RAYS

9. SO FAR
• Convex Lenses converge Light
• The thicker the lens the greater the
convergence
• Thicker lenses = More power = More
convergence
• Amount of convergence depends on the type
of incident rays

10. OPTICS 101: LENSES
CONCAVE LENS
Virtual Image
3M
(not really important for us)
Parallel light rays (incident)
Divergent light rays (incident) Divergent light rays (emergent)
3M Virtual Image
MORE DIVERGING POWER
AMOUNT OF DIVERGENCECE DEPENDENT ON NATURE OF INCIDENT RAYS

11. …
• Concave Lenses diverge Light
• The thicker the lens the greater the
divergence
• Thicker lenses = More power = More
divergence
• Amount of divergence depends on the type of
incident rays

12. RECAP
Convex lenses converge
Thicker the lens, the greater the power, more the
convergence
Amount of convergence depends on nature of incident
rays

13. RECAP
Diverging lenses diverge
Thicker the lens, the greater the power, more the
divergence
Amount of divergence depends on nature of incident
rays

14. Assimilate; Enjoy
Ask Questions

16. The Next few slides introduces a new term
“DIOPTER”
It will require some attention
If you think its too difficult, don’t worry. Most
ophthalmologists are in the same boat 

17. DIOPTER
• Measures the divergence (spread) or
convergence of light. The ‘power’ of light
• Calculated as n/d
– n = refractive index of medium (1 = air)
– d = distance from lens that measurements are
taken (in meters)
• Minus for Diverging light
• Plus for Converging light

18. DIOPTER
As the distance increase.
The spread of light also
increases. The power
contained in the light
beam decreases as it is
* Power = n/d spread ‘too thin’
* n=1
* ‘-’ sign for diverging light
P = -1/ 0.5. P =-2D P = -1/ 1. P =-1D P = -1/ 2. P =-.5D
d .5m 1m 2m

19. DIOPTER
• The Diopter also signifies the power of lens
• The more the power the more the converging
or diverging ability of the lens
• Minus for Diverging lens (just like light rays)
• Plus for Converging lens (just like light rays)

20. DIOPTER
Power of emergent rays:
Algebraically add power of light
rays & power of lens Pe = -1+ (+2) =+1D
+ = convergent rays
1D = Power of 1D Remember:
P = n/d
+2D
Can we find
out the image +1 = 1/d
location from D = 1*+1
this data? D=1m
1m 1m
Power of light rays at
lens: Image formed
at 1m from
P = -1/ 1. P =-1D
lens

21. DIOPTER
Power of emergent rays:
Algebraically add power of light
rays & power of lens Pe = -1+ (-2) =-3D
- = divergent rays
3D = Power of 3D Remember:
P = n/d
-2D
Can we find
out the image -3 = 1/d
location from D = 1/-3
this data? D= -.33m
.33m 1m
Image formed Power of light rays at
at .33m from lens:
lens P = -1/ 1. P =-1D

22. DIOPTER
• Measures power of light rays as well as lenses
• Is ‘+’ for Converging light rays and lenses
• Is ‘-’ for Diverging light rays and lenses
• Light ray lens interaction is calculated
algebraically

23. Assimilate; Enjoy
Ask Questions

24. THE OPTICS OF EYE
Aqueous Vitreous
LENS
Cornea
humor
humor
We only consider Cornea and Lens
The refractive index of aqueous humor equates to that of cornea
The refractive TRANSPARENT MEDIA ARE RESPONSIBLE of lens
index of vitreous humor equates to that FOR
Total Power ofREFRACTION AS(54D) ALLOW LIGHT TO PASS
the eye ~ 60D THEY
Cornea = 40D THROUGH TO THE RETINAin refractive index)
(36D) [Greater difference
Lens = 20D (18D)

25. OPTICS OF THE EYE
The lens is able to change its shape
The stimulus is a blurred image
The lens tries to bring the image into focus just like a camera
For near tasks (reading) the lens ‘thickens’ increasing power
This is called accommodation
Difference in refractive More
index Refraction
Air = 1.00 occurs at
Cornea = 1.337 Cornea-air
Lens = 1.38
interface

26. ACCOMODATION

27. ACCOMODATION

28. ACCOMODATION

29. RECAP
• Light rays coming from infinity (>6m) are focused by a resting (non-
accommodating) eye on the retina
• Light rays coming from a finite distance (<6m) produce a blur image on the
retina
• This blur image forces the eye to accommodate
• Accommodation involves constriction of ciliary muscles to make the lens
thicker (more power)
• Thicker lens then focuses the light rays back onto the retina
• Amount of accommodation depends on the distance of the near object
• The eye will always try to focus a blurred image on the retina
• Remembering this simple fact will help clear A LOT of confusion

