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Optics of Contact Lens
 This lecture presents the optical theory and
practice associated with contact lens fitting and
usage.
 Contact lenses and spectacles are also
compared as optical devices.
 BACK Vertex power is the distance of the
second principal focus from the back vertex of
the lens.
 Minus power – is less in contact lenses
compared to spectacles.
 Plus power – is higher in CL compared to
spectacles.
 Image size in an optical system is inversely
proportional to power of the lens.
SM = Image size with Spectacle
Image size without Spectacle
CLM = Image size with Contact Lens
Image size with Spectacle
 Image size in any optical system is directly
proportional to the focal length of the system.
 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.
 The reverse is the case in myopia. The contact
lens focal length is longer than the equivalent
spectacle focal length, and therefore the
contact lens image size is larger.
CLM = 1-d FSp
Examples with d = 14 mm
+ 10.00 D, CLM =
- 10.00 D, CLM =
0.86
1.14
RSM = Image size with Spectacle
Image size in Emmetropia
 SM is a real-world comparison of corrected (focused)
and uncorrected (blurred) retinal image sizes.
 CLM is a more realistic comparison of contact lens
corrected versus spectacle lens corrected retinal
image sizes.
 RSM is a hypothetical magnification comparing
image sizes in a corrected ametropic eye and a
theoretical emmetropic schematic eye.
 Spectacle wearing myopes accommodate less
than spectacle wearing hyperopes (2.114 D
versus 2.786 D).
 With contact lens wear, the accommodation
required in ametropia is approximately the
same as for an emmetrope (≈ 2.415 D).
 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).
 If a myope is switched from spectacles to
contact lenses the change may precipitate the
need for a near correction.
 If a hyperope is switched from spectacles to
contact lenses the need for a near correction
may be postponed
 A hyperope wearing contact lenses converges
less than when wearing spectacles.
 This is because of the base-out prism effect
induced by spectacles acting as an exercising
 A myope wearing contact lenses converges
more when wearing contact lenses than when
wearing spectacles.
 When wearing spectacles, they behave as a
base-in relieving Prism
Δ = h in cm
q in m (q is the distance from C of R to Fixation Plane)
(PD = 64, VD = 14)
CL Rx CL Convergence Spectacle
–5.00D 13.26Δ 14.97Δ
Plano 14.97Δ 14.97Δ
+5.00D 17.18Δ 14.97Δ
 Tear lens under a flexible lens is very thin and
has no power
 Tear lens under a rigid lens depends on
material rigidity and the fitting relationship
(apical touch) (Parallel) (apical Clearence)
- TL No TL + TL
 Cornea/tears interface is optically insignificant
bcz difference in refractive index of cornea
and Tear is insignificant
 Tear lens is “sphericalized” by the back
surface of a spherical lens
 This results in a major reduction of corneal
astigmatism with a spherical lens
 Almost 90% Corneal astigmatism can correct
with Spherical Rigid Lens
CL in-situ
Rigid Lens
Ocular Rx = BVPtrial + Powertear lens + Over-Rx
Soft Lens
Ocular Rx = BVPtrial + Over-Rx
1. Failure to vertex correct spectacle Rx to derive ocular
Rx
2. Failure to vertex correct over-Rx (if>+4.00 D)
3. K readings only represent the central 3 mm
4. K readings incorrect
5. Tear lens power varies slightly with Ks
6. BVPTRIAL may be incorrect - verify it
7. Subjective over-Rx is incorrect
8. BOZR of trial lens incorrect - verify it
9. Trial lens is decentred and/or tilted
10. Trial lens flexure in situ, rigid and soft
11. Corneal molding by the lens
12. Corneal shape not spherical or sphero-cylindrical
13. Variable toric tear lens due to trial lens
movement/decentration/tilting/rotation
14. Tear lens under a thick, high power, low water soft
lens
15. Environment-induced changes in thick, soft trial
lens
16. One or more of the above in combination
03 Optics of CL                             .ppt

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03 Optics of CL .ppt

  • 2.
  • 3.  This lecture presents the optical theory and practice associated with contact lens fitting and usage.  Contact lenses and spectacles are also compared as optical devices.
  • 4.
