knowledge purpose

1. ophthalmic lens material,
Tints and coatings
Kalpana Bhandari
M. Optom 2nd batch
Tilganga Institute of Ophthalmology

2. Ophthalmic lens material

3. Introduction
• Ophthalmic lens is the portion
of the transparent medium
bounded by two refracting
surfaces at least one of which is
curved.

4. History

5. Ophthalmic Lens Materials
o Natural media
o Glass
o Plastics

6. Natural media
• It includes quartz or rock made out of pure silica.
• Its hardness and low mist retaining property made it ideal
for spectacle lens.
• Not used in optical instrument since it is double refracting
medium.
• Clear natural crystal are very rare to find.
• So their use is almost stopped.

7. Glass material
• Glass material are often referred to as mineral
lenses.
• Also called super cooled liquid.
• Raw material
- quartz (sand )
-Soda (sodium carbonate)
-Lime(carbon dioxide)
- trace amount of potassium , arsenic ,antimony and
borax

8. Advantages:
• Absorb heat and cold
• Are scratch resistance
• Highest optical clarity
• Low aberration

9. Types of glass material
Crown glass
Flint
glass
Barium
crown glass
High
index
glass

10. Crown glass
• 70 % Silica , 14- 16 % sodium oxide , 11- 13 % calcium
oxide and small percentage of potassium , borax ,
antimony , arsenic.
• Used for single vision lenses
• Most common used in ophthalmic industry
• Refractive index- 1.523
• Abbe value - 59
• Specific gravity- 2.54
• Transparency- 91.6%

11. Flint glass
• Glass with abbe no. less than 50 are flint glass
• Ingredients: 45-65% lead oxide, 25-45% silica, 10%
soda and potassium oxide

12. Barium crown glass
• 25-40% barium oxide, 30% silica, trace amount of
lime, zinc, aluminium, boron, zirconium

13. High index glass
• Any lens have refractive index higher than 1.523 in
glass is called high index lens.
• R.I: Available in 1.6, 1.7, 1.8 and 1.9
• Abbe value: 33, 32, 31, 30.
• Contains high % of titanium oxide
• Transmission- less than 92%
• Useful for reducing thickness for high powered lenses

14. PLASTIC LENS MATERIAL
• It is a polymeric material of large molecular
wt.
• Also called as organic material.
• Most plastics are synthetic materials formed
by combining various organic ingredients with
inorganic materials such as carbon, hydrogen,
nitrogen, chlorine & Sulphur.

15. Plastic Materials
• Two main groups
Thermoplastic
Thermosetting

16. Thermoplastic materials
• Soften when heated
• Can be stretched, pressed, or molded without changing
chemical structure
• BUT less dimensionally stable and more susceptible to
deformation by heat
• Eg: acrylates (Plexiglas, Lucite, PMMA), cellulose acetate,
cellulose nitrate, polycarbonate, polystyrene, nylons,
vinyls (e.g., PVC)

17. Thermosetting materials
• Once hardened cannot be softened
• Stable, rigid, hard, flame resistant, insensitive to heat
and chemicals
Eg: CR-39, epoxies, phenolics, melamine compounds

18. Types of plastic material
plastic
material
CR 39
Polycarb
onate
High
index
plastic
Trivex
lenses

19. CR-39
• CR stands for Columbia Resin & it is 39th formula of a
thermosetting plastic developed by the Columbia resin project
of (PPG) industries back in 1940.
• For smaller, high velocity, sharply pointed objects, CR-39 lenses
perform better than chemically tempered glass
• CR-39 plastic lenses do not fog up as easily as glass lenses
• Good optical quality, with a high V-value and is easy to surface,
edge, drill and coat

20. • At present, CR39 is still the material of choice for standard
plastic lenses
• The main drawbacks of the material are its low refractive
index and low tensile strength
Refractive Index 1.498
Specific gravity 1.32
Abbe Value 59.3
Uv cutoff 355nm

