contact lens material

contact lens

1. RAJESHWORI NGAKHUSHI
Bachelor of Optometry

2. Content
Introduction
Lens design
Lens materials
Verification
Standards and
ordering

3. Introduction
A thin glass shell bounded by concentric and
parallel spherical segments ( Fick )
A contact lens, or simply contact, is a thin lens
placed directly on the surface of the eye.
considered medical devices and can be worn to
correct vision, or for cosmetic or therapeutic
reasons

4. Lens design
Design of contact lens is an important issue – it
optimizes the ocular response for the individual
and purpose is to achieve comfort, safety and
vision
Design of the RGP lens can be more complex than
the soft lens
Design matters - Most with physiologically poorer
materials and Least with better materials

5. Soft lens design

6. 94N27-9S.PPT
LENS PARAMETERSLENS PARAMETERS
tEA
tER
Ø0
Ø1
ØT Øa0
Ø0 = Back Optic Zone Diameter
(BOZD)
Øa0 = Front Optic Zone Diameter
(FOZD)
Ø1 = Back Peripheral Zone Diameter
(BPZD)
ØT = Total Diameter (TD)
tER = Radial Edge
Thickness
tEA = Axial edge Thickness

8. Soft lens design factors
• Geometric centre thickness
• Lens diameter (total diameter, TD)
• Back optic zone radius (BOZR)
• Back surface design
• Front optic zone radius (FOZR)
• Front surface design

9. Radial edge thickness
 Edge design
Material physical/mechanical properties
Material physiological properties
 Peripheral junctional thicknesses if transitions
exist

10. Soft contact lens design
1.DIAMETER :All soft lenses are fitted 1-2mm larger
than the horizontal visible iris diameter(HVID)
2.THICKNESS:
1.Along with central, mid peripheral and edge
thickness the overall lens thickness profile is also
important.
2. Local thickness is the only relevant thickness
when calculating local O
2 availability since there
is little tear mixing under a soft lens

11. 3.CURVATURE: the back and front optic zone Radii
are important to Rx determination other radii define
the physical design of the lens which also affects
lens behaviour .Corneal curvature -----flatter by 3-
5D
4.DESIGN: After defining centre thickness , front and
back radii in the optical zone, the remainder of the
lens design is defined by the radii of peripheral
curves , their widths , their numbers and the
junctional thickness.
Design—high prescription------aspheric design, multi
curve design

12. RELATIONSHIP WITH THE EYES: the
parameter of a contact lens should closely match the
dimensions of the ocular surface
 eg- corneal topography
HVID

13. 1.Material properties
Material properties are very significant in soft
lens design
Material properties of a soft lens have a
significant effect on fitting behaviour, comfort,
durability, etc
Water contents of 25 - 79% means material
properties vary greatly
Significance of material properties often leads
lens designers to develop material-specific lens
series.

14. 2.Center thickness consideration
1.Dk/t consideration- cornea’s O2 requirements
must be met
2.Pervaporation prevention: a high water material
with thin lens design, pervaporation corneal
dessication may result
3.Fitting considerations: too thin lens - excessive
flexing no dispersal of metabolic wastes due to
conformity overall lens performance is not good.
Lens wrinkiling causes ----corneal wrinkling and
staining

15. Minus lens series
Lenses of lower minus power (<2.00D) are made
thicker and with a larger FOZD to improve handling
For -3.00 to -6.00D,the lens series have constant
centre thickness

16. Plus lens series
Geometrical lens thickness cannot be decreased
since it is a function of BVP.
Reduction of FOZD is limited by vision issues –
not be tolerated by most wearers except with
small pupils

17. 3.Water content and thickness
thickness lens
Below 0.10mm Thin lens
Below 0.07mm Ultrathin lens
Below 0.05mm Superthin or hyperthin lens

