Why Understand Lens Technology? Everything Evolves. Refined Mediocrity Is Still Mediocrity. Copyright: Phernell C. Walker, II, AS, NCLC, ABOM

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1 Secrets to Designing Specialty Eyewear Contact Information: Phernell Walker, II, AS, Master in Ophthalmic Optics Phernell Walker, II, AS, (254) Why Understand Lens Technology? Everything Evolves The perfect lens produces a sharp retinal image. Many obstacles to achieving this: Fluctuating tear film, aspheric cornea, aging zoom lens, mounting the lens on patient s nose. Inherent lens design deficiencies. 3 4 Refined Mediocrity Is Still Mediocrity. Why Understand Lens Technology? The same lens characteristics that produce focus have negative effects (aberrations): Thickness, Curvature, Diameter New lens technologies reduce aberrations Improve image quality Need to be able to distinguish between quality and marketing hype 5 6 With Cosmetic Lens Design 1

2 Effects of Visual Acuity in Various Lens Designs Designing the most optically precise and cosmetically appealing lens goes beyond common myths and optical roulette. Each lens design will effect visual acuity differently. Visual acuity is the clarity of vision or the quality of the apparent image. It requires a knowledge of both geometric optics and a sense of cosmetic appeal. 8 How Do Ophthalmic Lenses Correct Refractive Errors? Emmetropia Vs. Ametropia Lens Substrates Design 1.498n 1.523n 1.530n 1.549n 1.586n 1.60n 1.67n 1.70n 1.74n Emmetropic Eye Emmetropia= optically perfect eye (ideal) Axial length = 24 mm Approximately diopters of focusing power The lens system focuses light on the fovea centralis, where an image forms Ametropic Eye (Refractive Error) Ametropiais the opposite of emmetropia A refractive error is present Light fails to focus images at the fovea centralis Patient experiences blurred vision Myopia Vs. Hyperopia Myopia: Light focuses in front of the retina The eye has too muchconvergence power Hyperopia: Hyperopia If the retina was transparent, light would form an image behind the retina The hyperopic eye lacks convergence power 12 With Cosmetic Lens Design 2

3 Astigmatism Astigmatism is the most common ametropia. It is a refractive error in which light focuses on two independent focal points. First Minus Lens Design Minus lens Bi-Concave Minus dioptric power distributed on the front and ocular surface This is most commonly the result of an irregular shaped cornea. First Plus Lens Design Second Evolution of Lens Design Bi-Convex Plus power lens Plus dioptric power distributed on the front and ocular surface Plano concave = flat base curve Plano convex = flat ocular surface Un-Equal Vertex Distance Challenges with First and Second Generation Lens Designs Failed to eliminate radial astigmatic error Differentiating vertex distance Poor eyelash clearance on plus lenses Unappealing geometric lens shape Increased surface reflections Difficult to produce With Cosmetic Lens Design 3

4 Minus lensesuse divergence power to increaselight s subtending arc Vergence Power (Refraction) Vergenceis the process of bending light Plus lensesuse convergence power to decreaselight s subtending arc Lens Aberration Aberration is the failure of a mirror or lens to bring light rays to a single focal point. Types of Aberration: Chromatic (Transverse) Spherical Coma (Comatic Flare) Radial Astigmatic Error Curvature of Field Distortion (Barrel and Pincushion) Chromatic Aberration Chromatic aberration or chromatism is the dispersion of white light into it s natural component colors: Red= 656n Orange= 610n Yellow = 588n Green = 510n Blue= 486n Indigo = 410n Violet = 380n Spherical Aberration Spherical aberration occurs when broad peripheral light rays focus at a different point than paraxial rays. Since pupils are only 3 to 5mm in diameter, the effect of spherical aberration is limited. Coma Coma occurs when broad light rays pass obliquely through a lens. Radial Astigmatic Error Radial astigmatic error is the result of narrow parallel light rays that pass obliquely through a lens. The rays create two opposing focal points. The axial ray does not intersect at the same point as the peripheral rays. Radial astigmatic error degrades visual acuity more than any other aberration. Consequently, R.A.E. is the primary aberration lens designers try to eliminate. With Cosmetic Lens Design 4

