Optics of Contact lenses
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Transcript of 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
1. 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
2. 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