OPTICS OF CONTACT LENSES

GREESHMA GBS,MPHIL OPTOMETRY

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¢

REFRACTIVE ANISOMETROPIA: HYPEROPIC

» Image sizes

CL

SL@feye

q

q

R

L

REFRACTIVE ANISOMETROPIA: MYOPIC

» Image sizes

R

LCL

SL@feye

q

q

AXIAL ANISOMETROPIA: HYPERMETROPIC

» Image sizes

R

LCL

SL@feye

q

q

AXIAL ANISOMETROPIA:MYOPIC

» Image sizes

R

LCL

SL@feye

q

q

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

Lid

Rigid Contact Lens

‘Prismatic’ Tear Lens

Cornea

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

REFRACTIVE INDICES OF CONTACT LENS MATERIALS

PMMA = 1.49 RGPs = 1.48 - 1.41 SCLs = 1.44 - 1.38

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

OPTICAL DISADVANTAGES OF CONTACT LENSES

• Lens decentration produces ‘ghosting’• When a toric lens rotates, a toric over refraction and decreased vision results• Moving lenses may produce disturbances of vision• In axial ametropia spectacles are better suited