5 Curvature of Field Curvature of Field is the inherent curvature of the image in the image plane and is a residual of a curved lens. Distortion Distortion occurs as the result of unequal magnification across a high powered lens. There are two types of distortion: Barrel The result is a blur in the periphery of the lens. Pincushion Barrel (Minus Lens) Pincushion (Plus Lens) Lens Formation Based on Base Curve Philosophy Tscherning s Ellipse With Cosmetic Lens Design 5

6 Calculating Dioptric Power Lens Dioptric Power Is Determined by Four Factors: Calculate the Dioptric Power of Thick Lenses Practical: D 1 + D 2 + (t) (D 1 ) 2 / n = D e Exact:[ D 2 / 1-(t/n) (D 2 ) ] + D 1 = D e Base Curve (Front Vertex Power) Ocular Surface (Anterior Vertex Power) Lens Thickness (Measured in Meters) Refractive Indice D 1 = Base Curve D 2 = Ocular Curve t = Thickness in Meters n = Refractive Index D e = Total Dioptric Power 1 = Constant Calculating Practical Lens Power Solution: A lens has a base curve of +9.00D, Ocular curve of -2.00D, 7mm thick and is made of plastic 1.60n. What is the lens power the patient will experience? D 1 + D 2 + (t) (D 1 ) 2 / n = D e Formula: D 1 + D 2 + (t) (D 1 ) 2 / n = D e (7mm) (9.00) 2 / 1.60 = D e (.007m) (81 ) / 1.60 = D e / 1.60 = D e = D e = D e (This is the power experienced by the patient ignoring vertex distance) Lens Thickness Formula Lens thickness can be calculated before the lens is manufactured using the following formula: ((R/2) 2 )(D e ) / ((n-1) (2000)) + t = T R = Lens Diameter D e = Total Dioptric Power n = Refractive Index t = Edge or Center Thickness T = Thickest Point of the Lens Calculate Lens Thickness Rx : OD DS PD 66 Frame size: 50x21 Ed 52. Lens Material 1.60n 1.0 center thickness. Formula: ((R/2) 2 )(D e ) / ((n-1) (2000)) + t = T ((59/2) 2 ) (-5.50) / ((1.60-1) (2000) = T ((29.5) 2 ) (-5.50) / ((0.60) (2000) = T (870) (-5.50) / (1200) = T (4785) / (1200) = T = 4.98mm Thickest Edge With Cosmetic Lens Design 6

7 True Vs Marked Surface Power A lens measure is used to determine surface power. Lens measures are calibrated for a refractive indiceof 1.530n. When using a lens measure the following formula must be used to achieve an accurate measurement: n = Refractive Index D 1 = Measured Surface Power (n-1) / (D 1 ) = D and 1 = Constant 37 Common Patient Complaints Distortion glasses just aren t right Glare difficulty with night vision\ Vision is clear but I m in a bowl Vision is clear but image is smaller Need to elevate chin to read Need to turn head to read 38 What Makes a Good Lens Design? Refractive Index and Image Quality The lens must be transparent. Able to reproduce a clear precise image. Economical to produce / purchase. Thin Light Strong 39 Refractive index -important role in image quality. As the refractive index of spherical (non-aspherical) lens increases, the image quality decreases in two areas: Chromatism Lateral Chromatism 40 Chromatism Vs. Lateral Chromatism Calculate Lateral Chromatism Chromatism-the dispersion of white light into it s natural component colors. The color dispersion increases as the material s index increases. Lateral chromatismis the increasing interval between red and violet wavelengths. It occurs in the meridionalplane and is expressed in diopters of prism. 41 Lateral Nu = (D e )(hcm) / Abbe Lateral Nu = Lateral Chromatism D e = Lens Dioptric Power (Specified Meridian) hcm= Centimeters from the OC Abbe= Material s V value 42 With Cosmetic Lens Design 7