19. Diagrams Representing the O2 Performance of
Low/High Water and Thick/Thin Lenses

20. TRANSMISIBILITY (Dk/t)
Dk H2O content
∝
O2 and CO2 transmissibilities 1/t
∝
corneal respiration is best served by a thin high
water lens.
Higher the H2O content, higher Dk/t
Greater the thickness, lesser the Dk/t
tc for minus lenses overestimates Dk/t
tc in plus lenses underestimates Dk/t

21. To prevent corneal oedema Holden & Mertz(1984)
derived a criteria of critical oxygen transmissibility
and EOP values
Equivalent
oxygen
percentage
Type of lens
wear
O2
transmissibility
Dk/t
9.9% Daily wear 24
17.9% EW 87
12.1% Compromised
lens wear
34.3

22. To achieve zero daytime edema
thickness are physiologically desirable they are
impractical
H2O
content
Daily wearExtended
wear
Compromise
EW
38% 0.033mm 0.009mm0.023mm
75% 0.166mm 0.117mm

23. Pervaporation
If the lens is too thin, corneal dehydration may result
due to bulk flow of water through the lens and
instability of water flow at the lens surface
Produces epithelial desiccation staining -
pervaporation staining
High water content lenses loose more water than less
water content due to temperature difference, pH and
tonicity

24. HIGH WATER CONTENT LENSES
 Lose more water than low water lenses (% of
total) on eye
 Lose water even when worn in a high humidity
environment
Experience on-eye lens shrinkage which affects
TD and BOZR.

25. Advantages of high water content
lenses
• Better comfort because of material softness.
• Faster adaptation.
• Longer wearing time.
• Extended wear.
• Easier to handle because of greater thickness.
• Better vision because of greater thickness.
• Better for intermittent wear.

26. Disadvantages of high water content
lenses
• Shorter life span and Greater fragility.
• More deposits, especially white spots.
• More discolouration.
• Reproducibility less reliable.
• Greater variation with environment.
• Fitting requires longer settling time.
• Greater variability in vision.
• More solutions problems.
• Lens dehydration and Corneal desiccation.

27. Advantages of low water content
lenses
• Greater tensile strength.
• Less breakage.
• Longer life span.
• Better reproducibility.
• Easier to manufacture.
• Can be made thinner.
• Less dehydration on the eye.
• Less discolouration with age.
• Fewer solutions problems.

28. Disadvantages of low water content
lenses
• A greater tendency to cause corneal oedema.
• A long-term tendency with thicker lenses (e.g.
with high powers) to cause vascularization

29. 4.Other Design Considerations
Centration Quality of vision, comfort and
mechanical effects of a lens on the eye, depend to
some extent on centration.
Movement - minimal amount of movement is
required for all soft contact lenses to remove debris
under the lens.

30. Design factors
Back surface designs
Front surface designs
Edge designs
Aspheric soft lenses
Lens design - limitations

31. Back Surface Designs
 Single curve - simplest design but not commonly
used.
Bicurve - second curve often 0.8 - 1.0 mm flatter
than BOZR and about 0.5 - 0.8 mm wide.
Blended multiple spherical curves (multicurve) –
fexible lenses don’t need a multicurve design
Aspheric – shapes cornea better

32. BACK PERIPHERAL CURVES
Presence or absence of back peripheral curves is
insignificant physiologically
Changes in back peripheral curves,especially
radical edge lift, affect lens movement
substantially

33. Front Surface Design
it tends to be ignored
important to lens fit and on-
eye behaviour
also influence the comfort of
the lens - especially true in
cases of higher Rxs because of
their greater thicknesses

34. Bicurve - with a peripheral curve chosen to
produce a thin edge.

Multiple blended peripheral spherical curves.
Continuous aspheric front surface curves are not
commonly used.

