Terminologies in contact lens dimension and manufacturing of RGP lenses
1. Terminologies in contact lens dimensions and
manufacturing of RGP lens
Mr. Sanjib Kumar Mishra Mr. Manoj Mahat
Ms Jenisha Bhattrai
2. Introduction to RGP
• Rigid Gas Permeable
• Hard lens that allows the passage of oxygen in substantial amount for the normal
corneal metabolism. -Philips.
• A lens that with normal lid pressure, body temperature and water content at
equilibrium with the wearing environment does not mould to the eye surface.
3. Lens parameters : ISO terminology
1.r0= Back Optic Zone Radius (BOZR)
2. r1= Back Peripheral Radius, First (BPR1)
3. r2= Back Peripheral Radius, Second (BPR2)
4. ra0= Front Optic Zone Radius (FOZR)
5.ra1= Front Peripheral Radius, First (FPR1)
6.tc= Geometric Centre Thickness
7.tpj1= Peripheral Junction Thickness, First
8.tpj2= Peripheral Junction Thickness, Second
4. 9. tER= Radial Edge Thickness
10.tEA= Axial edge Thickness
11.Ø0 = Back Optic Zone Diameter (BOZD)
12.Øa0 = Front Optic Zone Diameter (FOZD)
13. Ø1 = Back Peripheral Zone Diameter (BPZD )
14. ØT = Total Diameter (TD)
• Miscellaneous (not shown):
• Fv ´ = Back Vertex Power (BVP)
• Fv= Front Vertex Power (FVP)
7. 1. Total diameter
• Linear measurement of the greatest distance across the physical
boundaries of the lens.
• Expressed in millimeters
• RGP lenses typically have a diameter varying between 9.0 and 9.8
• An overall diameter of 0.5mm less or more may be selected if corneal
dimeter and palpebral aperture are small or wide respectively
• Diameter of lens and base curve have direct co-relation
• 1mm change in diameter is equivalent to 0.01mm change in radius
8. Effects of Changing the Lens Total Diameter
Affects other parameters:
• Centre of gravity
• Peripheral curve width
• Axial edge lift
• Edge profile
9. CHANGING LENS TOTAL DIAMETER
• Corneal coverage
• Tear exchange
• Lid interaction
10. CHANGING LENS TOTAL DIAMETER
• May also affect:
• Larger lens diameter places more of the lens edge under the lids in resting
position enhancing the comfort of RGP lens.
• Williams-Lyan et al.,(1993) showed that larger diameter(10mm vs 9.5 and
9.0mm) RGP lenses were more comfortable.
3 & 9 o'clock staining
• If an area is susceptible to staining(3-9 o’clock) then increasing diameter
will help to reduce this
• Occurs due to poor wetting in the horizontal axis
11. Centre of Gravity: Plus Lens
Changing the Lens Total Diameter
This diagram illustrates the effect that diameter reduction has on the location of the C of G of plus lenses. A
larger diameter plus lens places the
C of G more posteriorly. The more anterior the
C of G is located, the potentially less stable will be the fit because of the greater mislocation force (rotational
moment) produced by gravity.
12. The work of Carney and Hill (1987) has clearly demonstrated that the main design
change with RGP lenses that significantly affects the C of G is the lens TD. For plus
lenses, a diameter change of 0.1 mm produces a 7x greater effect on the position of
the centre of gravity than does a 0.01 mm change in the lens centre thickness.
13. This diagram illustrates the effect lens diameter can have on the location of the C of G of minus lenses.
A more anterior C of G is located in a smaller lens creating a less stable fit because of the greater
mislocation force (rotational moment) produced by gravity.
For minus lenses, a change in the lens TD produces a slightly reduced effect on the C of G compared
with plus power lenses.
