This document discusses the optics of contact lenses. It covers how contact lenses work by eliminating the cornea as a refractive surface and deriving their optical power from the curvature of the anterior and/or posterior surfaces. The document also describes the physical factors of contact lenses such as diameter, optical zone, curvatures, blend, and edge; and how power, tint, refractive error, curvature, and fit are determined. Special fitting considerations for conditions like keratoconus are also mentioned.

1. Optics of Contact lenses
Dr.C.R.Thirumalachar

2. 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

3. • 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 .

4. 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

6. • 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

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

10. 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

11. 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

13. – 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.

14. 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

15. • 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

16. • Special situations
– Keratoconus, Aphakia, Pseudophakia, Post
keratoplasty

17. Thank you