35. Front surface may also include bifocal or
multifocal components such as:
Continuous aspheric surface
 Concentric bifocal
 Flat-top segment

36. Edge Design and Thickness
Edge is already under both lids & has relatively little
effect on comfort
Edge thickness is governed by durability
considerations rather than comfort or physiology
concerns.
Too thick- discomfort
Too thin- tearing of the edge

37. Aspheric Soft Lenses
 ‘aspheric’ means a conicoid
 A mathematically regular nonspherical surface
usually take the form of a parabola, ellipse or
hyperbola and are defined by eccentricity.
Circle, e =0
Ellipse, e = 0.5
Parabola, e = 1
As eccentricity increases , the rate of peripheral
flattening or steepening increases exponentially

38. Contd.
e - Defines mathematically the departure of an
aspheric curve from a circle. Used to describe
both a lens form and the curvature of the cornea.
P value - Defines the rate of flattening with
eccentricity:p = 1 — e2.
closest mathematical approximation to the
topography of the human cornea is an ellipse.
Mean eccentricity = 0.45; p = 0.8.

39. ASPHERIC ADVANTAGES
• Better lens/cornea-peri-limbal fitting relationship
• Fewer base curve steps required
• Lens fit less sensitive to lens diameter changes
• Increased lens movement
• Bearing pressure more uniform

40. ASPHERIC DISADVANTAGES
• More expensive to manufacture
• Not as readily available
• Perceived to be more complex
• May decentre and move more than spherical
design

41. Manufacturing process may limit
lens design:
Method Limitations
Lathing
Molding-Anhydrous
Molding-Wet stabilized
Spin-casting
Molding & Lathing
Spin-casting & Lathing
Simple designs only
Few, but anisotropic expansion
on hydration changes lens shape
Almost none
Only simple back surface design
Possible
Lathing limitations
Lathing limitations

42. Rigid gas permeable Lens Design
Design is the cornerstone of any contact lens
fitting.
Ultimate goal of rigid lens design is to achieve
ideal fit
Essential for optimizing response

43. The desirable properties of an RGP lens are :
1.Optimal design
2.Material :
High Dk
Wettability
Deposit resistance
Stability
Ease of manufacture: manufacturing difficulties
with a particular material can be a barrier to its
usage.

44. DESIRED FITTING
Moderate edge width and clearance
Central and mid-peripheral alignment
Smooth movement
Centration

45. DESIRED PERFORMANCE
• Comfortable
• Clear vision
• Adequate wearing time
• Minimal ocular response
• Normal facial appearance

46. KEY DESIGN FEATURES
• Back surface design
• Back optic zone diameter
• Front surface design
• Lens thickness
• Edge configuration
• Lens diameter

47. Tricurve corneal lens
Ø
t
- total diameter
Ø1 - first back peripheral
zone diameter;
Ø0 - back optic zone
diameter;
 r
o
-

back optic zone radius
r
1
- back peripheral radius
r
2
- second back peripheral
radius

48. Continous non spherical design
Single continuous curve - approximates
cornea’s shape
Aspheric designs
Regular non spherical curves whose
centers of curvature appear to be off the
axis of symmetry

49. BACK SURFACE DESIGN
Controls Lens/Cornea Interaction
Affects both centration and movement
DESIGN FREEDOM
• Spherical or aspheric
• Single or multiple curves
• Fitting relationship

50. Back surface design – clinical
considerations

51. Back optic zone radius
Aspheric Spheric
Better alignment
Difficult to manufacture
Difficult to verify
 more decentration
Better vision
Better centration

52. Optic zone should be larger than the pupil size and
should cover it during the movement
Also dependent upon the overall diameter and the
peripheral curve and power

53. Optimal Back Surface Design:
• Alignment or a very slight tendency towards apical
clearance over the central 7 – 8 mm.
• Mid-peripheral alignment about 1 – 2 mm wide.
• Edge clearance about 0.5 mm wide.
• An obvious tear meniscus at the lens edge.