  • 5.  BACK Vertex power is the distance of the second principal focus from the back vertex of the lens.
  • 6.
  • 7.  Minus power – is less in contact lenses compared to spectacles.  Plus power – is higher in CL compared to spectacles.
  • 8.
  • 9.  Image size in an optical system is inversely proportional to power of the lens.
  • 10. SM = Image size with Spectacle Image size without Spectacle
  • 11. CLM = Image size with Contact Lens Image size with Spectacle
  • 12.  Image size in any optical system is directly proportional to the focal length of the system.  In hyperopia, the contact lens focal length is shorter than the equivalent spectacle focal length (shorter by the vertex distance in fact).
  • 13.  Consequently, the image size is smaller when contact lenses are worn.  The reverse is the case in myopia. The contact lens focal length is longer than the equivalent spectacle focal length, and therefore the contact lens image size is larger.
  • 14. CLM = 1-d FSp Examples with d = 14 mm + 10.00 D, CLM = - 10.00 D, CLM = 0.86 1.14
  • 15. RSM = Image size with Spectacle Image size in Emmetropia
  • 16.  SM is a real-world comparison of corrected (focused) and uncorrected (blurred) retinal image sizes.  CLM is a more realistic comparison of contact lens corrected versus spectacle lens corrected retinal image sizes.  RSM is a hypothetical magnification comparing image sizes in a corrected ametropic eye and a theoretical emmetropic schematic eye.
  • 17.
  • 18.
  • 19.
  • 20.
  • 21.  Spectacle wearing myopes accommodate less than spectacle wearing hyperopes (2.114 D versus 2.786 D).  With contact lens wear, the accommodation required in ametropia is approximately the same as for an emmetrope (≈ 2.415 D).
  • 22.  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).
  • 23.  If a myope is switched from spectacles to contact lenses the change may precipitate the need for a near correction.  If a hyperope is switched from spectacles to contact lenses the need for a near correction may be postponed
  • 24.
  • 25.  A hyperope wearing contact lenses converges less than when wearing spectacles.  This is because of the base-out prism effect induced by spectacles acting as an exercising
  • 26.  A myope wearing contact lenses converges more when wearing contact lenses than when wearing spectacles.  When wearing spectacles, they behave as a base-in relieving Prism
  • 27. Δ = h in cm q in m (q is the distance from C of R to Fixation Plane)
  • 28. (PD = 64, VD = 14) CL Rx CL Convergence Spectacle –5.00D 13.26Δ 14.97Δ Plano 14.97Δ 14.97Δ +5.00D 17.18Δ 14.97Δ
  • 29.
  • 30.  Tear lens under a flexible lens is very thin and has no power  Tear lens under a rigid lens depends on material rigidity and the fitting relationship (apical touch) (Parallel) (apical Clearence) - TL No TL + TL
  • 31.
  • 32.  Cornea/tears interface is optically insignificant bcz difference in refractive index of cornea and Tear is insignificant  Tear lens is “sphericalized” by the back surface of a spherical lens  This results in a major reduction of corneal astigmatism with a spherical lens  Almost 90% Corneal astigmatism can correct with Spherical Rigid Lens
  • 33. CL in-situ Rigid Lens Ocular Rx = BVPtrial + Powertear lens + Over-Rx Soft Lens Ocular Rx = BVPtrial + Over-Rx
  • 34. 1. Failure to vertex correct spectacle Rx to derive ocular Rx 2. Failure to vertex correct over-Rx (if>+4.00 D) 3. K readings only represent the central 3 mm 4. K readings incorrect 5. Tear lens power varies slightly with Ks 6. BVPTRIAL may be incorrect - verify it
  • 35. 7. Subjective over-Rx is incorrect 8. BOZR of trial lens incorrect - verify it 9. Trial lens is decentred and/or tilted 10. Trial lens flexure in situ, rigid and soft 11. Corneal molding by the lens
  • 36. 12. Corneal shape not spherical or sphero-cylindrical 13. Variable toric tear lens due to trial lens movement/decentration/tilting/rotation 14. Tear lens under a thick, high power, low water soft lens 15. Environment-induced changes in thick, soft trial lens 16. One or more of the above in combination