14. Reducing the Lens Total Diameter
Effect on the static fitting:
• Reduced edge width and clearance
Effect on dynamic fitting:
• Loosening of lens fit
• Increased lens movement
• Decentration more likely
16. Optic zone diameter
• It is the dimension of the central optic zone of lens meant for focusing
rays on retina
• Should be larger than pupil size and cover during movement
• Dependent upon overall diameter and peripheral curve
• Normal range:7.0-8.5mm
17. Changing the Back Optic Zone Diameter (BOZD)
• Increasing the BOZD increases apical clearance:-tightens fit - improves
• Decreasing the BOZD decreases apical clearance:-loosens fit-increases
Effects on movement and centration
• Small BOZD (7.40 mm) lenses showed slightly less movement but greater
decentration than larger (7.90, 8.40 mm) lenses
18. 2. Lens Thickness
• Effects of Changing the Lens Centre Thickness
A. Decreases transmissibility.
An inverse relationship between oxygen permeability and lens thickness
increasing oxygen transmission by decreasing the centre thickness (tc)
will reduce the level of corneal oedema during lens wear.
B. Moves C of G anteriorly.
Thinning a lens causes an inward shift of the C of G which translates into
improved lens stability.
19. C. Increases movement.
Due to the anterior shift of the C of G, greater movement
accompanied by a looser fitting lens may be exhibited.
Increased lens-lid interaction may also develop which, if excessive,
may cause instability of the lens fitting.
20. Changing the Lens Thickness
• Centre Thickness: Comfort Study
• Generally, thinner lenses are more comfortable. However, in a study
by Cornish and Sulaiman (1996), very thin lenses (0.08 mm) were
significantly more uncomfortable than thicker lenses (0.12 and 0.16
mm). They hypothesized this was due to deformation of the thinner
lens during a blink. The deformation would result from the greater
flexibility of the lens.
21. Changing the Lens Thickness
• The edge profile are ideally directed towards producing an edge with a well rounded,
centrally or posteriorly located apex to attain maximum comfort.
• Changes to lens edge thickness affects
Affects other parameters:
• Edge thickness
• Apex position and location
• Lid interaction
• Vertical centration
• Tear meniscus
• Lens removal
23. Edge Configuration
• Edge configuration can affect:
Comfort: Generally, the thinner, rounder and smoother the edge, the
Durability: If the edge is too thin, the risk of lens fragility is increased
and if it is too thick, comfort is adversely affected.
Tear meniscus: The edge clearance, apex location and material
wettability largely define the tear meniscus at the lens edge.
24. Edge Configuration
Comfort versus Edge Shape
• RGP lenses with rounded and square posterior edge profiles are more
• Comfort is determined by interaction of lens edge with the lid
25. • Edge Configuration
Comfort versus Apex Location:
• In a detailed study of custom fitted lenses, Orsborn (1988) found a
centrally located edge apex was more comfortable than either a
posterior or anterior location.
26. Back Vertex Power
• Measured as the position of the second principal focus from the back
vertex of the lens
Measurement of BVP of a contact lens requires a special stage on the focimeter (vertometer, lensometer) to
accurately locate the back vertex of a contact lens
27. • Front Vertex Power
• Front vertex power (also called the neutralizing power) is sometimes
used when specifying the power of haptic lenses.
• This is because the depth of a scleral shell may prevent the back
vertex from touching the focimeter stop.
• Under these circumstances the FVP can be determined using either a
special stop or placing a thin glass flat across the stop to locate the
front vertex of the lens.
• FVP can be calculated in a similar manner to that used for BVP.
28. Front Peripheral Radius (FPR):
• The lens peripheral profile is defined by the front peripheral radius
(FPR), the back peripheral radius (BPR) and their angular separation.
• FPR Can affect:
• Junction angle
• Junction thickness
• Front peripheral curve width
29. INCREASING JUNCTION THICKNESS
• The effects:
• Decreases comfort
• Increases upper lid interaction
• Superior/central lens decentration
•DECREASING JUNCTION THICKNESS
• The effects:
• Improves comfort
• Minimizes upper lid interaction
• Central/lower lens position
30. Edge Thickness:
• The edge thickness of an RGP lens is a key design feature affecting level of
comfort experienced by the patient.
• The edge thickness(measured as the radial edge thickness, tER) is
dependent on the lens BVP, lens design, material properties, manufacturing
• The manufacturer will attempt to determine the type of lenticulation and
edge design that is most appropriate.
