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5/11/2018 ABDUL SALAM 1
Abdul Salam
5/11/2018 ABDUL SALAM 2
Optic of contact lens
Objective
1 Explain Principles of Geometric,
Physical and Ophthalmic Optics.
2 Describe Magnification in contact
lens
3 Describe the Optical Considerations
4 Explain Accommodation,
Convergence and visual field
5
Explain Optical Advantages and
Disadvantages of Contact Lenses over
Spectacles
5/11/2018 ABDUL SALAM 3
The Optical Properties of Contact Lenses
5/11/2018 ABDUL SALAM 4
The Optical Properties of Contact Lenses
5/11/2018 ABDUL SALAM 5
Principal Foci
The Paraxial
Equation
Lens PowerConjugate PlanesBack Vertex Power
Principles of Geometric, Physical and Ophthalmic
Optics
Effect of (BVD)
5/11/2018 ABDUL SALAM 6
Principal Foci
‘Beta’ is one of the most important concepts
5/11/2018 ABDUL SALAM 7
The Paraxial Equation
The Thin Lens paraxial equation is presented
diagrammatically and the formula for calculating
the power of a lens, when the object and image
distances are known,
itis the algebraic sum of the vergence of the light
entering the lens and the dioptric power of the lens,
i.e. L′ = L+F.
5/11/2018 ABDUL SALAM 8
Fthin= F1 + F2
This translates to the simple
addition of the two lens surface
powers. The separation of the
surfaces (lens centre thickness)
is assumed to be negligible and
is ignored.
This is the general thick lens
formula. The power calculated is
sometimes referred to as the
‘equivalent power’ and the focal
length the ‘equivalent focal
length’.
Lens Power








 2121 FF
n
t
FFF
g
e
5/11/2018 ABDUL SALAM 9
Conjugate Planes
For each object position there is a corresponding
image position. This position may be real or
virtual, depending on the type of lens and the
object’s position in relation to the lens.
When an eye (emmetropic or corrected ametropic)
accurately focuses on an object, the retina and the
object are conjugate. If the object cannot be
viewed clearly, the retina and the object
are not conjugate.
5/11/2018 ABDUL SALAM 10
Conjugate Planes
5/11/2018 ABDUL SALAM 11
Back Vertex Power
from the principal planes. Since these planes
are theoretical constructs, their locations are
not obvious. Clinically, it is not practical to
measure true focal lengths of either contact
lenses or spectacles.
• In practice we measure the position of the
second principal focus from the back vertex of
the lens since this is accessible. The power so
measured is the Back Vertex Power
(or BVP).
5/11/2018 ABDUL SALAM 12
Effect of BVD
5/11/2018 ABDUL SALAM 13
Effect of BVD
5/11/2018 ABDUL SALAM 14
Effect of BVD
5/11/2018 ABDUL SALAM 15
Magnification is defined as the
ratio of image size to object size
Magnification
Magnification is defined as follows:
ABDUL SALAM
5/11/2018 16
Magnification
ABDUL SALAM
5/11/2018 17
Image size in any optical system is directly proportional to the
focal length of the system (or inversely proportional to the lens
power).
In hyperopia, the contact lens focal length is shorter than the
equivalent spectacle focal length (shorter by the vertex
distance in fact). Consequently, the image size is smaller when
contact lenses are worn.
Spectacles versus Contact Lenses
ABDUL SALAM
5/11/2018 18
Contact Lens Magnification
1
Similarly, myopes
experience a larger
image size
than they would
with spectacles of
equivalent
power.
With contact lenses, hyperopes
experience a smaller image size
than they would with spectacles
of equivalent power.
CLM = 1 - d F Sp
Examples with d = 14 mm
+ 10.00 D, CLM = 0.86
- 10.00 D, CLM = 1.14
5/11/2018 ABDUL SALAM 19
Accommodation: Spectacles versus Contact
Let’s look at each in detail
Accommodation: Emmetrope
Accommodation: +5.00 D Hyperope
Accommodation: –5.00 D Myope
5/11/2018 ABDUL SALAM 20
Accommodation: Emmetrope
For the emmetrope, the object is assumed to be at 40 cm. the bvd is 14mm.Find
the accomodative demand.