54. Back Surface Mid-Periphery
Should align flattening cornea
secondary and peripheral zones must have curves
which are flatter than the BOZR
Affects:
• Tear flow
• Stability of the fit
• Corneal mid-peripheral shape
• Centration

55. Back surface periphery affects
Fluorescein pattern at the periphery of the lens
eg. A flat and wide peripheral curve will result in
excessive edge clearance producing a bright band of
fluorescein
Tear exchange is greater with a wide and flat
peripheral curve
Excessive edge clearance results in an unstable fit
with excessive lens movement

57. Peripheral or edge curve
Radius - 2.50 mm flatter than BOZR
Width - 0.30 to 0.50 mm
Affects:
• Peripheral fluorescein appearance
• Centration
• Tear exchange
• Lens fit
• 3 & 9 staining

58. Edge width and tear reservoir

59. Edge configuration
Position of apex – centrally located apex was more
comfortable
Should not exhibit any high point
The topography of lens just inside the lens edge aka
blend of junctions, influences the edge profile,
thickness, junction angles..
Affects
Comfort
Durability
Tear meniscus

60. Edge shapes of lenses: (a) posterior; (b)
central; (c) anterior;
(d) blunt; (e) sharp

61. Rounded edge – most comfortable
Edge profile rough or square at the anterior side –
least comfortable
Posterior design – square
 Comfort is determined by interaction of lens edge
with the lid

62. Edge shape vs comfort

63. IDEAL FITTING
Centre - aligned
Mid-periphery - align/min. clearance
Pheripheral curve - 0.3-0.5 mm wide
AEL - 75-100μm clearance

64. LENS THICKNESS
Determined by:
•Rigidity
• Permeability
• Back vertex power
CONSIDERATIONS
‘On-eye’ lens flexure
 Correction of corneal astigmatism
 Dk/t

65. Center thickness
Each lens material has a critical thickness –
minimum ct which can be made of a particular
lens material so that the lens does not flex on the
eye
Ct – more in higher dk lenses

66. Suggested minimum thicknesses for different
materials (BVP-3.00D)
Material tc (mm)te (mm)
PMMA
CAB
Silicon acrylate
Fluorosilicon
acrylate
0.10
0.16
0.15
0.14
0.12
0.12
0.13
0.15

67. More stable and comfortable – center of gravity is
posteriorly located
Can be made stable by the diameter of the lens,
mass by lenticular design or adding minus
carrier lenses

68. Lenticulation affects:
 Centre thickness - In plus lenses only.
 Lens mass - true for all lenses.
 O2 transmission - true for all lens types
comfort

69. influence comfort, movement and centration

70. Junction angle & thickness
Affects
• Comfort
• Lens movement
• Centration
• Lens bulk

71. Lens diameter
Determined by:
Corneal diameter
 HVID of patient
Inter-palpebral aperture
 Lens power (minus/plus)

72. Lens diameter
Affects:
• Centre of gravity
• Stability
• Option to have larger
BOZD/FOZD
• Comfort
• 3 & 9 staining

73. Centre of Gravity

74. OTHER DESIGN ISSUES
Tints
Handling
Aid to colour defectives
Lens Markings
For ‘piggyback’ fits

75. Contact lens
material

76. Introduction
Glass was used exclusively for some yrs
PMMA began to replace glass in 1940s –
toughness, optical properties and physiological
inactivity

77. IDEAL CONTACT LENS
MATERIAL
• Meets cornea’s oxygen requirements
• Physiologically inert
• Excellent in vivo wetting
• Resists spoilation
• Dimensionally stable
• Durable
• Optically transparent
• Requires minimal patient care
• Easily machineable

78. IMPORTANT MATERIAL
PROPERTIES
• Oxygen permeability
• Wettability
• Scratch resistance
• Rigidity (RGPs)
• Flexibility (SCLs)
• Durability
• Deposit resistance

79. OPTICAL PROPERTIES
• Refractive index
• Spectral transmission
• Dispersion
• Scatter

80. Rigid contact lens
Material used is PMMA
Stable materials
Resists warpage , wets well and clean easily
Lack of permeability to oxygen – tear exchange
phenomenon
Backbone of all rigid lens materials
Trial lens

