Optics of Contact lenses

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Optics of Contact lenses

• Dr.C.R.Thirumalachar

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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 .

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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

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• 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

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• Combined lens
• Curvature of both surfaces contribute
• Both glass lens fluid lens give dioptric power.

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Physical factors of CL

• Overall diameter of Hard lenses- 8mm, about
  1.5-2mm less than corneal diameter
• GP soft lenses allow larger diameters
• Larger lenses are more stable
• Larger lenses vaulting effect
• Optical zone- central zone of 5-6mm
• Average of maximum minimum pupil size

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• 3. Curvatures
• Ant. Curvatures
• 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

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• 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.

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• 4. Blend
• Smooth area of transition of radius of curvature
  from one curve to other
• 5. Edge-Polished blended union of ant post
  surfaces
• Too sharp- may dig into corneal epithelium
• Too thick- may irritate the lids
• Edge lift or Z factor- comfort stability

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• 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

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• Special situations
• Keratoconus, Aphakia, Pseudophakia, Post
  keratoplasty

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• Thank you