Ce diaporama a bien été signalé.
Nous utilisons votre profil LinkedIn et vos données d’activité pour vous proposer des publicités personnalisées et pertinentes. Vous pouvez changer vos préférences de publicités à tout moment.
Optics Of Ametropia
Dr.Mohammad Dmour- PGY1
Supervised by : Dr.Yehya AL-Zorqan
Ophthalmology Department- Islamic Hospital
Introduction
• In contrast to emmetropia
the ametropic eye fails to
bring parallel light to a focus
on the retina, i.e. th...
Myopia
• In the myopic eye, the second principal focus lies
in front of the retina .
What is the difference between axial and refractive myopia?
In axial myopia , the refractive power of the eye is normal (a...
Hypermetropia
• In the hypermetropic eye, the second principal
focus lies behind the retina.
Is there a difference between refractive and axial hyperopia ?
Yes, in axial hyperopia , the refractive power of the eye i...
Hypermetropia
• Phakic patients can overcome some or all of their
hypermetropia by using accommodation for
distance vision...
Hypermetropia
TOTAL
MANIFEST
FACULTATIVE
ABSOLUTE
LATENT
What are four types of hyperopia ?
• 1. Manifest hyperopia : Without cycloplegia , this is the most
plus correction the ey...
A patient requires +1.00 D to see at distance. Manifest
refraction reveals she will tolerate up to +2.00 D. Cycloplegic
re...
w Absolute = +1 D
w Manifest = +2 D
w Cycloplegic = +5 D
w Facultative = 2 – 1 = +1 D
w Latent = 5 – 2 = +3 D
Astigmatism
• The refractive power of the astigmatic eye varies
in different meridians. The image is formed as a
Sturm's c...
• The conoid of Sturm is the three-dimensional envelope
of light rays formed by an astigmatic lens acting upon
the rays of...
astigmatism
• Regular astigmatism – principal meridians
are perpendicular (at 90° to each other)
▫ With-the-rule astigmati...
astigmatism
• Simple astigmatism
▫ Simple myopic astigmatism – first focal
line is in front of the retina, while the
secon...
Astigmatism
Anisometropia
• When the refraction of the two eyes is different, the
condition is known as anisometropia.
• Small degrees...
Anisometropia
• The myopic patient with anisometropia is less
likely to develop amblyopia because both eyes
have clear nea...
Far Point
• The far point (FP) of an eye is the position of an
object such that its image falls on the retina of
the relax...
The far point in emmetropia is infinity
The far point in myopia lies a finite distance in front of the eye.
The far point in hypermetropia is virtual, as only
converging light can be focused on the retina.
Optical Correction of Ametropia
• The purpose of the correcting lens in ametropia
is to deviate parallel incident light so...
Optical Correction of Ametropia
• The focal length, f, of the correcting lens is
approximately equal to (») the distance, ...
Optical Correction of Ametropia
• where F is the power of the lens in dioptres; f is
the focal length of the lens in metre...
Effective Power of Lenses
• The power of the correcting lens must be adjusted to
take into account its position in front o...
Effectivepoweroflenses
Formula to calculate the new focal length of lens at the
new distance:
F2= 1/ f1- d or
F2= F1/ 1- D...
Effectivepoweroflenses
In uncorrected hyperopia the
image of an object falls behind the
retina.
The purpose of convex lens...
Correction of hypermetropia.
Correction of myopia.
Practical Application:
Back Vertex Distance
• For any lens of power greater than 5 dioptres, the
position in front of the ...
Practical Application:
Back Vertex Distance
• Therefore any high powered lens should be placed in the
back cell of the tri...
Example 1
• Refraction shows that an aphakic patient
requires a +10.0 D lens at BVD 15 mm. He
needs a contact lens (F2)
Example 2
• Likewise a high myope whose spectacle
correction is –10.0 D at BVD 14 mm
requires a contact lens (F2)
Happily, tables exist which give the value of F2 when F1 and d are known !
Optics of ametropia
Optics of ametropia
Optics of ametropia
Prochain SlideShare
Chargement dans…5
×

