OPT506 - L10 - Optics of CL 2024-25 PDF

Optics of Contact Lens Dr. Asma Zahidi Learning outcomes 1. Understand the effect of vertex distance on the power of the C L 2. Compare the image sizes produced by spectacle and contact lens and 3. Explain the effect of contact lens and spectacle wear on the field of view 4. Compare t...

Optics of Contact Lens Dr. Asma Zahidi Learning outcomes 1. Understand the effect of vertex distance on the power of the C L 2. Compare the image sizes produced by spectacle and contact lens and 3. Explain the effect of contact lens and spectacle wear on the field of view 4. Compare the difference in convergence with CL and spectacle wear 5. Compare the difference in accommodation with CL and spectacle wear 6. Understand the neutralization of regular / irregular astigmatism with a tear lens. Optical principles of CL 1. Power of the C L 2. Image size 3. Field of view 4. Convergence 5. Accommodation 6. Neutralization of regular / irregular astigmatism with a tear lens. Power of Contact Lens COMPENSATING FOR VARIATIONS IN VERTEX DISTANCE F ′Spec F ′CL = (1 − dF ′Spec ) A common calculation is estimating the Back vertex power (BVP) of a CL based on the eye’s spectacle Rx. The CL BVP is the effectivity of the spectacle Rx in the corneal plane. The separation of the two planes is known as the vertex distance CORRECTING HYPEROPIA CLs vs. Specs Note that CLs have a A SHORTER distance over which to bring light to a focus than do spectacle lenses, i.e. fC Lens < fSpec Lens B Therefore, the BVP of a CL must be GREATER than the equivalent spectacle to correct HYPEROPIA C D CORRECTING MYOPIA CLs vs. Specs Note that CLs have a LONGER A distance over which to bring light to a focus than do spectacle lenses, i.e. fC Lens > fSpec Lens B Therefore, the BVP of a CL must be LOWER than the equivalent spectacle to correct MYOPIA A hint of differences in image C size according to mode of correction (specs vs CLs) are apparent D Image size SPECTACLES vs CONTACT LENSES: MAGNIFICATIONS Spectacle Magnification (SM) = Corrected ametropic image size Uncorrected ametropic image size SM is a comparison of a clear retinal image with a blurred retinal image Corrected image size with CLs Contact Lens Magnification (CLM) = Corrected image size with specs CLM is a more realistic comparison of two clear retinal image sizes SPECTACLES vs CONTACT LENSES CONTACT LENS MAGNIFICATION (CLM) This diagram is a derivative of the previous slide CONTACT LENS MAGNIFICATION (CLM) Referring to right angled triangles CFhC & SFhS Image size with CL CLM = Image size with spectacles FhC CLM = FhS But image sizes are directly proportional to the focal lengths Therefore... f ' CL F ' Spec CLM = or CLM = f ' Spec F ' CL However, we have already shown that: F ' Spec F ' CL = (1 − dF ' Spec ) F ' Spec CLM = F ' Spec (1 − dF ' Spec ) Simplifying... CLM = 1 − dF ' Spec CONTACT LENS MAGNIFICATION (CLM) From its derivation, CLM is a comparison of CL & spectacle lens image sizes CLM = 1 − dFSpec Calculating examples using a vertex distance of 14 mm (d = 14 mm): For a +10 D HYPEROPE, CLM = 0.86 (i.e. 14% SMALLER) For a –10 D MYOPE, CLM = 1.14 (i.e. 14% LARGER) These CLMs show that hyperopes experience a smaller image size with CLs than with spectacles of equivalent Conversely, myopes experience a larger image size than with spectacles  IMAGE SIZE (CL MAGNIFICATION) With contact lenses, hyperopes experience a smaller image size than with spectacles Similarly, myopes experience a larger image size than with spectacles CL vs Specs Spec vs No RX CL vs No RX Spec vs No Rx Myopia Hyperopia CL vs Spec -/+ 5.00 D +8% -7% -/+ 10.00 D +13% -11% -/+ 15.00 D +22% -16% CL vs No Rx RELATIVE SPECTACLE MAGNIFICATION (RSM) RSM: The ratio of image size in the corrected ametropic eye to the image size in the NORMAL emmetropic eye. However, RSM is complicated by the need to consider the nature of the ametropia, i.e. does it have an axial or a refractive origin? Note: fEye = –(g + d2) The Optics of CLs: RSM – REFRACTIVE - Hyperopia REFRACTIVE ANISOMETROPIA: HYPEROPIA (Spec image is LARGER) The Optics of CLs: RSM – AXIAL- Hyperopia AXIAL ANISOMETROPIA: HYPEROPIA (Spec image is about the SAME) REFRACTIVE