• For a given RGP lens design, as the minus BVP increases, the unfinished
edge thickness also increases.
• Based on an edge thickness calculation, the lenticulation process should be
chosen that provides the most effective reduction in the finished edge
31. • An edge thickness of about 0.12 mm is considered to be optimal for
comfort and lens durability.
• A thinner lens edge will be prone to breakage, whereas a thicker
edge may reduce comfort.
• The lenticulation process involves either a straight line taper using a
conical modification tool, or a lathe-cut peripheral curve.
• A taper may generally be used up to an edge thickness of 0.20 mm.
• For thicker edges, the use of a lathed edge curve will be more
33. Base curve
• Curve on the back surface of the lens to fit the front surface of cornea
• Base curve should align with the curvature of cornea
• The base curve radius (BCR) to be selected for a given patient
depends upon several factors, including corneal curvature, lid-to-
cornea relationship and fluorescein pattern evaluation.
34. • As the cornea is aspheric and tends to flatten at a greater rate at an
increasing distance from the center, the selection of a base curve radius
that is slightly flatter than “K” should result in an alignment fitting
• A simple rule of thumb is to flatten the BCR by 0.25D for every increase
in overall diameter of 0.5mm and to steepen the BCR by 0.25D for
every decrease in overall diameter of 0.5mm.
• Gas permeable lenses will always move along the steeper meridian of
the cornea as well as to the steepest region of the cornea (i.e., apex).
• Range of normal values:7.0-8.5mm
35. Changes to the back surface design will affect the:
• Fluorescein pattern: Changes to the back surface geometry can be
observed by the effect the varying regional differences in the post-
lens tear film thickness have on fluorescein brightness.
36. • Centration :An alignment fit is desirable to achieve good lens
centration, but this is influenced by the back surface curvature of the
RGP lens in relation to the curvature and sphericity of the corneal
• Movement: due to interaction between the RGP lens and the lid
forces during blink( particularly the upper lid).
• Tear exchange :The back peripheral surface geometry is crucial to
allowing a good tear exchange. A tight back periphery can greatly limit
38. BOZR/BOZD RELATIONSHIP
• This Rule of Thumb is a useful guide to determining the effect of
BOZD changes to the BOZR while trying to maintain the same central
• In a study by Atkinson (1984), an 0.7 mm change in BOZD is required
for every change of 0.5 mm BOZR to maintain the same fluorescein
48. • The terminology for a standard tricurve lens in ISO 8320-1986
• Example:7.90:7.80/8.70:8.60/10.75:9.20 tc 0.15 BVP -3.00D Tint
• 7.90 = back optic zone radius (BOZR) r0
• 7.80 = back optic zone diameter (BOZD Ø0
• 8.70 = first back peripheral radius r1
• 8.60 = first back peripheral zone diameter Ø1
• 10.75 = second back peripheral radius r2
• 9.20 = total diameter ØT
• 0.15 = geometric centre thickness tc
• -3.00 = back vertex power (BVP)
53. Cellulose Acetate Butyrate
Flexible, water resistant.
Can be molded or lathed
Hydroxyl group result in 2% water content and reasonable wettability.
Material stability lower than PMMA.
Dk in range 4 to 8.
54. Butyl Styrene
DK of 25
The refractive index of 1.553 is the highest of any RGP material.
The specific gravity of 0.95 is the lowest of any RGP material.
The combination of high refractive index and low specific gravity
offers the thinnest, lightest lens possible. This makes the material
ideal for high RX
55. Siloxane Acrylate
A PMMA backbone gives material its dominant physical properties
Si-O-Si bond results in significant increase in oxygen permeability but
reduction in material rigidity.
56. Unlike PMMA lens materials which are made of single component, SA material
contain methacrylate, wetting agents and cross linking agents.
Dks in low to medium range are available
Higher oxygen permeability than the preceding lenses.
The lower rigidity of SAs allows lenses to conform more closely
to the shapes of corneas on which they are placed .
Allows Optic Zone diameter to increase.