8 Lateral Chromatism Example Increasing Refractive Index How much lateral chromatismwill a patient experience looking 8mm above the OC (0 pantoscopictilt) of a DS, 1.70n lens? Decrease the Base Curve Formula: Lateral Nu = (D e )(hcm) / Abbe Lateral Nu = (-9.50) (.8cm) / 30 Lateral Nu = 7.60 / 30 Lateral Nu = 0.25 Prism diopters Proper Base Curve Selection If the new prescription is within 1 diopter of the previous prescription and the patient is comfortable with the view through the lenses, keep the same base curve (unless the refractionist specifies otherwise) If the Rx has changed by more than 1 diopter, change the base curve. True Vs. Marked Surface Power A lens measure (Geneva lens clock) is used to determine surface power. Lens measures are calibrated for a refractive index of 1.530n. When using a lens measure the following formula must be used to achieve an accurate measurement: (n-1) / (D 1 ) = D 1 If the glasses are half eyes, decrease the base curve 2 diopters due to the increased vertex. Always increase parabolic angle when decreasing the base curve. 45 n = Refractive Index D 1 = Measured Surface Power and 1 = Constant 46 The Truth About the Relationship Between Field of View and Curvature Decreased Field of View Resulting From a Plus Lens and a Steep Base Curve High Minus Lenses Increase the Patient s Field of View. Flatter Base Curves Increase Field of View. Higher Plus Lenses Decrease Field of View. Steeper Base Curves Decrease Field of View With Cosmetic Lens Design 8

9 Increased Field of View Resulting From a Minus Lens and a Flat Base Curve Plate Height Plate height shows the lens profile. Flatter base curves creates a more cosmetically appealing lens Understanding Different Progressive Lenses Today lens manufactures offer multiple progressive lens designs. Though many designs are available, the basic optical fundamentals remain the same. What Exactly is a Progressive Lens? Progressive lenses designed to allow presbyopicpatients the ability to see at multiple focal lengths without residual image jump (base down prism effect), without a restrictive focal length no demarcation lines How Does a Progressive Lens Work? Traditional nonprogressive lenses use rotationally symmetric surfaces with a specific focal point or radius of curvature. Progressive lenses use conic sections blended together to create free-form surfaces, which result in multiple focal points. Asymmetrical Surfaces With Cosmetic Lens Design 9

10 Progressive Design Considerations Refractive indices Prescription Pantoscopictilt of the frame Pupil distance Progressive Design Considerations Lens center and edge thickness Ocular vertex pole (distance from the cornea to the lens) Front vertex pole (distance from the lens to the object) Object's angular position in the eye's field of vision Equi-thinning (ramifications of equithinning) Linear Power Law Equation Linear Power Law Equation The increase in dioptric power (per millimeter) through the corridor (umbilical line) can be calculated using the linear power law equation. D e = D add / h umbilical D e = Dioptric shift in plus power D add = Add power h umbilical = Length of the progressive corridor What is the amount of dioptric shift through the umbilical corridor of a linear progressive design with the following RX: OD: DS OS: x 180 Add: D e = D add / h umbilical D e = / 22 D e = The power dioptric power shift equates to a little more than an eighth diopterper 1mm downward shift Linear Power Law Equation Astigmatic Nature of Isocylindrical Dioptric Power and Magnification As the add power increases, positive radial astigmatic dioptric power is introduced in the lens design resulting in skewed aberration and an increase in magnification With Cosmetic Lens Design 10

11 Horizontal Symmetry Horizontal symmetryensures your vision will be identical in both eyes anywhere on the lens (even with two different prescriptions) while maintaining normal stereopsis. Reading Power Threshold Beware of some manufacturer s claims of minimum optical center fitting heights. These claims must take into account the Reading Power Threshold,or simply put, that point in which optimum add power is achieved (100% of the prescribed additional power) while maintaining an acceptable field of view Minimum Fitting Cross Height Digitally Surfaced Lenses Digitally surfaced lenses use Morphing Technology. This technology allows for a varying corridor length and width based on parameters such as the prescription, frame measurements, lens substrate and other factors (i.e. vertex, vertex pole, etc ) One-Size-Fits-All Multifocal Hmmm.. One-Size Fits All Multifocal What a great concept! It s too bad it doesn t work. 65 Inside Every Optician is An Artist As you can see from my original Picasso below, Mrs. Peanut-butter, that s how a progressive lenses work 66 With Cosmetic Lens Design 11