21. Characteristics of CR-39
• Lightness
• Impact resistance
• Chemical inertness
• Resistance to pitting
• Resistance to fogging
• Tintability
• Versatility in optical design

22. Disadvantages of CR-39
• Surface abrasion
• Warpage upon glazing
• Increased thickness
• Inferior photochromic properties
• Index variability with temperature

23. Polycarbonate
• The first alternate plastic material was polycarbonate
• It is a high index plastic lens
• Gives extra level of protection to the lens wearers
• Usually preferred for children and sportswear
Refractive index 1.586
Abbe no 30
Specific gravity 1.20

24. Advantages of polycarbonate
• Light weight.
• Most impact resistance material
• 30-40% thinner than CR-39
• Absorbs nearly 100% of UV-A and UV-B radiation
• Can be hard coated for scratch resistance
• Center thickness can be ground to 1.2mm

25. Disadvantages of polycarbonate
• Low V-value (30)
• Poor chemical resistance
• Susceptibility to cracking, particularly on contact with
metal
• Material requires a unique dry edging process in the
glazing lab
• Applying a robust, tintable, hardcoat is difficult to achieve

26. High index plastic
• Plastic are made up of Thermosetting plastics.
• These lenses have higher than standard refractive
index.
• Most cosmetically acceptable lenses.
• It enable the lens to bend light “faster” than other.
• Hence they are lighter and thinner than standard
lenses.

27. High index plastic
Advantages
• Cosmetically good
• Magnification is reduced
• Thinner and lighter than other lenses

28. High index plastic Cont...
• Available in range of lens type including
photochromatic , multifocal etc.
• Impact resistant.
Disadvantages
• Greater distortion
• Not available in larger blank size
• More chromatic dispersion
• Off axis aberration are seen
• Expensive

29. Trivex
• Trivex is a very new lens material that exhibits three very
good lens characteristics
I. Lightest weight of all plastic materials.
II. Impact resistance at least equal to polycarbonate
III. Excellent abbe value.
• It also has similar U.V. absorption as polycarbonate
Like polycarbonate, Trivex is extremely difficult to break.
• It is the lens of choice for drill mounted lenses because it
does not crack or split at the drilled hole.

30. Trivex
Advantages
• Impact resistance
• Lighter in weight
Disadvantages
• More scratch

31. Comparison
Property Trivex Polycarbonate
Refractive Index 1.53 1.6
Abbe value 43-45 30
Weight Lighter Heavier
Thinness Less thin Thinner
Impact resistant Yes Yes
Optical clarity Very good Not good
Cost More than polycarb Lower than Trivex

32. Comparison

33. NXT material
• Bullet proof material developed in 1900 under US
government contract.
• Light weight material that is extremely strong and also
compatible to photochromic pigments and with
polarization.
• Used in sun and sport eye wear, helmet visors for bikes,
airline cockpit door view port, ballistic police shields,
vehicle door armor.
• Reported to be compatible with low powered sphere and
cylinder prescription lens power.

34. Properties of lenses
Optical
properties
Mechanical
properties
Electrical
properties
Chemical
properties
Thermal
properties

35. Optical properties
Defines how the material interacts with light.
Includes
1. Abbes number
2. Refractive index
3. Reflectance
4. Absorption

36. 1.Abbes number
• Measure of the degree to which light is
dispersed when entering a lens material
• Abbes number is reciprocal of dispersion
• Also called as constringence / V number
• Higher the abbes number lower is the
chromatic aberration in the lens.

37. 2.Refractive index
• Ratio of velocity of light in vacuum to the
velocity of light in a given medium.
• Higher the R.I of material the thinner the lens
can made.
• Materials with an index between 1.523 and
1.57 are considered as mid-index, while 1.58
and greater is considered as high-index.