59. Fluoro-siloxane Acrylate
Developed to increase Dk of RGP materials and to increase
resistance to surface deposition.
The element F is added to basic SA chemistry to enhance
A lower surface charge results
60. DKs of 40-100 are more achievable.
DKs are high enough for extended wear to be a possibility
FAs are generally more flexible than SAs
Surfaces are relatively easily scratched.
62. No surface charge thus reducing the likelihood of lens spoilage
High flexibility, conforming to the corneal shape.
This results in stable vision and comfort
A low refractive index means thicker lens for a given prescription
High specific gravity means a heavier lens for a given prescription
Low yields and high costs
64. The wet ability of the fluorocarbon lens surface is only average
On eye flexibility and conformity reduces the correction of cornea
induce d astigmatism.
65. Manufacturing Methods
There are three main manufacturing methods:
1.Moulding - mainly the mass production of lenses of limited parameters at
2.Lathe cutting - the production of individually specified lenses.
3. Hybrid lens manufacture - a combination of the two.
67. Step2-Once the cavity is formed the molding material is injected into the
mould at very high pressure to ensure that the complete cavity is filled.
Stage3 - Once cooled, the bolsters are moved apart to allow access to the
Stage 4 - The cast is removed from the bolster, using a robotic system and
then it is transferred onto a conveyor where it is stocked for the next stage
68. Stage 5 and 6
A precise quantity of contact lens material monomer is placed on the female
Immediately after dosing the mould, the male cast mould is placed on the
female to form an individual contact lens cavity containing the monomer and
Female mold defined front surface of contact lens and male mold defines
the back surface
69. Stages 7 and 8 –
The combined female and male mould is transferred from the conveyor
system into a polymerization process.
Each contact lens material uses a different polymerization process to cure
Step 9 and 10-
Once fully cured the casts are mechanically bent and separated to reveal the
70. Stage 11 –
The lens is hydrated and washed in a hydrating fluid such as saline, and a
cleaning solution, in a suitable bath.
This process also removes any excess monomers that have not been
Stage 12- Quality inspection and final packaging
74. 2. Lathing
Lathe cutting is a technique favored throughout the world as a method for
the manufacture of both specialist and durable RGP.
75. a. Lens Design
Machining parameters are determined taking into account the specified
parameters ,designs and material property of the lens.
Depending on the level of automation, the machining parameters are
transferred into the contact lens laboratory.
76. b. Annealing
All casting introduces strain, which needs to be released from the material
to ensure consistent results in rigid lens fabrication.
This is done by annealing which involves heating the material to a
temperature between 140 and 150°C for a short period and slowly returning
it to room temperature.
77. c. Stress testing
Any stress present in the blank will later cause distortion to the finished lens.
Blanks are assessed for stress by viewing with a polarized light strain tester
79. Back surface lathing
The material blank is held in the spindle of the lathe by the spindle collets.
Then the cutting process is started ensuring that the correct data have been
uploaded into the lathe.
The final cui creates the final back surface geometry and removes only
0.005--0.02 mm of material
80. The feed rate, depth of cuts and spindle speeds are preprogrammed according
to the particular characteristics of the material being machined.
When the generation of the back surface is complete, the blank is removed
from the collet and replaced with the next blank.
Marks are produced on lenses by laser light focused to
create a series of axis alignment marks
Or, by engraving using diamond tool held against the
surface with a specific force and moved to create the
82. Substance measurement
During the front lathing process, the lathe probes the back surface
And then it calculates the amount of material to remove by subtracting the
desired centre thickness from the substance thickness
83. Back surface polishing
Polishing is carried out in
by mounting the blank in a
The polishing tool may be formed from a spherical
convex tool covered with foam and a polishing cloth held
on to the tool by an
84. Front Surface blocking
It allows the front lathe to cut the front surface accurately.
The blank must be presented on a chuck designed to fit within the
spindle collet and remain optically concentric
85. Front surface lathing
The cutting process is similar to that of the back surface.