12 Power Grid Emerging Trends for Single Vision Lenses 5 0 Fitting Cross D +2.00D +2.50D % Fitting Cross % Fitting Cross % Aspheric and Atoric lenses -the new Buzzwords for single vision lenses. Free-form Progressive Lenses. Trivexis emerging onto the scene. Lens designers are revisiting ophthalmic glass due to technological advances Aspheric vs. Atoric Lenses Aspheric lensesuse rotational Asphericity(Sagittal). Results in non-stable vision due to the change in surface power as the eye rotates behind the lens. Atoric lensesuse linear Asphericity(Tangentially). The result is optimized vision in every meridian as the eye rotates behind the lens. Atoric Lenses Revisiting Ophthalmic Glass Lens Designs to Accommodate Drill Mounts Lantelglass up to 1.90n Thinner lenses Unsurpassed optics New ways to harden surface for improved safety Rimless eyewear -more popular than ever. Fashion demands have challenged technology to create a lens that can handle drill mounts. Lenses are secured by only two points of tension for each lens. Traditional lenses simply cannot handle the stress of these new frame designs and often crack. Polycarb is impact resistant but not heat resistant With Cosmetic Lens Design 12

13 MR-10 Resin MR-10 Resin MR-10 -designed by Carl Zeiss& Seiko. centralized 10mm aspheric button reduces radial astigmatic error, chromatic aberration, and distortion. Heat resistant won t develop spider cracks Won t warp Refractive Index = 1.67 n refractive Index Abbe Value = 32 Center Thickness = 1.0mm (minus lenses) Specific Gravity = 1.36 (gcm 3 ) Purpose = Rimless eyewear Cutting Edge Lens Treatments Lens treatments have evolved from yesterday s choices of: What color tint do you prefer? Would you like a solid or gradient tint? Today we have a plethora of advanced lens treatments that dwarfs yesterdays choices! 75 Basic Physics of Thin Films Anti-reflective coatings work on the principle of destructive wave interference. As light encounters a lens, a percentage of the light reflects off both the base and ocular curves (front and back surfaces). The amount of light reflectance is dependant upon several factors including the lens refractive index and the surrounding refractive index (air 1), which can be determined using Fresnel s equation: % Reflection = 100 [(n-1) 2 / (n+1) 2 ] 76 Substrate to Reflection Factor Example: Light reflected off each surface of a lens with a 1.70n refractive index Formula: % Reflection = 100 [(n-1) 2 / (n+1) 2 ] % Reflection = 100 [ (1.70-1) 2 / ( ) 2 ] % Reflection = 100 [ (.70) 2 / (2.70) 2 ] % Reflection = 100 [(.49) / (7.29)] % Reflection = (100) (.0672) % Reflection = 6.72 % each surface (13.44% combined total) Refractive Index and Reflection Correlation As the refractive index increases so does the amount of reflections. By adding a layer(s) of a metal oxide, typically a thickness which is ¼ the wavelength of incident light, a secondary wave front is created which cancels reflections of a specific wave length. This is known as destructive wave interference With Cosmetic Lens Design 13

14 V Coatings Vs. Broadband Treatments V Coatings Broad band treatments designed at (multi-coatings) eliminate 550nm reflections across the entire yellow/green) visible spectrum (380 to Very thin 750nm), maximizing the Limits percentage of available light. amount of The result is more than 99% light entering of available light reaches the eye retina with minimal Less reflections, ghost images and expensive reduced blur. 79 Conclusion All lens types and designs work. Some work better than others. The best lens design is the one that will maximize your patient s visual acuity and comfort, at a reasonable price. 80 References: Pure Optics By Phernell Walker, II, AS, Contact Information: Phernell Walker, II, AS, pureoptics@earthlink.net (254) Secrets to Designing Specialty Eyewear Phernell Walker, II, AS, Master in Ophthalmic Optics With Cosmetic Lens Design 14