Optics of Contact lenses
Dr.C.R.Thirumalachar
Introduction
• Major refraction of eye occurs at
AIR/CORNEA INTERFACE.
• Spectacle lenses- most common method
of correcting ref. errors
• Disadv of spectacles- too many, though
advantages are considerable
• Development of contact lenses – quest for
better looks and better vision
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Idea of contact lenses- Leonardo Da Vinci
Herschel & Fick evolved its practical use
Blown, Ground, moulded lenses
Lathe cutting, injection moulding & spin
moulding
• Evolved better materials , polymer
chemistry –Hard, soft, semisoft & gas
permeable .
Optics
• Contact lens placed in contact with cornea
with a thin fluid film in between
• It eliminates cornea as ref. surface
• Afocal contact lens:
– ant & post curvatures of CL same as cornea
– No optical power
– Surface irregularities of cornea are taken care
of
• Fluid lens
– Curvature of posterior surface of CL derives
the power of CL
• Glass lens
– Post surface of CL same curvature as cornea
– CL power derived by curvature of ant surface
of CL
• Combined lens
– Curvature of both surfaces contribute
– Both glass lens & fluid lens give dioptric
power.
Physical factors of CL
1. Overall diameter of Hard lenses- 8mm,
about 1.5-2mm less than corneal
diameter
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GP & soft lenses allow larger diameters
Larger lenses are more stable
Larger lenses – vaulting effect
2. Optical zone- central zone of 5-6mm
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Average of maximum & minimum pupil size
3. Curvatures
Ant. Curvatures:
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C.A.C (Cental ant.Curvature)- ant surface of
optical zone
P.A.C. (Peripheral ant curvature): slope on
the periphery of ant surface
I.A.C. (Intermediate anterior curvature) for
high power plus & minus lenses in between
CAC & PAC
– Posterior curvatures:
– CPC (central post curve )– Base curve to fit
the front surface of cornea
– I.P.C.(Intermediate)- flatter than CPC
– P.P.C.(Peripheral)- flatter than IPC
These are meant to serve as tear fluid reservoir.
These form ski for contact lens movement.
CL can have bicurve, tricurve or even multi
curve contour design.
4. Blend
Smooth area of transition of radius of curvature
from one curve to other
5. Edge-Polished & blended union of ant & post
surfaces
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Too sharp- may dig into corneal epithelium
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Too thick- may irritate the lids
Edge lift or Z factor- comfort & stability
• Power of CL determined by central ant & post surfaces
at O.Z. determined by the ammetropic correction
required
• Tint- to reduce the glare for cosmesis
• Ref. error determined by retinoscopy
• Curvature determined by keratometry
• Fit can be assessed by fluorescein pattern- flat , steep,
ideal
• Special situations
– Keratoconus, Aphakia, Pseudophakia, Post
keratoplasty
Thank you