Optics of ametropia

Optics of ametropia

  • Identifiez-vous pour voir les commentaires

Optics of ametropia

  1. 1. Optics Of Ametropia Dr.Mohammad Dmour- PGY1 Supervised by : Dr.Yehya AL-Zorqan Ophthalmology Department- Islamic Hospital
  2. 2. Introduction • In contrast to emmetropia the ametropic eye fails to bring parallel light to a focus on the retina, i.e. the second principal focus of the eye does not fall on the retina.
  3. 3. Myopia • In the myopic eye, the second principal focus lies in front of the retina .
  4. 4. What is the difference between axial and refractive myopia? In axial myopia , the refractive power of the eye is normal (about 60 D), but the eye is too long. In refractive myopia , the refractive power of the eye is too strong (more than 60 D), while the length is normal. Both situations create a focal point in front of the retina.
  5. 5. Hypermetropia • In the hypermetropic eye, the second principal focus lies behind the retina.
  6. 6. Is there a difference between refractive and axial hyperopia ? Yes, in axial hyperopia , the refractive power of the eye is normal (about 60 D), but the eye is too short. In refractive hyperopia , the refractive power of the eye is too weak (less than 60 D), while the length is normal (aphakia is the extreme example). Both situations move the focal point behind the retina.
  7. 7. Hypermetropia • Phakic patients can overcome some or all of their hypermetropia by using accommodation for distance vision. They then have to exercise extra accommodation for near vision. • Because the amplitude of accommodation declines with age , these patients require reading glasses at a younger age than emmetropic patients.
  8. 8. Hypermetropia TOTAL MANIFEST FACULTATIVE ABSOLUTE LATENT
  9. 9. What are four types of hyperopia ? • 1. Manifest hyperopia : Without cycloplegia , this is the most plus correction the eye can accept without blurring of vision. the ( strongest convex lens correction accepted for clear distance vision) . • 2. Absolute hyperopia : Without cycloplegia , this is the least amount of plus correction required for clear vision at distance • 3. Latent hyperopia : is the remainder of the hypermetropia which is masked by ciliary tone and involuntary accommodation. ( the difference between manifest hyperopia and hyperopia measured with cycloplegia) • 4. Facultative hyperopia : This is the difference between absolute and manifest hyperopia.
  10. 10. A patient requires +1.00 D to see at distance. Manifest refraction reveals she will tolerate up to +2.00 D. Cycloplegic refraction is +5 D sphere. What are the absolute, manifest, cycloplegic, facultative , and latent hyperopia ?
  11. 11. w Absolute = +1 D w Manifest = +2 D w Cycloplegic = +5 D w Facultative = 2 – 1 = +1 D w Latent = 5 – 2 = +3 D
  12. 12. Astigmatism • The refractive power of the astigmatic eye varies in different meridians. The image is formed as a Sturm's conoid.
  13. 13. • The conoid of Sturm is the three-dimensional envelope of light rays formed by an astigmatic lens acting upon the rays of light from a point object. • Instead of single focal point there are two focal points separated by focal interval. The distance between two focal points is called sturms conoid interval.
  14. 14. astigmatism • Regular astigmatism – principal meridians are perpendicular (at 90° to each other) ▫ With-the-rule astigmatism – the vertical meridian is steepest (a rugby ball or American football lying on its side). ▫ Against-the-rule astigmatism – the horizontal meridian is steepest (a rugby ball or American football standing on its end). ▫ Oblique astigmatism – the steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees. • Irregular astigmatism – principal meridians are not perpendicular (are not at 90° to each other ) and cannot be corrected by spectacles. Based on axis of the principal meridians
  15. 15. astigmatism • Simple astigmatism ▫ Simple myopic astigmatism – first focal line is in front of the retina, while the second is on the retina. ▫ Simple hyperopic astigmatism – first focal line is on retina, while the second is located behind the retina. • Compound astigmatism ▫ Compound myopic astigmatism – both focal lines are located in front of the retina. ▫ Compound hyperopic astigmatism – both focal lines are located behind the retina. • Mixed astigmatism – focal lines are on both sides of the retina. Based on focus of the principal meridians
  16. 16. Astigmatism
  17. 17. Anisometropia • When the refraction of the two eyes is different, the condition is known as anisometropia. • Small degrees of anisometropia are commonplace. Larger degrees are a significant cause of amblyopia. • A disparity of more than 1 D in the hypermetropic patient is enough to cause amblyopia of the more hypermetropic eye because accommodation is a binocular function, i.e. the individual eyes cannot accommodate by different amounts. The more hypermetropic eye therefore remains out of focus.