ANISOMETROPIA: MYOPIA (Spec image is SMALLER) AXIAL ANISOMETROPIA: MYOPIA (Spec image is about the same) RSM: APPLICATIONS Useful in clinical decision making in cases of anisometropia any resulting aniseikonia must be considered origin of the ametropia, i.e. axial or refractive, is central unfortunately, the origin is difficult to determine & can even be a combination of both If K readings mirror the ametropia, probable refractive. e.g.  steep cornea in moderate - high myopia  flatter cornea in hyperopia RSM: SUMMARY AXIAL ametropia: correct with SPECTACLES REFRACTIVE ametropia: correct with CLs Implication: CLs are not always the correction of choice Clinical judgement is required Refractive surgery procedures, e.g. LASIK, are REFRACTIVE in origin (cornea-based corrections) AMETROPIAS: AXIAL OR REFRACTIVE Higher ametropias (±4 to 8 D) are more likely to be AXIAL spectacle correction may be better in cases of axial anisometropia Importantly/fortunately, most ametropias are approximately isometropic, not anisometropic ‒∴other considerations may apply, e.g. cosmetic, occupational, & safety issues HOW DO SM, CLM, & RSM RELATE TO ONE ANOTHER? All are retinal image size comparisons SM compares corrected (focused) with uncorrected (blurred) images – difficult to deal with! CLM compares images with CL & spectacle corrections – a more realistic situation RSM compares corrected with a theoretical, emmetropic, schematic eye - hypothetical (axial, refractive, mixed, or unknown?) WHAT ABOUT APHAKIA? APHAKIA is considered to be refractive in origin IOL implantation is considered to be the ideal correction. Eye → pseudophakic (location of Rx nearly ideal) ‒IOLs impart optical & physiological advantages If IOLs are not implanted, CLs are preferable WHAT ABOUT ASTIGMATISM? Astigmatism is classed as a refractive ametropia (one meridian can be considered ‘normal’, the other meridian the ‘cause’ of the astigmatism) Spectacle lenses can induce significant meridional aniseikonia, especially in high astigmatism CLs are the correction of choice in astigmatism despite some fitting issues Field of view FIELDS-of-VIEW: FIELD LIMITATIONS IN SPECTACLES: HYPEROPIA As CLs move with the eye, no such limitations apply to their use FIELDS-of-VIEW: FIELD LIMITATIONS IN SPECTACLES: MYOPIA As CLs move with the eye, no such limitations apply to their use Convergence CONVERGENCE: EMMETROPIA / CLs CONVERGENCE: HYPEROPIA – SPECS vs CL WEARER (or EMMETROPE) CONVERGENCE: MYOPIA – SPECS vs CL WEARER (or EMMETROPE) Accommodation ACCOMMODATION WITH SPECTACLES ACCOMMODATION WITH CONTACT LENSES ACCOMMODATION: EMMETROPE Vergence of near pencil of light @ cornea, 347.33 mm (333.33mm+14mm) away 1 from reading material: 0.34733 = 2.88D Ocular Rx @ cornea: Plano (emmetrope) Accommodation required: 2.88 D Accommodation in CLs is the same: 2.88 D ACCOMMODATION: +5.00 D HYPEROPE K = +5.00D A Reading distance (l1) Vertex Pencil of light after distance (d) cornea (l2) 14mm K = Power of spectacle lens L1 = Vergence of light from object to spectacle plane L1’ = Vergence of light after spectacle plane L2 = Vergence of light after the cornea A = Accommodation ACCOMMODATION: +5.00 D HYPEROPE K = +5.00D A Reading distance (l1) Vertex Pencil of light after 333.33mm distance (d) cornea (l2) 14mm Step 1 – Find the vergence of near pencil of light after the spectacle lens L’1 =L1 + K = -3.00D + (+5.00D) = +2.00D *-3.00D is obtained from the object distance which is 333.33mm away from the spectacle plane ACCOMMODATION: +5.00 D HYPEROPE K = +5.00D A Reading distance (l1) Vertex Pencil of light after 333.33mm distance (d) cornea (l2) 14mm Step 2 – Find the vergence of near pencil light after the cornea L2 = 1/l2 l2 =l’1 – d =(1/+2.00D) – 14mm = 500mm – 14mm = 486mm L2 = 1/l2 = 1/0.486m = +2.058D ACCOMMODATION: +5.00 D HYPEROPE K = +5.00D A Reading distance (l1) Vertex Pencil of light after 333.33mm distance (d) cornea (l2) 14mm Step 3 – Find the ocular Rx at the cornea k = n/K (*K is the power of the spectacle lens) = 1/+5.00D = 200mm f’sp = k-d = 200mm – 14mm = 186mm Fsp = n/f’sp = 1/ 0.186m = +5.376D ACCOMMODATION: +5.00 D HYPEROPE K = +5.00D A Reading distance (l1) Vertex Pencil of light after 333.33mm