Except that an initial probing routine is used to determine the back
reference surface of the lens blank as previously described
Front surface polishing
Same as that of back surface polishing
86. Back surface blending
This is carried out on multicurve RGP lenses
in order to smooth the transitions. it is necessary to blend between each
The contact lens is held on a suction holder against the vertical spindle
carrying the convex tool.
Using plenty of polishing compound, the lens is moved from side to side
and removed for frequent inspection
87. Edge polishing
Most laboratories use an
automatic edge polisher to
polish RGP lenses.
The lens is held in a suction cup and rotated on a revolving
sponge, which contains polish.
88. Final checking
The checking of the final lens includes thickness, diameter, radius, back
vertex power measurements, Surface and edge quality.
Lens cleaning and packaging
Once an RGP lens completes final inspection, it is cleaned using a
laboratory contact lens cleaning solution.
89. Cleaning removes all traces of polish and detritus likely to affect the surface
wet ability of the lens.
The lens is then rinsed in purified water, to ensure that it is perfectly clean,
and placed into its final container either as a dry lens or with a disinfecting
91. Concave and convex toric surfaces can be produced in
one of two ways:
Manual methods such as crimping. (historical interest
Lathe cutting, which can be subdivided into:
Direct lathing using oscillating tool technologies
Toric generation using fly-cutting techniques
automatic crimping techniques
92. Oscillating tool technology
It is a method for oscillating the diamond-cutting tool on a contact lens
The oscillations are sinusoidal and the period is synchronized to the
rotation of the lathe spindle such that the lathe cuts a toric surface directly
onto the contact lens blank.
93. Toric generation using fly cutting technique
Here the position of blank and cutting tool are reversed.
When the spindle rotates the fly cutter tip describes in the vertical plane
and at right angles to the spindle axis
This process defines a circle whose radius is the distance from the centre of
the spindle to the tip of the diamond
94. The radius of the vertical circle, which is the flatter radius of the toric
surface, can be altered by:
• Changing the size of the cutting tool.
• Offsetting the axis of the fly-cutting tool centre line in relation to the
centre line of the radius cartridge.
95. Automated crimping technique
It induces toricity by a double-toothed jawed collet crimping the blank in situ.
The degree of crimp and hence toricity is controlled by computer
96. Once the requisite amount of crimp is achieved, the lens blank is
machined using conventional spherical lathing.
Front surface toric lenses can be generated in a similar fashion to the back
Complex designs are difficult.
Intensive labor is required.
High cost per lens
Variable surface finish
99. Manufacturing RGP bifocals
Concentric and progressive bifocals are made by using conventional lathing or
Implanted segments have a high refractive index segment incorporated in a
lens button of conventional material. They are usually D shaped or crescent
Diffractive Concentric zones are molded onto the lens back segment.
101. Hybrid Process
Lenses are manufactured using the cast-moulding
technique with a defined back surface geometry and
a generic front surface.
Once the mould is split, the cast lens is then lathe-
cut while remaining attached to the back surface
102. Preliminary lens assessment
BOZR and lens diameter are amongst the most
BVP is measured to determine that the final
prescription is as ordered.
103. CT is measured as it determines the physiology of the lens.
Edge profile has been shown to be critical to lens comfort.
The overall quality of workmanship needs to be assessed to
predict the optimum performance and comfort on the eyes.
104. Wet state
BOZR is critical. It influences the lens fit and optics of the tear lens.
The BOZR will be approximately 0.03mm flatter after hydration.
Image quality is assessed to determine the optical quality of the product.
105. Changes from dry to hydrated state
Expansion resulting from hydration and its uniformity determine the
outcome of hydration step.
Any untoward hydration effect has the potential to produce a toric or
even irregular lens shape.