81. Properties:-
 excellent biocompatibility
 good optical properties
 scratch resistance
 good manufacturing properties
Fairly wettable when clean.
 Easy to care for.
 Rigid.
 0.2 - 0.5% water when hydrated fully.
 Almost zero oxygen permeability.
Produces ‘spectacle blur’

82. Gas permeable lenses
Essentially rigid lenses
Material used are:
Cellulose acetate butyrate
Silicone acrylate
Fluoropolymers( teflon)
styrene

83. Cellulose acetate
Cellulose is combined with acetic and butyric
acids ( 13% acetyl, 37% butyryl and 1-2%free
hydroxyl groups)
Low oxygen permeability
dk range of 4-8
Lack of dimensional stability i.e. Warpage,
scratching and coating
No longer available

84. Silicone acrylate
Silicone and oxygen are combined to make into
siloxane
Combined with PMMA to produce a gas
permeable lens
Most successful rigid gas permeable material
introduced in 1970
Dk value range 12 to 60 are achievable

85. Negative charge
Tended to become coated with proteinaceous
materials from the tears
Scratches easily may cause flexure problems if
made thinner
Pure silicone – o2 permeability is high but poor
wettability
Polycon II 14.2

86. Fluoro-Siloxane Acrylates
(FSAs)
Fluorine monomer added to SA material
Lower surface charge
 Withstand high heat and chemical attack
O2 permeability is like silicone but more wettable
Dks 40 to 100+ (med-high)
Surface easily scratched
 Greater lens flexure

87. Perfluoroethers
consists of: Fluorine, Oxygen, Carbon and
Hydrogen
Dk 90+ (high)
Neutral surface charge
Greater flexibility ‘on eye’
Low refractive index
 High specific gravity

88. SOFT CONTACT LENS
MATERIALS

89. PHEMA
Incorporation of hydroxyl group into PMMA gives 2-
hydroxy ethylmethacrylate and makes it more
hydrophilic
close relative of poly(methyl methacrylate)
Water content is approximately 38%

90. Other variants to improve PHEMA are:
PVP Poly Vinyl Pyrrolidone
MA Methacrylic Acid
MMA Methyl Meth Acrylate
GMA Glyceryl Meth Acrylate
DAA Di Acetone Acrylamide
PVA Poly Vinyl Alcohol

91. Convenient to consider the polymers that have
been used as contact lens materials under four
heading:
1.Thermoplastics – capable of being shaped or
moulded under heat or pressure
Eg: PMMA
Polyethylene and polyvinyl chloride
Copolymer of tetrafluoroethylene
Poly(4-methyl pent-1-ene)
Cellulose acetate butyrate(CAB)-

92. Synthetic elastomers
Not only fexible but show rubber like behaviour
Intermediate characteristics b/w thermoplastic
and hydrogel materials.
Oxygen permeabilities 100x-1000x more than
PMMA
Hydrophobic – surface treatment
Ethylene propylene terpolymer(EPT)
Silicone rubber or poly(dimethyl siloxane)

93. Hybrid RGPs
have a rigid GP central optical zone, surrounded
by a peripheral fitting zone made of a soft contact
lens material.
second generation silicone hydrogel hybrid
contact lens called Duette. The lens features a
highly oxygen-permeable GP center (Dk 130),
surrounded by a soft silicone hydrogel "skirt" for
comfort (Dk 84; 32 percent water).

94. Hydrogels
Called as soft, elastic, water containing gels
Witcherle and coworkers– first developed
hydrogels polymers (PHEMA)
Made from HEMA, lightly cross linked with
ethylene glycol dimethacrylate. (spin cast)
Monomers commonly used in hydrogel contact
lens materials include
N-vinyl pyrrolidone (NVP),
Methacrylic acid (MA) and
Poly-2-hydroxyethyl me-thacrylate
(polyHEMA).