30. EMMTEROPIA
EMMTEROPIA = Equal Measure = No Refractive Error = Desirable optical system
RESTING EYE
ACCOMMODATING EYE

31. REFRACTIVE ERRORS
• A fault in the mechanism of Refraction
• Produces a blur image on the retina
• Can be for far (infinite; >6m)objects
• As well as for near (finite; <6m) objects
• AMMETROPIA = Not Equal Measure

32. NEAR VISION ERROR
(READING ERRORS)

33. PRESBYOPIA
• Presby = Old + Hard (Rigid)
• Opia = sight
• An age related phenomena (40 Years)
• Weakness in ciliary muscles
• Can’t contract as much as before
• Lens can’t accommodate as well
• Thus can’t focus for near objects
• Becomes worse with advancing age

34. PRESBYOPIA
What do old
people do to read
without glasses
As the object is moved further
The light rays spread more decreasingnullify power
The eye will try to accommodate to their the blur
(Diopter; Power = n/d) more accommodative effort left
It will fail as there is no
Light striking the cornea will have lower power
(muscles are too weak)
Less accommodative effort then can help focus the light rays

35. CORRECTING PRESBYOPIA WITH
LENSES
A Convex lens corrects presbyopic error. HELP BRING THE LIGHT TO FOCUS
WHAT TYPE OF LENS CAN
In fact any refractiveTHE RETINA? the light focuses behind the retina is corrected
ON error in which
by a convex lens
As muscle weakens with increasing age so does the power of correcting lens
A lens that corrects a refractive error is called a correcting lens

36. DISTANT VISION ERRORS
(infinity; 6m)

37. DISTANT VISION ERRORS: RULES
• ALL ERRORS ARE DEFINED WITH THE
FOLLOWING CONDITIONS
– Light rays are parallel (coming from distance)
– The eye is at rest (not accommodating)
– The error is defined based on where the light
focuses
• Hyperopia/ Hypermetropia (Long measure)
• Myopia (Short measure)

38. DISTANT VISION ERROR I: HYPEROPIA
(also known as hypermetropia)

39. How does
eye react
HYPERMETROPIA
to ‘blur’ voluntarily (unknowing to the patient) accommodates
*The eye
to the blurred image
For a box of candy:
*Most hyperopes don’t even know they have a refractive error
A common of lens canpatient’s vision a short error?
What type hyperopic be used to correct this using
Help fix this cause of hypermetropia iswithout eye ball
*They are latent hyperopes
lenses …focusing
*Only when they start getting older and they loose their
accommodative power do they find out the truth behind the
Convex Lens
*Their hyperopia then becomes retina
It
manifest
accommodates!
Parallel light
rays…
…while eye is at rest…
(Not Accommodating)

40. RECAP: HYPEROPIA
• A distance vision refractive error
• Parallel light rays focus behind the retina in a resting eye
• The eye automatically compensates for the error by
accommodating
• Corrected with a convex lens, just like presbyopia (a near
vision refractive error)
• Just because both these types of errors are corrected by
the same lens mean they are the same error!

41. DISTANT VISION ERROR II: MYOPIA

42. …accommodation MYOPIA
*Inof any help?
Myopia accommodation would create a blurrier image!
*As it is not possible to relax accommodation
*Accommodation has of be used involuntary correction of myopia
A common cause no role in to long eye ball
What type of lens can myopia is acorrect this error?
*A corrective aid must be used to fix myopia …focusing in
front of the
Concave Lens retina
The eye always
reacts to a blurred
image!
Parallel light
rays…
…while eye is at rest…
(Not Accommodating)

43. RECAP: MYOPIA
• A distance vision refractive error
• Parallel light rays focus in front of the retina in a
resting eye
• The eye CAN NOT automatically compensate for
the error by accommodating
• Corrected with a concave lens, unlike presbyopia
(a near vision refractive error) and hyperopia

44. WAIT THERE IS MORE…
• It is possible to reshape the surface of the cornea with laser
• The cornea can be reshaped into a convex or concave lens
using ultra-violet laser
• The laser used is called EXCIMER
• Three types of procedures:
– Photo Refractive Keratectomy (PRK; Common)
– Laser insitu Keratomelusis (LASIK)
– Laser Intra-epithelial Keratomelusis (LASEK)
• Oh, and Contacts lenses can also used!