Solution
We will add bvd to working distance
40 + 1.4 = 41.4
𝟏
𝟒𝟏.𝟒
= 0.0241
0.0214 ×100 = +2.415 D.
Under similar circumstances, the eye of a contact
lens wearer is subjected to approximately the
same accommodative demand
5/11/2018 ABDUL SALAM 21
The vergence of light at the corneal plane for a 40 cm working distance after
passing through a +5.00 D spectacle lens placed 14 mm in front of the cornea is
+2.591 D.
Solution
We will subtract bvd from working distance
40 - 1.4 = 38.6
𝟏
𝟑𝟖.𝟔
= 0.0259
0.0259 ×100 = +2.591
The ocular Rx at the corneal plane is +5.376 D and therefore the
accommodative demand is
+5.376 – (+2.591) = +2.785 D.
Accommodation: +5.00 D Hyperope
5/11/2018 ABDUL SALAM 22
The vergence of light at the corneal plane for a 40 cm working distance after
passing through a –5.00 D spectacle lens place 14 mm in front of the cornea is
–6.787 D.
Solution
The ocular Rx at the corneal
plane is –4.673 D and therefore the accommodative
demand is:
–4.673 – (–6.787) = +2.114 D.
Accommodation: –5.00 D Myope
5/11/2018 ABDUL SALAM 23
Accommodation: Summary
Spectacle wearing myopes accommodate less than spectacle wearing
hyperopes
With contact lens wear, the accommodation required in
ametropia is approximately the same as for an emmetrope
The accommodative demand of a myope is greater in
contact lenses (2.415 D) than with spectacles (2.114 D).
The accommodative demand of a hyperope is
greater with spectacles (2.786 D) than with
contact lenses (2.415 D).
Accommodation Summary
ABDUL SALAM
5/11/2018 24
A hyperope wearing contact
lenses converges less
than when wearing spectacles.
This is because of
the base-out prism effect
A myope wearing
contact lenses
converges more
when wearing contact
lenses than when
wearing
spectacles.
Convergence in Spectacles versus Contact Lenses
5/11/2018 ABDUL SALAM 25
Convergence in Spectacles versus Contact Lenses
5/11/2018 ABDUL SALAM 26
Hyperopia
Field Limitations:
The ring ‘scotoma’ that is produced by the differences
between the field limitations imposed by the frame/lens
combination and the optics of a plus lens is shown.
Because the field of view is decreased by a plus lens,
some parts of the field are not seen at all. This produces a
‘ring scotoma’ (absence of vision) whose exact shape
depends on the shape and power of the lens and the
profile/thickness of the spectacle frame.
As a contact lens moves with the eye no such
limitations or scotoma results.5/11/2018 ABDUL SALAM 27
Myopia
Field Limitations:
The ring ‘diplopia’ that is produced by the differences
between the field limitations imposed by the frame/lens
combination and the optics of a minus lens is shown.
Because the field of view is relatively larger with a minus
lens than with a plus lens, some parts of the field are
seen both clearly through the lens as well as blurred
outside the lens.
5/11/2018 ABDUL SALAM 28
Optical Advantages of Contact Lenses
There are several benefits of contact lens :
No astigmatism of
oblique pencils
No distortion No chromatic
aberration
No limitations
on the field of
view
ABDUL SALAM
5/11/2018 29
Optical Disadvantages of Contact Lenses:
There are several disadvantages of contact lens:
7
6
5
4
3
2
1 Lens decentration produces ‘ghosting’ or flare from
the peripheral zone of the lens.
ABDUL SALAM
5/11/2018 30
Optical Disadvantages of Contact Lenses:
There are several disadvantages of contact lens:
7
6
5
4
3
2
1
• toric lens rotates When a toric lens rotates, a toric over-
refraction and decreased vision may result
• ghosting’
ABDUL SALAM
5/11/2018 31
Optical Disadvantages of Contact Lenses:
There are several disadvantages of contact lens:
• toric lens rotates
• ghosting
7
6
5
4
3
2
• Narrow range of refractive indices the lens designer often has
no choice of refractive index.