An almost ideal fluorescein fitting pattern is presented in slide 35. It demonstrates the little or no fluorescein pattern under the central and mid- peripheral zones of the lens indicating corneal alignment, the edge clearance desired, the tear meniscus at the lens edge and good lens centration
Anteriorly placed c of g cause loosing of lens and viceversa for posteriorly placed
Larger the diameter larger the edge lift and clearance
Diameter and edge width inverse relation if BOZD varies
Daimeter and edge width direct relation if BOZD constant
shows a large diameter lens. The large BOZD and narrow edge width indicates the need for their alteration when the lens TD is reduced. When this is done, an acceptable fluorescein pattern is noted and good centration is maintained
Thin edged lens – ‘lens –lid ‘interaction decrease, comfort increaset
Vertical cebtration; thick lens ride low and vice-versa
When thickness change to edge profile result from decreasing TD there will be reduced tear meniscus and difficult in lens removal
luorescein patterns of a flat and steep fitting RGP lens on the same cornea.
n these examples of a series of lenses with a fixed BOZD of 7.00 mm, as the corneal radius of curvature (based on keratometry readings - K) flattens, the central TLT decreases, even though the fitting relationship with the flattest corneal meridian is maintained.
This highlights the association between the BOZR and the BOZD with regard to the central TLT as disclosed by fluorescein.
ncreasing the radius of the back peripheral curves increases the AEC (axial edge clearance)
This diagram shows the competing issues involved in selecting edge widths. While a wide edge will increase the volume of tears stored in the edge reservoir, the stability of lens fit may be adversely affected. A balance between these forces is required.
Hence also called acrylic.
A class of thermoplastic material derived from special grade of wood cellulose.
Hydroxyl group…….hence well tolerated.
DK…from half to approximately the same as PMMA, HEMA orPHEMA
The combination of high refractive index and low specific gravity offers the thinnest, lightest lens possible. This makes the material ideal for high RXs
OZD= By offering improved physiology and to a lesser extent lower rigidity larger lenses could be fitted.
Surface charge and surface chemistry
Relatively soft surfaces
Possibly due to the result of induced internal stresses being released, leading to surface and MATRIX FALIURE
Less rigid- less confirmation- reduced correcton of astigmatism
Flexure problems – silicone content-less rigidity
Influence of age and their environment, lens care products and the stresses placed on them by storage cases or astigmatic eyes
FSA and SA are more prone to surface burning and solvent damage during manufacture.
These 2 points constitute a significant disadvantage, Together they mean a heavier, thicker lens, the reverse of what is required
Success rate during manufacture is lower
which not only has the benefit of low cost mass production, but also goes some way to producing lenses that are more specific.
The majority of contact lens applications require two injection-moulding machines configured to create either female or male casts. The former is used to form the front surface of the contact lens and the latter the back surface.
(examples include ultraviolet light and temperature exposure) and an annealing process may be employed to reduce any stresses in the material matrix
The 1st fig shows the two operating faces of a typical injection molding machine…the two bolsters effectively close together to form a cavity where mould is injected at high pressure…then the bolsters are separated by robotic system to reveal the cast….Now the contact lens polymer is placed on female mould…immediately afer dosing the male mould is placed over the female mould and are squeezed together…..the mold along with the cast are placed for polymerization and then hydrated and inspected for the parameters.
This does not cause any difficulty when done with sheet material but with thick material the temperature has to be low enough to prevent depolymerization of the surface but heated long enough to ensure the centre of the block reaches at least I3O°C
Before discussing the lathe cutting, it is important to understand the machining part….
The above fig. shows lathe machine…the leg portion carries the weight of the machine… The tray portion collects the waste products…bed portion consists of carriage which supports tool post which contains cutting material…it can be rotated to any point….Head stock contains all the gears…..The green portion is the spindle which is hollow and is able to rotate….The chuck is a lens holding part…The chuck portion moves along with the spindle…The tail stock portion holds the working rod and shows no movement
Too tight a collet, which causes the blank to be crimped during cutting, will result in a torie surface once the cui blank is released from the collet. Too loose a collet, which allows the blank to slip during machining, will produce a poor-quality surface.
Overpolishing may result in localized heating of lens blank with surface alteration of its properties and also results in poor wettability
There are three ways to achieve this:
• Static. • Dynamic. • Optical.
Feed rates and depths of cut will again affect the overall surface quality and material
The closed loop control systems to drive the oscillating tool:r. are all software controlled and synchronized with the spindle using spindle encoders