Optics of contact lens

Basic optics of contact lens

1. Optics of contact lens Moderator Presenter Dr. S anjeev Bhattarai Aayush chandan

2. Presentation Layout Introduction Basic optics Various optical properties of CL Advantage & Disadvantages Of CL References

3. What is contact lens ? It is an artificial device placed on cornea or sclera for optical or therapeutic purpose .

4. History of contact lens In 1508 , Leonardo da vinci first conceived & sketched prototypes of modern contact lens Experimented by neutralizing his own refractive error by placing his face in a container of water

5. Basic optics Although thin in appearance , CL are treated in geometrical optics as a thick lens Unlike thin lenses , the refraction of light as it passes through the thickness of the lens must be taken into consideration Thin lens Thick lens    

6. Conjugate planes Conjugate planes are planes perpendicular to the optical axis whose positions are related to each other by the image forming properties of the lens .

7. Principal plane The principal planes are two hypothetical planes in a lens system at which all the refraction can be considered to happen . For a given set of lenses & separations , the principal planes are fixed & do not depend upon the object position

8. Back vertex power The true focal lengths (f’ & f) are measured from the principal planes . Since these planes are theoretical constructs their locations are not obvious . Clinically it is not practical to measure true focal lengths of either contact lens or specs. In practice we measure the position of second principal focus from the back vertex of the lens since this is accessible .The power so measured is the Back Vertex Power (BVP)

9. The E ffectivity Relationship Vergence at any point is the inverse of the distance from the point to which it is converging or from which it is apparently diverging . When light is travelling in a medim other than air , the vergence is n/d i.e the reduced vergence . Vergence at D = 1/l Vergence at D’= 1/l-d = L/1-dL

10. So from the effectivity relationship we can have Back vertex power F v ’ F v ’=   Where , Feq = F1+F2-(t/n)F1F2

11. Front vertex power Also called as neutralizing power FVP can be calculated in a similar manner to that of BVP F V = + F1 F V = Used when specifying the power of haptic lens because the depth of a scleral shell may prevent the back vertex from touching the focimeter stop  

12. Optics of contact lens Various optical properties of contact lens include Vertex distance correction Magnification Accommodation Convergence Tear lens Field of vision Aberration Neutralization of astigmatism Over refraction

13. Vertex D istance Correction Vertex distance is the distance between back surface of lens & front surface of cornea . Formula for vertex correction is F CL = If distance(d) is not measured an assumption based on wearer’s anatomy can be made In Asian people , figures of 10-14 mm generally apply For Caucasians value of 12-15 mm are more likely  

14. Spectacle magnification Concerns the change in the retinal image size of a single eye brought about by a correcting lens(either spectacle lens or a contact lens) SM = Mathematically, SM =   Where, F1=power of front surface of lens Fv =BVP of composite lens t = lens thickness n=index of refraction of lens material h=distance from back vertex of lens to entrance pupil of eye

15. Spectacle magnification is always greater than unity for a plus lens & less than unity for a minus lens(for both spectacle and CL) Although the amount of magnification is different for a CL than that for spectacle lens because of much shorter distance between lens & entrance pupil of the eye

16. Contact lens magnification Contact Lens Magnification is the ratio of the image sizes in an ametropic eye corrected by a contact lens (CL) and a spectacle lens (SL ). CLM = Realistic since focused image are used in both the numerator and denominator  

17. Spectacle vs Contact Lens Image size in any optical system is directly proportional to the focal length of the system (or inversely proportional to lens power). So , in hyperopia f CL < f SL (shorter by vertex distance)consequently , Image size is smaller when CL are worn. In Myopia , f CL > f SL , image size is larger

18. In comparing spectacle and contact lens image sizes : CLM = 1 - d F SP Examples with d = 14 mm + 10.00 D, CLM = 0.86 - 10.00 D, CLM = 1.14 With contact lenses, hyperopes experience a smaller image size than they would with spectacles of equivalent power . Similarly, myopes experience a larger image size than they would with spectacles of equivalent power . Both of these outcomes are desirable and together constitute perhaps the biggest single advantage of contact lenses over spectacles .

19. Relative spectacle magnification Defined as the ratio of the retinal image size(for an object at infinity) of the corrected ametropic eye to that of standard emmetropic eye. RSM = To compare retinal image size of one eye with that of another eye. RSM = =  

20. RSM in Refractive ametropia If the source of the ametropia is assumed to be refractive RSM = 1 + d 2FSP With Spectacles ( d 2 ≈ d = vertex distance ): [RSM ≠ unity] With Contact Lenses, d 2 = 1.55 mm. In this context 1.55 mm is treated as being negligible (≈ 0 ). [RSM ≈ unity] Clearly, if anisometropia results from ametropia (unilateral or bilateral) which is refractive in origin, contact lenses would be the correction of choice because they produce negligible differences between the corrected image size and the normal emmetropic image size

21. RSM in Axial ametropia RSM = 1 - g F SP With Spectacles ( g ≈ 0 i.e. d 2 ≈ - f ) [RSM ≈ UNITY] With Contact Lenses g = f eye - ( d + 1.55) [RSM ≠ UNITY] Knapp’s Rule For axially ametropic eye , if the correcting lens is placed so that its secondary principal point coincides with anterior focal point of the eye , the size of retinal image is same as if it were standard emmetropic eye Clearly, if anisometropia results from ametropia (unilateral or bilateral) which is axial in origin, spectacles would be the correction of choice

22. How do SM, CLM and RSM Relate to One Another ? SM is a real-world comparison of corrected (focused ) and uncorrected (blurred) retinal image sizes CLM is a more realistic comparison of contact lens corrected versus spectacle lens corrected retinal image sizes RSM is a hypothetical magnification comparing image sizes in a corrected ametropic eye and a theoretical emmetropic schematic eye .

