optics of contact lenses
Post on 24-Jan-2017
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THE EFFECTIVITY RELATIONSHIP
D D¢
d l - dl
LIGHT
Vergence @ D = , Vergence @ D¢ = =1l
1l - d
L(1 - dL)
AMETROPIA THE FAR POINT
The Far Point is that point which is conjugate with the fovea centralis of the unaccommodated (relaxed) eye.
• It lies at:• Infinity in emmetropes• Behind the eye in hyperopes• In front of the eye in myopes
THE CORRECTON OF AMETROPIA
• A correcting lens is that lens which has its primary focus at the Far Point of the unaccommodated ametropic eye
HYPEROPIASIGNIFICANCE OF VERTEX DISTANCE
Far point
C P¢
Relaxed eyeCorrected with a spectacle lens
C P¢
Sf ¢
SP
f ¢
CL
Corrected with a contact lens
Uncorrected eye
SC P¢
MYOPIASIGNIFICANCE OF VERTEX DISTANCE
C P¢
Relaxed eye
C P¢
S
ƒSP
ƒ CL
Corrected with a contact lens
Farpoint
Uncorrected eye
C P¢
S
Corrected with a spectacle lens
Starts from 4.00D
CORRECTION FOR VERTEX DISTANCE
FC/L=
d = Distance form back vertex of spectacle lens to corneal apex
FSp
(1 - d FSp)
SPECTACLE MAGNIFICATION
Corrected ametropic image size Uncorrected ametropic image size
• ratio compares a clear image to a blurred image.
SM =
CONTACT LENS MAGNIFICATION
Image corrected with contact lenses Image corrected with spectacles
CLM =
SPECTACLES VS CONTACT LENSES
• Image size α Focal Length α 1
lens power
Myope
Hyperope
hCL
hspec
hspec
hCL
+
-
CONTACT LENS MAGNIFICATION• In comparing spectacle and contact lens image sizes:
• CLM = 1 -d FSp (d = vertex distance)
• Examples with d = 14 mm + 10.00 D, CLM = 0.86 - 10.00 D, CLM = 1.14
• Therefore, with contact lenses, hyperopes experience a smaller image size than with spectacles
• Similarly, myopes experience a larger image size than with spectacles
GULLSTRAND-EMSLEY SCHEMATIC EYE SPECTACLES VS CONTACT LENSES
LIGHT
CS P
d2
f
g
F
d
CP = 1.55
P¢
f¢F¢
RSM APPLICATIONS
• Useful in anisometropia• Aetiology of the ametropia is unknown• If K readings mirror ametropia, major cause is
probably refractive
RELATIVE SPECTACLE MAGNIFICATION
• Axial ametropia: correct with spectacles
• Refractive ametropia: correct with contact lenses
AMETROPIA: AXIAL OR REFRACTIVE
• High ametropias usually axial• Axial anisometropia best corrected with
spectacles• Most ametropes are approximately
isometropic• Choice of correction is then usually based on
other considerations
HOW DO SM, CLM & RSM RELATE TO ONE ANOTHER?
• SM is a comparison of corrected vs uncorrected retinal image sizes
• CLM is a comparison of CL corrected vs spectacle lens corrected, retinal image sizes
• RSM compares image sizes in a corrected ametropic eye and a theoretical emmetropic schematic eye
APHAKIA
• Aphakia is considered refractive• If IOLs are not implanted, contact
lenses are preferable
ASTIGMATISM
• High corneal astigmatism is classed as a refractive ametropia
• Spectacle lenses will cause significant meridional aniseikonia
14mm= 400 mm
= -2.50 D
+5.00 D
Vertex DistanceReading Position
ACCOMODATION: SPECTACLES VS CONTACT LENSES
+5.00 D (spectacle) Hyperope
ACCOMMODATION: EMMETROPE
• Vergence of near pencil @• Cornea 414 mm away: -2.415 D• Ocular Rx @ cornea: Plano• Accommodation required: 2.415 D
14mm= 400 mm
= -2.50 D
+5.00 D
Vertex DistanceReading Position
ACCOMODATION: SPECTACLES VS CONTACT LENSES
+5.00 D (spectacle) Hyperope
ACCOMMODATION: +5.00 D HYPEROPE
Vergence of near pencil after spectacle lens:
2.50 D (-2.50 + (+5.00))
+Vergence @ cornea 14 mm away:
+2.591 D
Ocular Rx @ cornea: +5.376 D
Accommodation requiredAt spectacle plane
2.785 D
cf. Accommodation in CLs 2.415 D
ACCOMMODATION:-5.00 D MYOPE
• Vergence of near pencil after spectacle lens: -7.50 (-2.50 + (-5.00))
• Vergence @ cornea 14 mm away: -6.787 D
• Ocular Rx @ cornea: -4.673 D
• Accommodation required: 2.114 D• cf. Accommodation in CLs 2.415 D
INCIPIENT PRESBYOPIA
• 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
HYPEROPIA – CONVERGENCE SPECTACLES VS CONTACT LENSES
Apparent object position (spectacles)
Contact lens-wearing hyperope converges
LESS
Near object
Distance PD
MYOPIA - CONVERGENCESPECTACLES VS CONTACT LENSES
Apparent object position (spectacles)
Contact lens-wearing myope converges MORE
Near object
Distance PD
CONTACT LENSES ON A CORNEA
• 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
• If a rigid lens decenters, the tear lens will acquire a prismatic component
NEUTRALIZATION OF ASTIGMATISM
• Cornea/tears interface is optically 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
OPTICAL ADVANTAGES OF CONTACT LENSES
• No astigmatism of oblique pencils• No distortion• No chromatic aberration• No limitations on the field of view• No spectacle frame diplopia
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