  18. 18. Anisometropia • The myopic patient with anisometropia is less likely to develop amblyopia because both eyes have clear near vision. However, when one eye is highly myopic it usually becomes amblyopic. • However, myopic patients who have been anisometropic all their lives may tolerate higher degrees of anisometropia and achieve binocular vision with more than 2 D difference between the two eyes.
  19. 19. Far Point • The far point (FP) of an eye is the position of an object such that its image falls on the retina of the relaxed eye, i.e. in the absence of accommodation. • The distance of the far point from the principal plane of the eye is denoted by r, which according to sign convention carries a negative sign in front of the principal plane and a positive sign behind the principal plane.
  20. 20. The far point in emmetropia is infinity
  21. 21. The far point in myopia lies a finite distance in front of the eye.
  22. 22. The far point in hypermetropia is virtual, as only converging light can be focused on the retina.
  23. 23. Optical Correction of Ametropia • The purpose of the correcting lens in ametropia is to deviate parallel incident light so that it appears to come from the far point in myopia or to be converging towards the virtual far point in hypermetropia. • The light will then be brought to a focus by the eye on the retina. Thus the far point of the eye must coincide with the focal point of the lens.
  24. 24. Optical Correction of Ametropia • The focal length, f, of the correcting lens is approximately equal to (») the distance, r, of the far point from the principal plane when the correcting lens is close to the principal plane • Thus the power of lens, F, required is
  25. 25. Optical Correction of Ametropia • where F is the power of the lens in dioptres; f is the focal length of the lens in metres; and r is the distance of the far point from the principal plane in metres.
  26. 26. Effective Power of Lenses • The power of the correcting lens must be adjusted to take into account its position in front of the eye. • Suppose a lens of focal length f1 at a given position in front of the ametropic eye corrects the refractive error; then a different lens of focal length (f1 – d) is required when the correction is moved a distance d towards or away from the eye. • The value of d is positive if the lens is moved towards the eye, and negative if moved away from the eye. The usual sign convention applies to the lens.
  27. 27. Effectivepoweroflenses Formula to calculate the new focal length of lens at the new distance: F2= 1/ f1- d or F2= F1/ 1- Df1 Where, F1= power of the original lens in diopters F2= power of lens in diopters at new position f1= focal length in meters of original lens d= distance moved in meters. It is taken positive if moved toward the eye and negative if moved away from the eye.
  28. 28. Effectivepoweroflenses In uncorrected hyperopia the image of an object falls behind the retina. The purpose of convex lens is to bring the image forward. If the correcting lens is itself moved forward the image will move still forward.ie- the effectivity of the lens is increased. Thus a weaker lens is required to project the image onto the retina in uncorrected myopia the image falls in front of the retina. The purpose of the concave lens is to bring the image behind. If the correcting lens is itself moved forward the image moves still forward.ie- the effectivity of the lens is reduced. Thus a stronger lens is required to project the image onto the retina MyopiaHypermetropia
  29. 29. Correction of hypermetropia.
  30. 30. Correction of myopia.
  31. 31. Practical Application: Back Vertex Distance • For any lens of power greater than 5 dioptres, the position in front of the eye materially affects the optical correction of ametropia. This is especially true in aphakia where high power lenses are prescribed. • For this reason the refractionist must state how far in front of the eye the trial lens is situated so that the dispensing optician can adjust the lens power if a contact lens is to be used, or if spectacles are to be worn at a different distance, e.g. because of a high- bridged nose or deep-set eyes.
  32. 32. Practical Application: Back Vertex Distance • Therefore any high powered lens should be placed in the back cell of the trial frame and the distance between the back of the lens and the cornea measured. This is called the back vertex distance (BVD and must be given with all prescriptions over 5 dioptres. • The measurement may be made with a ruler held parallel to the arm of the trial frame. Other means include a small rule which is slipped through a stenopaeic slit placed in the back cell of the trial frame until it touches the closed eyelid. Two millimeters must be added to the measurement to correct for the thickness of the lid.
  33. 33. Example 1 • Refraction shows that an aphakic patient requires a +10.0 D lens at BVD 15 mm. He needs a contact lens (F2)
  34. 34. Example 2 • Likewise a high myope whose spectacle correction is –10.0 D at BVD 14 mm requires a contact lens (F2)
  35. 35. Happily, tables exist which give the value of F2 when F1 and d are known !

×