23. Accommodation Spectacle Refraction VS ocular Refraction The power of the correcting lens , specified at the spectacle plane is termed spectacle refraction The power of the correcting lens , specified at the first principal plane of the eye is termed ocular Refraction +10D -10D Spectacle refraction = +10D Spectacle Refraction = -10D Vertex Distance = 15mm Vertex Distance = 15mm Secondary focal length = 10cm-1.5cm=+8.5cm Secondary focal length = -10cm-1.5cm=-11.5cm Ocular Refraction = =+11.76D Ocular Refraction = = -8.70D +10D -10D Spectacle refraction = +10D Spectacle Refraction = -10D Vertex Distance = 15mm Vertex Distance = 15mm Secondary focal length = 10cm-1.5cm=+8.5cm Secondary focal length = -10cm-1.5cm=-11.5cm

24. Spectacle accommodation VS ocular accommodation For sake of convention , accommodation is usually considered to take place at the spectacle plane .( spectacle accommodation ) However because it represents a change in ocular refraction , accommodation actually takes place at the first principal plane ( ocular accommodation ) The amount of ocular accommodation required of an eye can be determined by the use of formula ocular accommodation = V d - V n For emmetrope , V d = = 0 V n = = -2.41D (0.09D less than 2.50D for spectacle plane)  

25. +10D -10D Ocular refraction ( V d )= +11.76D Ocular refraction ( V d )=-8.70D Vergence of light at the spectacle plane = -2.50+10=+7.50D Vergence of light at the spectacle plane = -2.50-10=-12.50D Distance from spectacle plane to image = 1/+7.50=+0.133m Distance from spectacle plane to image = 1/-12.50=-0.08m Distance from principal plane to image =+0.133-0.015=+0.118m Distance from principal plane to image = -0.08-0.015=-0.095m V n = = +8.47D V n = Ocular accommodation = V d -V n = +11.76-(+8.47)=+3.29D Ocular accommodation = V d -V n = -8.70-(-10.53)=+1.83 +10D -10D Ocular refraction ( V d )= +11.76D Ocular refraction ( V d )=-8.70D Vergence of light at the spectacle plane = -2.50+10=+7.50D Vergence of light at the spectacle plane = -2.50-10=-12.50D Distance from spectacle plane to image = 1/+7.50=+0.133m Distance from spectacle plane to image = 1/-12.50=-0.08m Distance from principal plane to image =+0.133-0.015=+0.118m Distance from principal plane to image = -0.08-0.015=-0.095m Ocular accommodation = V d -V n = +11.76-(+8.47)=+3.29D Ocular accommodation = V d -V n = -8.70-(-10.53)=+1.83

26. The 10D hyperope , therefore must accommodate about 1D more than emmetrope & 10D myope needs to accommodate about 0.50D less than emmetrope Because a contact lens fits on the cornea rather than about 13mm in front of it and therefore is less than 2mm in front of the first principal plane of the eye , the 10D hyperope has to accommodate less while wearing CL than while wearing spectacles and the 10D myope has to accommodate more while wearing CL than while wearing Spectacle

27. In summary Spectacle wearing myopes accommodate less than spectacle wearing hyperope With CL wear , the accommodation required in ametropia is approximately the same as emmetrope Accommodative demand of a myope is greater in CL than with Spectacle Accommodative demand of a hyperope is greater with spectacle than with CL

28. Incipient presbyopia If a myope is switched from spectacles to contact lenses the change may precipitate the need for a near correction If a hyperope is switched from spectacles to contact lenses the need for a near correction may be postponed

29. Convergence Monocular convergence = for PD=64mm VD=14mm CR=13.5mm   Where h=IPD/2 q= distance from the plane of fixation to the centre of rotation of the eye CL Spectacle -5.00D 14.97 13.26 Plano 14.97 14.97 +5.00D 14.97 17.18

30. A hyperope wearing CL converges less than when wearing spectacles because of the BO prismatic effect induced by speactacles acting as an exercising prism w hich forces more convergence than vertex distance would suggest. Since CL moves with the eye, no such prismatic consideration applies A myope wearing CL converges more than wearing Spectacle because of BI relieving prism effect & eye converges less than Vertex distance would suggest .

