Optics and design of soft contact lens.pptx

Contact lens

1. Optics and Design of Soft contact lens Presented by: Ashi Lakher B Optometry Final year

2. Basics: Although thin in appearance , contact lens in geometrical optics is considered as thick lens. Thickness of the lens must be taken into consideration. F t = F 1 + F 2 +(t/n) * F 1  F 2

3. Optics of Contact lens Various optical properties of contact lens include  Effectivity relationship Magnification Accommodation  Convergence  Tear lens  Field of vision  Aberration 

4. Effectivity: There is non-zero vertex distance of any spectacle correction, the far point will lie at slightly different distances from the two types of correcting lens .Thus the spectacle and contact lens powers required to correct a particular eye will differ.

5. Myopes: Minus lens is more effective when moved closer to eye Will need less power in CL than Spec Hyperopes: Plus lens is less effective when moved closer to eye Will need more power in CL than Spec

6. Magnification Spectacle magnification, as its name implies, describes the ratio of the image size in the corrected ametropic eye to that in the uncorrected eye. The component of spectacle magnification are Power Factor and shape factor.

7. Myopes: - Will have less minification with CLs - Often report things look larger with CLs Hyperopes  - Will have less magnification with CLs - Often report things appear smaller (“more normal”) with CLs

8. Accommodation  Vertex distance of lens affects the amount of accommodative stimulus. Myopes: - Stimulus to accommodate INCREASES as lens moves towards eye Hyperopes: - Stimulus to accommodate DECREASES as lens moves towards eye This effect increases with the increase in lens power.

9. Convergence  Spectacles optically centred for distance vision but which are used for all distances of gaze exert a prismatic effect during off-axis viewing.            Myope – BI prismatic effect            Hyperope – BO  prismatic effect Contact lenses which move with the eyes remain centred for all distances, and positions of gaze cause no such prismatic effect.

10. A contact lens wearing myope has to exert more convergence than with spectacle. A contact lens wearing hyperope has to exert less convergence than with spectacle.

11. Tear lens Tear lens is formed between the posterior surface of  CL and anterior surface of cornea. The tear layer between contact lens and the corneal surface is an optical lens in its own. Soft contact lens conforms to the corneal curvature and forms plano  tear lens.

12. Field of vision  The field of view larger for contact lens wear is macular FOV. Because contact lens are closer to the entrance pupils and lack frames (spectacle frames reduce the field of corrected vision by about 20°), contact lenses provide a larger field of corrected vision .

13. Aberration  Because a CL wearer always looks through a point at or near optical centre of the lens , Chromatic aberration (longitudinal &Transverse) doesn’t present a problem  The aberration of Spherical aberration & Coma occur only for large aperture optical system. As with spectacle lenses , aberration do not present a problem for CL of moderate power because eye looks through only a small position of lens.

14. .The aberration of Oblique astigmatism & Curvature of Image occur when a narrow pencil of light from an object passes obliquely through spherical surface . Because CL fits the eye in such a way that foveal line of sight always passes through a point at or near the optical centre of lens , aberration do not present a problem for CL wearer . Distortion result as gradual change in magnification brought about by the lens from centre towards periphery . Because eye always fixates through the centre of CL & pupil of eye is very small , distortion is not  a problem for CL wearer

15. Lens design of soft contact lens

16. Design of contact lens is an important issue because it optimizes the ocular response for individual and its purpose is to achieve comfort, safety and vision. Design matters most with physiologically poorer material and least with better material.

17. Lens parameters: Total diameter: Maximum external diameter of finished contact lens Optic zone diameter: It is the dimension of central optical zone meant for focusing rays on retina. Should be larger than pupil size and cover during movement. 

18. Base curve:  • Curve on the back surface of the lens to fit the front surface of cornea  • It is measured by its radius of curvature in mm or diopter . Peripheral curve: Secondary curve just outside the base curve at the edge of contact lens. Typically flatter than base curve to approximate the normal flattening of peripheral cornea.

19. Sag height: Maximum distance from a chord, which is perpendicular to the axis of rotation of a surface, to the curved surface. Critical for determining good fit. Effect of sagittal height and Diameter on lens fit: Increasing sag height independent of diameter - Steeper =Tight fit Decreasing sag height independent of diameter - Flatter =Loose fit

20. Soft lens design factors • Geometric centre thickness  • Lens diameter (total diameter, TD) • Back optic zone radius (BOZR)  • Back surface design  • Front optic zone radius (FOZR)  • Front surface design Radial edge thickness Edge design Material physical/mechanical properties Material physiological properties Peripheral junctional thicknesses if transitions exist

21. Diameter: All soft contact lens are fitted larger than HVID. Thickness: Along with central, mid peripheral, and edge thickness ,Overall lens thickness is also important parameter. Local thickness is the only relevant thickness when calculating local O 2  availability since there is little tear mixing under soft contact lens. Curvature: Front optic zone radii and back optic zone radii are important in Rx determination. Other radii define the physical design of lens which also affects lens behaviour.

22. Material properties Very important in soft lens design because have effect on fitting behaviour, comfort, durability etc. Center thickness consideration: Dk/t - must meet the corneal O 2  requirement of intended lens use. Pervaporation prevention Fitting consideration

23. Water content and thickness

24. Fitting characterstics - BOZR BOZR less significant in soft than RGP. Flexible material – greater conformity, thinner post lens tear film. Large changes in lens parameter required to produce clinically significant alteration to lens behaviour. Eg : Soft lens BOZR need change of 0.3mm        RGP lens need change of 0.05-0.1mm

25. Design factor   Back Surface Design : Monocurve    Simplest design but uncommon. Bicurve   Common design  Second curve often 0.8-1.0mm flatter than BOZR. Blended multiple spherical curve (Multicurve) Uncommon Aspheric  Shapes cornea better.

26. Back peripheral curve: Presence or absence of back peripheral curve insignificant physiologically. Change in back peripheral curve specially radical edge lift affects lens movement substantially.

27. Front surface design Front surface design tend to be ignored but is important to lens fit and comfort. It depends on the manufacturing process. Bicurve – most common with peripheral curve chosen to produce thin edges. Multiple blended peripheral curve Continuous aspheric front surface curve.

28. Edge design: Less consideration given because of size of soft lens. Edge is already under both lids hence has little effect on comfort. Too thick edge – Discomfort Too thin edge – Tearing of edge.

29. Aspheric soft lens A mathematically regular non spherical surface. Based on conic section. Conic section relevant to soft lens are Circle (e= 0) Ellipse (e=0.5) Parabola (e=1)

30. Eccentricity value of a conic section is a description of the flattening rate of the lens periphery. Another factor , 'P' =Shape factor         P = (1-e 2 ) Q = -e 2      = p-1 As eccentricity increases , rate of peripheral flattening increases exponentially

31. Soft aspheric: Aspheric requires fewer back curve step to cover range of fit. Bearing pressure more uniform . Better lens corneal -peri-limbal fitting relationship. Disadvantage: More expensive to manufacture May decenter and move more than spherical design.