95. Some Examples of Hydrogel materials, by Water
Content
Group 1
Low Water Content
Nonionic
Group 2
High Water
Content
Nonionic
Group 3
Low Water Content
Ionic
Group 4
High Water
Content
Ionic
Crofilcon
Dimefilcon A
Genfilcon A
Hefilcon A & B
Hioxifilcon B
Iotrafilcon A
Isofilcon
Mafilcon
Polymacon
Tefilcon
Tetrafilcon A
generally show
lower levels of
protein deposit
Alphafilcon A
Altrafilcon
Ofilcon A
Omafilcon A
Scafilcon A
Surfilcon A
Vasurfilcon A
Xylofilcon A
Heat and
sorbic acid should
be avoided for
disinfection
because of the risk
of lens
discolouration.
Balafilcon A
Bufilcon A
Deltafilcon A
Droxifilcon A
Etafilcon A
Ocufilcon A
Phemfilcon A
Bufilcon A
Etafilcon A
Focofilcon A
Methafilcon A, B
Ocufilcon B
Ocufilcon C
Ocufilcon D
Ocufilcon E
Perfilcon A
Phemfilcon A
Tetrafilcon B
Vifilcon A
show the highest
level of protein
deposition ,
heat and sorbic
acid should be
avoided for lens
disinfection.

97. Verification
Contact lens verification undergoes two stages,
laboratory and clinical
Laboratory
During the final phase of manufacture, an
overall parameter check is performed to ensure
the lenses do not differ significantly from the
parameters ordered by the practitioner.
Clinics
Verification of lenses upon receipt, rather than
during the dispensing visit, is advisable

98. Why Verify Contact Lens Parameters?
Ensure correct lens is dispensed
Assess changes in contact lens with wear
To ensure that proper over-refraction and trial fitting
examination has been conducted
To correlate with the manufacturer’s parameter to
actual lens parameter
Prior to initial dispensing of CLs, the clinician
should verify that all parameters of the lenses are
as ordered and that they meet established (e.g.,
ANSI) standards.

99. Rigid and soft lenses have similar parameters which
require verification by the practitioner.
 Radii of curvature
Linear parameters
 Edge profile
 Power
 Lens quality
Rigid and soft contact lenses should be hydrated in
a soaking solution for 12 - 24 hours before
verification procedures are conducted.

100. INSTRUMENTS
 Radiuscope
 Keratometer (modified)
 Toposcope
 Moiré fringe deflectometer
 Radius checking device
 Topographical mapping system
 Electrical conductivity method
 Microspherometer

101. Drysdale’s Principle

102. based on the theory that when a curved reflecting
surface is positioned so that the real image created by
the instrument is located at its centre of curvature
an image will be formed in the same plane as the
aerial object.
real image/aerial object is formed at the first focal
plane and an aerial image is formed at the second
focal plane
distance between the real image at the lens surface
and the aerial image

103. keratometer
used to measure the BOZR of a contact lens by using
special attachments.
Used with special contact lens holder which utilizes
the front surface silvered mirror and a lens support

104. Toposcope
Moire’ fringes were used - measuring radii and diameters
of corneal lenses.
target consist of a series of straight lines
shape and orientation of the fringes formed were a
function of the relationship between the two sets of lines
Straight parallel fringes indicate a spherical surface,
curved fringes indicate an elliptical surface. Any warpage
or dimples in the surface was indicated by irregularly
shaped fringes

105. Measures spherical, toric and aspheric contact lenses
Quality of lens surface can also be assessed

106. Power verification
verified with a lensometer
A smaller lens stop however, is recommended to
reduce the amount of light passing through the lens
focusing the light source more through the central
area of the lens.
May be recorded as being a greater positive or
smaller negative value than it actual value
True back vertex focal length is actually greater -
Back surface of lens is not in the plane of the stop

107. Lens must be centered
concave side down on
the focimeter stop -
BVP
with lens convex side
down - FVP
focimeter

108. Verification procedures
Diameters and linear
parameters
Measuring
magnifier
V gauze
Cast, dividers and
transparent rule
Micrometer &
spheres

109. Measuring magnifier
An adjustable eye piece through which an engraved
scale is viewed
Held with the concave surface towards the scale ( 20
mm long )

