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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:
Phernell Walker, II, AS, NCLC, ABOM
Email: pureoptics@earthlink.net
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
Copyright: Phernell C. Walker, II, AS,
NCLC, ABOM
Lens Cosmesis; Blending Optical Theory
With Cosmetic Lens Design 2
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
Copyright: Phernell C. Walker, II, AS,
NCLC, ABOM
Lens Cosmesis; Blending Optical Theory
With Cosmetic Lens Design 3
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
Copyright: Phernell C. Walker, II, AS,
NCLC, ABOM
Lens Cosmesis; Blending Optical Theory
With Cosmetic Lens Design 4
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.
Copyright: Phernell C. Walker, II, AS,
NCLC, ABOM
Lens Cosmesis; Blending Optical Theory
With Cosmetic Lens Design 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.
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
Copyright: Phernell C. Walker, II, AS,
NCLC, ABOM
Lens Cosmesis; Blending Optical Theory
With Cosmetic Lens Design 6
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
Copyright: Phernell C. Walker, II, AS,
NCLC, ABOM
Lens Cosmesis; Blending Optical Theory
With Cosmetic Lens Design 7
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
� n = Refractive Index
� 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
Copyright: Phernell C. Walker, II, AS,
NCLC, ABOM
Lens Cosmesis; Blending Optical Theory
With Cosmetic Lens Design 8
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
� n = Refractive Index
� 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
Copyright: Phernell C. Walker, II, AS,
NCLC, ABOM
Lens Cosmesis; Blending Optical Theory
With Cosmetic Lens Design 9
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
Copyright: Phernell C. Walker, II, AS,
NCLC, ABOM
Lens Cosmesis; Blending Optical Theory
With Cosmetic Lens Design 10
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
Linear Power Law Equation
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
Linear Power Law Equation
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
Copyright: Phernell C. Walker, II, AS,
NCLC, ABOM
Lens Cosmesis; Blending Optical Theory
With Cosmetic Lens Design 11
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
Copyright: Phernell C. Walker, II, AS,
NCLC, ABOM
Lens Cosmesis; Blending Optical Theory
With Cosmetic Lens Design 12
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
Copyright: Phernell C. Walker, II, AS,
NCLC, ABOM
Lens Cosmesis; Blending Optical Theory
With Cosmetic Lens Design 13
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
Copyright: Phernell C. Walker, II, AS,
NCLC, ABOM
Lens Cosmesis; Blending Optical Theory
With Cosmetic Lens Design 14
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
Phernell Walker, II, AS, NCLC, ABOM
Contact Information:
Phernell Walker, II, AS, NCLC, ABOM
Email: pureoptics@earthlink.net
www.pureoptics.com
(254) 338-7946
Secrets to Designing Specialty
Eyewear
Phernell Walker, II, AS, NCLC, ABOM
Master in Ophthalmic Optics
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