ABDUL SALAM
5/11/2018 32
Optical Disadvantages of Contact Lenses:
There are several disadvantages of contact lens:
• toric lens rotates
• ghosting
• Narrow range of refractive indices
8
7
6
5
4
3
• In axial ametropia spectacles are better suited
ABDUL SALAM
5/11/2018 33

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optic of contact lens

  • 1. 5/11/2018 ABDUL SALAM 1 Abdul Salam
  • 2. 5/11/2018 ABDUL SALAM 2 Optic of contact lens
  • 3. Objective 1 Explain Principles of Geometric, Physical and Ophthalmic Optics. 2 Describe Magnification in contact lens 3 Describe the Optical Considerations 4 Explain Accommodation, Convergence and visual field 5 Explain Optical Advantages and Disadvantages of Contact Lenses over Spectacles 5/11/2018 ABDUL SALAM 3
  • 4. The Optical Properties of Contact Lenses 5/11/2018 ABDUL SALAM 4
  • 5. The Optical Properties of Contact Lenses 5/11/2018 ABDUL SALAM 5
  • 6. Principal Foci The Paraxial Equation Lens PowerConjugate PlanesBack Vertex Power Principles of Geometric, Physical and Ophthalmic Optics Effect of (BVD) 5/11/2018 ABDUL SALAM 6
  • 7. Principal Foci ‘Beta’ is one of the most important concepts 5/11/2018 ABDUL SALAM 7
  • 8. The Paraxial Equation The Thin Lens paraxial equation is presented diagrammatically and the formula for calculating the power of a lens, when the object and image distances are known, itis the algebraic sum of the vergence of the light entering the lens and the dioptric power of the lens, i.e. L′ = L+F. 5/11/2018 ABDUL SALAM 8
  • 9. Fthin= F1 + F2 This translates to the simple addition of the two lens surface powers. The separation of the surfaces (lens centre thickness) is assumed to be negligible and is ignored. This is the general thick lens formula. The power calculated is sometimes referred to as the ‘equivalent power’ and the focal length the ‘equivalent focal length’. Lens Power          2121 FF n t FFF g e 5/11/2018 ABDUL SALAM 9
  • 10. Conjugate Planes For each object position there is a corresponding image position. This position may be real or virtual, depending on the type of lens and the object’s position in relation to the lens. When an eye (emmetropic or corrected ametropic) accurately focuses on an object, the retina and the object are conjugate. If the object cannot be viewed clearly, the retina and the object are not conjugate. 5/11/2018 ABDUL SALAM 10
  • 12. Back Vertex Power from the principal planes. Since these planes are theoretical constructs, their locations are not obvious. Clinically, it is not practical to measure true focal lengths of either contact lenses or spectacles. • In practice we measure the position of the second principal focus from the back vertex of the lens since this is accessible. The power so measured is the Back Vertex Power (or BVP). 5/11/2018 ABDUL SALAM 12
  • 13. Effect of BVD 5/11/2018 ABDUL SALAM 13
  • 14. Effect of BVD 5/11/2018 ABDUL SALAM 14
  • 15. Effect of BVD 5/11/2018 ABDUL SALAM 15
  • 16. Magnification is defined as the ratio of image size to object size Magnification Magnification is defined as follows: ABDUL SALAM 5/11/2018 16
  • 18. Image size in any optical system is directly proportional to the focal length of the system (or inversely proportional to the lens power). In hyperopia, the contact lens focal length is shorter than the equivalent spectacle focal length (shorter by the vertex distance in fact). Consequently, the image size is smaller when contact lenses are worn. Spectacles versus Contact Lenses ABDUL SALAM 5/11/2018 18
  • 19. Contact Lens Magnification 1 Similarly, myopes experience a larger image size than they would with spectacles of equivalent power. With contact lenses, hyperopes experience a smaller image size than they would with spectacles of equivalent power. CLM = 1 - d F Sp Examples with d = 14 mm + 10.00 D, CLM = 0.86 - 10.00 D, CLM = 1.14 5/11/2018 ABDUL SALAM 19
  • 20. Accommodation: Spectacles versus Contact Let’s look at each in detail Accommodation: Emmetrope Accommodation: +5.00 D Hyperope Accommodation: –5.00 D Myope 5/11/2018 ABDUL SALAM 20
