secrets to designing specialty contact information: eyewear › files ›...

Copyright: Phernell C. Walker, II, AS,

NCLC, ABOM

Lens Cosmesis; Blending Optical Theory

With Cosmetic Lens Design 1

Secrets to Designing Specialty

Eyewear

Phernell Walker, II, AS, NCLC, ABOM

Master in Ophthalmic Optics

Contact Information:


Email: [email protected]

www.pureoptics.com

(254) 338-7946

Why Understand Lens

Technology?

� 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

Everything Evolves

4

Refined Mediocrity Is Still Mediocrity.

5

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

6


NCLC, ABOM



Designing the most optically precise

and cosmetically appealing lens goes

beyond common myths and optical

roulette.

It requires a knowledge of

both geometric optics and a

sense of cosmetic appeal.

Effects of Visual Acuity in Various Lens Designs

Each lens design will effect visual acuity differently.

Visual acuity is the clarity of vision or the quality of

the apparent image.

8

How Do Ophthalmic Lenses

Correct Refractive Errors ?

Lens DesignLens Substrates

•1.498n

•1.523n

•1.530n

•1.549n

•1.586n

•1.60n

•1.67n

•1.70n

•1.74n

Emmetropia Vs. Ametropia

Emmetropic Eye

� Emmetropia =

optically perfect eye

(ideal)

� Axial length = 24 mm

� Approximately 63.00

diopters of focusing

power

� The lens system

focuses light on the

fovea centralis, where

an image forms

Ametropic Eye

(Refractive Error)

� Ametropia is 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 much convergence power

Hyperopia

12

Hyperopia:

� If the retina was transparent, light

would form an image behind the retina

� The hyperopic eye lacks convergence

power


NCLC, ABOM



Astigmatism

Astigmatism is the most

common ametropia. It is a

refractive error in which

light focuses on two

independent focal points.

This is most commonly the

result of an irregular

shaped cornea.

First Minus Lens Design

Bi-Concave

� Minus lens

� Minus dioptric power distributed on the

front and ocular surface

First Plus Lens Design

Bi-Convex

� Plus power lens

� Plus dioptric power distributed on the front

and ocular surface

Second Evolution of Lens Design

� Plano concave = flat

base curve

� Plano convex = flat

ocular surface

Un-Equal Vertex DistanceChallenges 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


NCLC, ABOM



Vergence Power

(Refraction)

Vergence is the process of bending

light

� Minus lenses use

divergence power to

increase light’s

subtending arc

� Plus lenses use

convergence power to

decrease light’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.

The axial ray does not intersect at the same

point as the peripheral rays.

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.

Radial astigmatic error degrades visual acuity more than any

other aberration. Consequently, R.A.E. is the primary

aberration lens designers try to eliminate.


NCLC, ABOM



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.

The result is a blur in the periphery of the lens.

Distortion

Distortion occurs as the result of Distortion occurs as the result of

unequal magnification across a high unequal magnification across a high

powered lens. powered lens.

There are two types of distortion:There are two types of distortion:

��BarrelBarrel

��PincushionPincushion

Barrel (Minus Lens)

27

Pincushion (Plus Lens)

28

Lens Formation Based on Base

Curve Philosophy Tscherning’s Ellipse


NCLC, ABOM



Calculating Dioptric Power

Lens Dioptric Power Is Determined by Four

Factors:

� Base Curve (Front Vertex Power)

� Ocular Surface (Anterior Vertex Power)

� Lens Thickness (Measured in Meters)

� Refractive Indice

Calculate the Dioptric Power of

Thick Lenses

Practical: D1 + D2 + (t) (D1)2 / n = De

Exact:[ D2 / 1- (t/n) (D2) ] + D1 = De

� D1 = Base Curve

� D2 = Ocular Curve

� t = Thickness in Meters

� n = Refractive Index

� De = Total Dioptric Power

� 1 = Constant

Calculating Practical Lens Power

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?

D1 + D2 + (t) (D1)2 / n = De

33

Solution:

Formula: D1 + D2 + (t) (D1)2 / n = De

+9.00 + -2.00 + (7mm) (9.00)2 / 1.60 = De

+7.00 + ( .007m) (81 ) / 1.60 = De

+7.00 + .567 / 1.60 = De

+7.00 + .35 = De

+7.35 = De (This is the power experienced by the

patient ignoring vertex distance)

34

Lens Thickness Formula

Lens thickness can be calculated before the

lens is manufactured using the following

formula:

((R/2)2 )(De) / ((n-1) (2000)) + t = T

R = Lens Diameter

De = Total Dioptric Power

n = Refractive Index

t = Edge or Center Thickness

T = Thickest Point of the Lens

35

Calculate Lens Thickness

Rx : OD -5.50 DS PD 66 Frame size: 50x21 Ed 52.

Lens Material 1.60n 1.0 center thickness.

Formula: ((R/2)2 )(De) / ((n-1) (2000)) + t = T

((59/2)2) (-5.50) / ((1.60-1) (2000) + 1.0 = T

((29.5)2) (-5.50) / ((0.60) (2000) + 1.0 = T

(870) (-5.50) / (1200) + 1.0 = T

(4785) / (1200) + 1.0 = T

3.98 + 1 =

4.98mm Thickest Edge

36


NCLC, ABOM



True Vs Marked Surface Power

A lens measure is used to determine surface power.

Lens measures are calibrated for a refractive indice of

1.530n.

When using a lens measure the following formula must

be used to achieve an accurate measurement:

(n-1) / 0.530 (D1) = D1


� D1 = Measured Surface Power

� 0.530 and 1 = Constant37

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?