31. Patient Change in accommodative demand Change in accommodative convergence and phoria (at near vision) Change in prismatic effects and phoria (at near vision) High myopia with exophoria at near(low AC/A ratio) Increase Increase; decreased exophoria Lack of BI effect; *increased exophoria High myopia with esophoria at near(high AC/A ratio) Increase Increase ; *increased esophoria Lack of BI effect; decreased esophoria High hyperopia with exophoria at near(low AC/A ratio) Decrease Decrease ; increased exophoria Lack of BO effect ; *decreased exophoria High hyperopia with esophoria at near (high AC/A ratio) Decrease Decrease ; *decreased esophoria Lack of BO effect ; increased esophoria

32. The tear lens Tear lens is formed between the posterior surface of CL & anterior surface of cornea SCL conforms to the corneal curvature & forms a plano tear lens If RGP is used , ‘tear lens’ depends on the relationship between the curvature of the lens back surface & cornea & to lesser extent , the material’s rigidity BC equal to K reading : plano power tear lens BC steeper than K : plus power tear lens BC flatter than K : minus power tear lens

33. Tear lens power with rigid lenses A rule of thumb can be derived for tear lens under rigid lenses n tear = 1.336 n lens = 1.490 n air = 1.000 r =7.80mm flatter(r=7.85mm) Steeper(r=7.75mm) For convenience consider that CL & TL are separated by thin layer of air TL front surface power ( FS Tears )= = +43.076923   For (r=7.85) FS Tear = +42.802548 for (r=7.75mm) FS Tear = +43.354839 ∆ = -0.274375D ∆ = +0.277916D

34. Rule of thumb ∆0.05mm in BOZR ≈ ∆0.25D in BVP required to offset ∆ in tear lens power

35. FOV The pheripheral FOV is the field of view for steadily fixating eye , subtended at the entrance pupil & is given by equation Tan Φ = y(E-F) Macular FOV is the field of view for moving eye , subtended at centre of rotation of eye Tan θ = y(S-F) where Φ =one-half of the angular FOV Θ = one-half of the angular FOV Y= one-half of the lens aperture (in m) E= the vergence of light at the entrance pupil of the eye S=the vergence of light at the centre of rotation of eye F= the power of the correcting lens

36. For a spectacle lens of aperture 50mm and VD=15mm from entrance pupil For a CL of aperture 7mm and VD=3mm from entrance pupil In both examples above , the peripheral FOV is about 20° smaller with a CL than with Spectacle Spectacle lens Contact lens +5D 2 Φ =114.06° 2 Φ =97.93° -5D 2 Φ =121.66° 2 Φ =99.63°

37. Although these result could vary somewhat depending on VD of spectacle lens & aperture size of spectacle lens & CL , it is apparent that a CL doesn’t necessarily provide a longer peripheral FOV than spectacle lens The FOV is larger for a CL wear is not the peripheral FOV but Macular FOV The macular FOV can be shown to be larger for minus lens than plus lens

38. Field limitation . Hyperopia Myopia The ring scotoma is produced by the difference between the field limitations imposed by the frame/lens combination & optics of plus lens . The ring diplopia is produced by the difference between the field limitations imposed by the frame/lens combination & optics of plus lens As CL moves with the eye , no such limitation or scotoma results As CL moves with the eye no such limitation or diplopia results

39. 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

40. 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 apt to be problem for CL wearer

41. Neutralization of astigmatism Since refractive indices of the cornea & tear are not vastly different , the effectiveness of optical interface between two is much reduced If an astigmatic cornea is fitted with RGP ,tear lens is sphericalized by back surface of lens It is usually difficult to fit spherical lenses on cornea with 3D of corneal astigmatism. Some claim that 2D is a more realistic upper limit If astigmatism is to be corrected with SCL, toric lenses must be used

42. Approximately 90% of corneal astigmatism is neutralized by spherical RGP lenses

43. Over Refraction With RGP Oc Rx = BVP trail + P TL + Over Rx With SCL Oc Rx = BVP Trail + Over Rx

44. Reasons for Discrepancies Failure to correct for vertex distance of spectacle Rx when deriving the ocular Rx Failure to correct the over-Rx for vertex distance when it is > ±4D K readings only represent the central zone (approximately 3 mm) and give no indication of the shape of the periphery . Tear lens power varies slightly with K readings Trial lens BVP may be incorrect Subjective over-Rx is incorrect BOZR of the trial lens is incorrect Trial lens is decentred and/or tilted .

45. Trial lens flexure in situ for both rigid and soft lenses can mislead the practitioner Corneal molding by the lens Corneal shape not spherical or spherocylindrical Variable toric tear lens due to lens movement,decentration , tilting, rotation Environment-induced changes in a thick, soft trial lens

46. Advantages and Disadvantages of CL Advantages Disadvantages No astigmatism of oblique pencils. Lens decentration produces ‘ghosting’ or flare from the peripheral zone of the lens No distortion When a toric lens rotates, a toric over-refraction and decreased vision may result. No chromatic aberration Moving or generally unstable lenses may produce disturbances of vision No limitations on the field of view In axial ametropia , usually spectacles are better suited No spectacle frame diplopia . ( Myopia ). No spectacle frame scotoma . (Hyperopia). No prismatic imbalance in anisometropia Corneal irregularities/astigmatism reduced by 90%.

47. References IACLE module Clinical Optics ; Fannin & Grosverner Internet