  • 21. Accommodation: Emmetrope For the emmetrope, the object is assumed to be at 40 cm. the bvd is 14mm.Find the accomodative demand. Solution We will add bvd to working distance 40 + 1.4 = 41.4 𝟏 𝟒𝟏.𝟒 = 0.0241 0.0214 ×100 = +2.415 D. Under similar circumstances, the eye of a contact lens wearer is subjected to approximately the same accommodative demand 5/11/2018 ABDUL SALAM 21
  • 22. The vergence of light at the corneal plane for a 40 cm working distance after passing through a +5.00 D spectacle lens placed 14 mm in front of the cornea is +2.591 D. Solution We will subtract bvd from working distance 40 - 1.4 = 38.6 𝟏 𝟑𝟖.𝟔 = 0.0259 0.0259 ×100 = +2.591 The ocular Rx at the corneal plane is +5.376 D and therefore the accommodative demand is +5.376 – (+2.591) = +2.785 D. Accommodation: +5.00 D Hyperope 5/11/2018 ABDUL SALAM 22
  • 23. The vergence of light at the corneal plane for a 40 cm working distance after passing through a –5.00 D spectacle lens place 14 mm in front of the cornea is –6.787 D. Solution The ocular Rx at the corneal plane is –4.673 D and therefore the accommodative demand is: –4.673 – (–6.787) = +2.114 D. Accommodation: –5.00 D Myope 5/11/2018 ABDUL SALAM 23
  • 24. Accommodation: Summary Spectacle wearing myopes accommodate less than spectacle wearing hyperopes With contact lens wear, the accommodation required in ametropia is approximately the same as for an emmetrope The accommodative demand of a myope is greater in contact lenses (2.415 D) than with spectacles (2.114 D). The accommodative demand of a hyperope is greater with spectacles (2.786 D) than with contact lenses (2.415 D). Accommodation Summary ABDUL SALAM 5/11/2018 24
  • 25. A hyperope wearing contact lenses converges less than when wearing spectacles. This is because of the base-out prism effect A myope wearing contact lenses converges more when wearing contact lenses than when wearing spectacles. Convergence in Spectacles versus Contact Lenses 5/11/2018 ABDUL SALAM 25
  • 26. Convergence in Spectacles versus Contact Lenses 5/11/2018 ABDUL SALAM 26
  • 27. Hyperopia Field Limitations: The ring ‘scotoma’ that is produced by the differences between the field limitations imposed by the frame/lens combination and the optics of a plus lens is shown. Because the field of view is decreased by a plus lens, some parts of the field are not seen at all. This produces a ‘ring scotoma’ (absence of vision) whose exact shape depends on the shape and power of the lens and the profile/thickness of the spectacle frame. As a contact lens moves with the eye no such limitations or scotoma results.5/11/2018 ABDUL SALAM 27
  • 28. Myopia Field Limitations: The ring ‘diplopia’ that is produced by the differences between the field limitations imposed by the frame/lens combination and the optics of a minus lens is shown. Because the field of view is relatively larger with a minus lens than with a plus lens, some parts of the field are seen both clearly through the lens as well as blurred outside the lens. 5/11/2018 ABDUL SALAM 28
  • 29. Optical Advantages of Contact Lenses There are several benefits of contact lens : No astigmatism of oblique pencils No distortion No chromatic aberration No limitations on the field of view ABDUL SALAM 5/11/2018 29
  • 30. Optical Disadvantages of Contact Lenses: There are several disadvantages of contact lens: 7 6 5 4 3 2 1 Lens decentration produces ‘ghosting’ or flare from the peripheral zone of the lens. ABDUL SALAM 5/11/2018 30
  • 31. Optical Disadvantages of Contact Lenses: There are several disadvantages of contact lens: 7 6 5 4 3 2 1 • toric lens rotates When a toric lens rotates, a toric over- refraction and decreased vision may result • ghosting’ ABDUL SALAM 5/11/2018 31
  • 32. Optical Disadvantages of Contact Lenses: There are several disadvantages of contact lens: • toric lens rotates • ghosting 7 6 5 4 3 2 • Narrow range of refractive indices the lens designer often has no choice of refractive index. ABDUL SALAM 5/11/2018 32
  • 33. Optical Disadvantages of Contact Lenses: There are several disadvantages of contact lens: • toric lens rotates • ghosting • Narrow range of refractive indices 8 7 6 5 4 3 • In axial ametropia spectacles are better suited ABDUL SALAM 5/11/2018 33