� The lens must be transparent.

� Able to reproduce a clear precise

image.

� Economical to produce / purchase.

� Thin

� Light

� Strong39

Refractive Index and Image Quality

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

� Chromatism - the dispersion of white

light into it’s natural component

colors. The color dispersion increases

as the material’s index increases.

� Lateral chromatism is the increasing

interval between red and violet

wavelengths. It occurs in the

meridional plane and is expressed in

diopters of prism.41

Calculate Lateral Chromatism

Lateral Nu = (De)(hcm) / Abbe

� Lateral Nu = Lateral Chromatism

� De = Lens Dioptric Power (Specified Meridian)

� hcm = Centimeters from the OC

� Abbe = Material’s V value

42


NCLC, ABOM



Lateral Chromatism Example

How much lateral chromatism will a patient experience

looking 8mm above the OC (0 pantoscopic tilt) of a -

9.50 DS, 1.70n lens?

Formula: Lateral Nu = (De)(hcm) / Abbe

Lateral Nu = (-9.50) (.8cm) / 30

Lateral Nu = 7.60 / 30

Lateral Nu = 0.25 Prism diopters

43

Increasing Refractive Index

Decrease the Base Curve

44

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.

� 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

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) / 0.530 (D1) = D1


� D1 = Measured Surface Power

� 0.530 and 1 = Constant

46

The Truth About the Relationship Between

Field of View and Curvature

� 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.

47 48

Decreased Field of View

Resulting From a Plus Lens and a Steep Base Curve


NCLC, ABOM



49

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.

50

Understanding Different

Progressive Lenses

Today lens manufactures offer

multiple progressive lens designs.

Though many designs are

available, the basic optical

fundamentals remain the same.

51

What Exactly is a Progressive Lens?

Progressive lenses –

� designed to allow presbyopic patients the ability to see at multiple focal lengths

� without residual image jump (base down prism effect),

� without a restrictive focal length

� no demarcation lines.

52

How Does a Progressive Lens Work?

� Traditional non-

progressive

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.

53

Asymmetrical Surfaces

54


NCLC, ABOM



Progressive Design Considerations

� Refractive indices

� Prescription

� Pantoscopic tilt of the frame

� Pupil distance

55

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 equi-thinning)

56

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.

De = D add / h umbilical

De = Dioptric shift in plus power

D add = Add power

h umbilical = Length of the progressive corridor

57


What is the amount of dioptric shift through the umbilical corridor of a linear progressive design with the following RX:

OD: -3.00 DS

OS: -2.75 –0.25 x 180

Add: +2.75

De = D add / h umbilical

De = +2.75 / 22

De = 0.125

The power dioptric power shift

equates to a little more than

an eighth diopter per 1mm

downward shift.

58


59

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.

60


NCLC, ABOM



Horizontal Symmetry

Horizontal symmetry ensures your

vision will be identical in both eyes

anywhere on the lens (even with

two different prescriptions) while

maintaining normal stereopsis.

61

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.

62

Minimum Fitting Cross Height

63

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…).

64

“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


NCLC, ABOM



Power Grid

67

0 0.5 1 1.5 2 2.5 3 3.5-25

-20

-15

-10

-5

0

5

Fitting Cross

95%

0 0.5 1 1.5 2 2.5 3 3.5-25

-20

-15

-10

-5

0

5

Fitting Cross

95%

0 0.5 1 1.5 2 2.5 3 3.5-25

-20

-15

-10

-5

0

5

Fitting Cross

95%

+1.50D

+2.00D

+2.50D

Emerging Trends for Single Vision Lenses

� Aspheric and Atoric lenses - the new

“Buzzwords” for single vision lenses.

� Free-form Progressive Lenses.

� Trivex is emerging onto the scene.

� Lens designers are revisiting ophthalmic glass

due to technological advances.

68

Aspheric vs. Atoric LensesAspheric lenses use rotational Asphericity (Sagittal). Results in non-stable vision due to the change in surface power as the eye rotates behind the lens.

Atoric lenses use linear

Asphericity (Tangentially). The

result is optimized vision in

every meridian as the eye

rotates behind the lens.

69

Atoric Lenses

70

Revisiting Ophthalmic Glass

� Lantel glass – up to 1.90n

� Thinner lenses

� Unsurpassed optics

� New ways to harden surface for

improved safety

71

Lens Designs to Accommodate Drill Mounts

� 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

72


NCLC, ABOM



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

73

MR-10 Resin

Refractive Index = 1.67n refractive Index

Abbe Value = 32

Center Thickness = 1.0mm (minus lenses)

Specific Gravity = 1.36 (gcm3)

Purpose = Rimless eyewear

74

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 / (1.70 + 1)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)

77

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.

78


NCLC, ABOM



V Coatings Vs. Broadband Treatments

� V Coatings –

designed at

550nm –

yellow/green)

� Very thin

� Limits

amount of

light entering

eye

� Less

expensive

� Broad band treatments

(multi-coatings) eliminate

reflections across the entire

visible spectrum (380 to

750nm), maximizing the

percentage of available light.

� The result is more than 99%

of available light reaches the

retina with minimal

reflections, ghost images and

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

81

References: Pure OpticsPure OpticsPure OpticsPure Optics

By


Contact Information:


Email: [email protected]

www.pureoptics.com

(254) 338-7946

Secrets to Designing Specialty

Eyewear


Master in Ophthalmic Optics

secrets to designing specialty contact information: eyewear › files ›...

Documents