110. V gauge
Made of metal or plastic
V shaped channel cut into the material
Channel may vary in width from 6 to 12.50mm

111. Cast, dividers &
transparent rule
Micrometer &
spheres:

112. Thickness verification
Dial thickness gauze
Centre or edge thickness may
be determined with a suitable
thickness gauze which usually
incorporates a dial gauge
calibrated to 0.01mm
Centre thickness is measured
at a common geometric and
optical centre

113. LENS THICKNESS VERIFICATION:

114. Edge profile verification:
Instruments/techniques:
• Edge molding
• Projection magnifier
• Ehrmann profilometer
• Palm test
• Radiuscope (modified)

115. LENS AND SURFACE QUALITY
ASSESSMENT:
for RGP AND SCL
Instruments:
Magnifying 10x loupe
Projection magnifier
 Contact lens optical quality analyzer (CLOQA)
 Dark field microscope
 Moire fringe deflectometer

116. Differences between verification soft and RGP
lenses
Hydrogel contact lenses are flexible
If exposed to atmosphere, they dehydrate and alter
their contour. Verification in air is inaccurate due to-
Shrinkage of Hydrogel on dehydration
Accumulation of surface moisture
So, artifact liquid cells are used to measure
parameters of soft lenses
But RGP lenses can be measured in air

117. Contact lens standards

118. American National Standard Institute
(A.N.S.I.) 1999
Contact Lens Tolerances
Power Tolerance
O to 5.00D +/- 0.12D
SPHERE
POWER
5.12 to 10.00D+/- 0.18D
10.12 to 15.00D+/- 0.25D
15.12 to 20.00D+/- 0.50D

119. POWER TOLERANCE
CYLINDER
POWER
0 to 2.00D +/- 0.25
2.12 to 4.00D+/- 0.37
Over 4.00D +/- 0.50

120. Power Tolerance
Cylinder axis0.50 to 1.50D+/- 8°
Above 1.50D +/- 5°
Parameter Tolerance
Bifocal
refractive
Add power +/-0.25D
Seg height +/-0.10mm
For toric lens cylindrical axis is specified in
relation to the base apex meridian

122. The terminology for a standard tricurve lens in ISO 8320-1986
symbols is:
Example:7.90:7.80/8.70:8.60/10.75:9.20 tc 0.15 BVP
-3.00D Tint light blue
7.90 = back optic zone radius (BOZR) r0
7.80 = back optic zone diameter (BOZD) 00
8.70 = first back peripheral radius r2
8.60 = first back peripheral zone diameter 02
10.75 = second back peripheral radius r2
9.20 = total diameter 0T
0.15 = geometric centre thickness tc
-3.00 = back vertex power (BVP)

123. The ISO 11539 standard for the classification of
contact lenses, describes the use of a six part code to
describe a material type
Prefix – stem – series suffix – group suffix – Dk range
– modification code
Prefix and series suffix – administrated by United
States Adopted Names (USAN) council and such are
only relevant for materials with FDA approval.
Two types of stem – filcon stem - materials which
contain >10% water by mass (hydrogels )

124. Focon stem – materials which contain <10% water by
mass ( non hydrogel )
Group suffixFilcon ( hydrogel) Focon ( non hydrogel)
I
II
III
IV
Low water content non ionic <50% EWC
<1% ionic monomer
High water content, non-ionic
>50%EWC, <1%ionic monomer
Low water content, ionic <50%EWC,
>1% ionic monomer
Low water content, non ionic>50%,>1%
ionic monomer
No silicon + no fluorine
CAB
Silicone + no fluorine
Silicon acrylate
Silicon + fluorine
Flurosilicon acrylate
No silicon + flurine
Flurocarbon

125. Contact lens ordering

126. Soft contact lens
In addition to the trade name of the lens being
ordered
An order for soft contact lens should include the
following parameters:
Base curve radius
Overall diameter
And power

127. Thank
you!!!