distance (d) cornea (l2) 14mm Step 4 - Find the accommodation with specs A = Ocular Rx at cornea – L2 = 5.376D – 2.058D = 3.318D Accommodation in CLs: 2.88 D (determined previously) Difference: 0.44 D MORE in specs ACCOMMODATION: –5.00 D MYOPE K Reading distance (l1) Vertex distance (d) 14mm 333.33mm Pencil of light after cornea (l2) A ACCOMMODATION: –5.00 D MYOPE Step 1 – Find the vergence of near pencil of light after the spectacle lens L’1 =L1 + F K = -3.00D + (-5.00D) = -8.00D *-3.00D is obtained from the object distance which is 333.33mm away from the spectacle plane Reading distance (l1) Vertex distance Step 2 – Find the vergence of near (d) 14mm 333.33mm pencil light after the cornea Pencil of light after cornea (l2) L2 = 1/l2 l2 =l’1 – d A =(1/-8.00D) – 14mm = -125mm – 14mm = -139mm L2 = 1/l2 = 1/-0.139m = -7.194D ACCOMMODATION: –5.00 D MYOPE Step 3 – Find the ocular Rx at the cornea k = n/K (*K is the power of the spectacle lens) K = 1/-5.00D = -200mm f’sp = k-d = -200mm – 14mm = -214mm Fsp = n/f’sp = 1/ -0.214m Reading distance (l1) Vertex distance (d) 14mm = -4.673D 333.33mm Pencil of light after cornea (l2) Step 4 - Find the accommodation with specs A A = Ocular Rx at cornea – L2 = -7.194D – (-4.673D) = 2.521D Accommodation in CLs: 2.88 D (determined previously) Difference: 0.36 D LESS in specs ACCOMMODATION: SPECTACLES vs CONTACT LENSES READING DISTANCE: 333.33 mm ( –3.00 D ) ( from spectacle lens ) ACCOMMODATION (D) Accommodation cf. an Emmetrope or CL wearer 5.5 5 With specs 4.5 4 LESS MORE 3.5 3 With CLs 2.5 With CLs 2 1.5 With specs -20 -15 -10 -5 0 +5 +10 +15 +20 SPECTACLE Rx ( @14 mm ) ACCOMMODATION: INCIPIENT PRESBYOPIA If a myope is switched FROM spectacles TO contact lenses, the change may PRECIPITATE the need for a near correction in myopia, specs → CLs can ↑ any existing problem If a hyperope is switched FROM spectacles TO contact lenses, the change may POSTPONE the need for a near correction in hyperopia, specs → CLs can ↓ any existing problem NEAR VISION IN CLs: SUMMARY Convergence: CL-wearing MYOPES converge MORE (cf. spectacles) CL-wearing HYPEROPES converge LESS (cf. spectacles) Accommodation: CL-wearing MYOPES accommodate MORE (cf. spectacles) CL-wearing HYPEROPES accommodate LESS (cf. spectacles) At NEAR: CL-wearing MYOPES CONVERGE more & ACCOMMODATE more than HYPEROPES Tear lens  TEAR LENS: NEUTRALIZATION OF ANTERIOR CORNEAL ASTIGMATISM The tears ↔ cornea interface is less significant optically nTears = 1.336 & nCornea = 1.376 Tear lens is ‘sphericalized’ by the back surface of a spherical rigid CL These factors combined → a major ↓ in corneal astigmatism when a spherical rigid CL is worn However, gross differences in CL & corneal curvatures are dysfunctional/physically unacceptable, ∴practical limits apply to the amount of corneal astigmatism that can be corrected by a simple rigid spherical CL practical limits depend on the topography of the cornea but a limit of about 2.0 - 3.0 D is widely accepted  Tear Lens & Corneal Astigmatism NEUTRALIZATION OF ANTERIOR CORNEAL ASTIGMATISM Ks (mm)( ∆0.3 ) 8.4/8.7 7.5/7.8 6.8/7.1 Astigmatism ( D ) 1.544 1.928 2.336 With CL in situ ( D ) 0.164 0.205 0.249 Residual ( % ) 10.64 10.64 10.64 n′ − n Meridional Power = r RULE-OF-THUMB: Approximately 90% of anterior corneal Using the first column's data in air: n′ − n 1.376 − 1.000 astigmatism is neutralized by the presence of a spherical Power1 = r = 8.4 = +44.762 D rigid CL. n′ − n 1.376 − 1.000 However, all other sources of ocular astigmatism remain Power1 = = = +43.218 D r 8.7 unaltered. If the anterior cornea is astigmatic but the AstigmatismAir=1.544 manifest ocular Rx is spherical, a spherical rigid CL in situ will Using the first column's data in situ: disclose the ‘internal astigmatism’ as a residual astigmatism n′ − n 1.376 − 1.336 Power1 = = = +4.762 D with an axis at 90 ° to the corneal astigmatism & of about r 8.4 n′ − n 1.376 − 1.336 90% its magnitude Power1 = = = +4.598 D r 8.7 Astigmatismin situ =0.164 D Summary Myopia Hyperopia Power Decreased Increased Imagesize Larger Smaller Field of view Larger Larger Convergence Increased Decreased Accommodation Increased Decreased Vision (quality) Increased Same Question