38. 3.Reflection
• Light reflectance result in the loss of lens
transparency on the lens surface.
• The higher the refractive index the greater the
proportion of light reflected from the surface. That’s
why antireflection coating is must in high index
lenses. Refractive index % of light reflected
1.5 7.8%
1.6 10.4%
1.7 12.3%
1.8 15.7%
1.9 18.3%

39. 4. Absorption
• The amount of light which goes through a lens can
be reduced because of absorption by the lens
material.
• This is negligible in case of a non–tinted lens, but
constitutes an function of a tinted or photo
chromatic lens.

40. Mechanical properties
1. Specific gravity
2. Impact resistance
3. Scratch resistance

41. 1. Specific gravity
• Ratio of 1 cubic cm of a material to 1 cubic cm of
water.
• Higher the specific gravity of a lens material higher
will be the density and heavier will be the lens.
• Give a rough idea about the relative weight of
various lens.

42. 2. Impact resistance
• Susceptibility of lens material to fracture under stresses
applied at high speed.
• Relative impact resistance of various material will vary
depending on the size and shape of the missile in the
test.
• The standard test employed by FDA is Drop ball test

43. Drop ball test
• 5/8 inch steel ball
• Weight approx. 0.56 ounch
• Dropped from a height of 50 inches
• Strike within a 5/8 inch diameter
circle located at the geometrical
center of the lens
• To pass this test lens must not
fracture
• Polycarbonate > high index plastic
lenses > CR -39 > ordinary glass
lenses

44. Tempering
• Any treatment that places the lens under
compression provides the lens with greater impact
resistance than that possessed by ordinary annealed
glass.
Types
• Thermal tempering
• Chemical tempering

45. Thermal tempering
• Finished lenses is heated to 650 degree and rapidly cooled
by blast of air on both sides.
• surface quickly becomes rigid
with cooling.
• Surface compression defines
Strength and center tension
defines break pattern.
• Birefringence under polariscope characterized as maltese
cross patterns.

46. Chemical tempering
• Ion exchange method .
• Exchange small ions in the glass with larger ions in the
molten salt bath below strain temperature .
• Crown glass lenses the Bath consists of 100% kno3 and
sodium ions Na+ in the lens is replaced by larger
potassium ion present in bath.
• Corning photochromatic lens bath consists of 40 %
NaNo3 and 60% KNo3
• Temperature much less than thermal tempering (400-
470degree ).
• Lower temperature means less lens warpage and fewer
irregularities.

47. Chemical tempering

49. 3.Scratch resistance
• Glass lenses are abrasion resistance.
• Plastic lenses needed to be coated with an additional
resin to approach the scratch resistance of glasses.
• Premium hard coating >Ziess> Crizal > Teflon

50. Electrical properties
• Characterize the effects of electromagnetic
waves and electricity on the material.

51. Chemical properties
• The reaction of the materials to the chemical
substances usually found during lens
manufacture.
Thermal properties
• Characterized changes of state and effect of
temperature on material

52. Lens selection consideration
Lens
selection
Abbe no
Impact
resistant
Lens
design
Other
properties
Specific
gravity
Refractive
index

53. Lens selection in High Errors
• High index lens should be used
• Use of plastic lens with less specific gravity to reduce weight
• Use of aspheric lens
• Use of lenticular lens
• Use of material with high abbe no. to reduce dispersion of
light

54. Comparisons between glass & plastic
Glass lens Plastic lens
1 Heavy Light weight
2 Low impact resistance High impact resistance
3 Highly scratch resistant Lower scratch resistant
4 Cheaper Costlier
5 3 piece cant be used 3piece can be used
6 Aberration is low Aberration is high
7 Higher tendency to fog Lower tendency
8 Less transmission 2% greater transmission
9 Coating isn’t a necessity Coating is must
10 Not preferred for children Preferred for children

55. • Due to inherent lightness and safety, plastics has almost
completely replaced glass as the first choice of spectacle
lens material.
• Today in most of the developed world, some 95% of
spectacle lenses are made from plastic materials.
• However glass is still in use with very high indices in
excess of 1.80, and to photochromic lenses with
specialized properties, such as Corning’s CPF glasses.

56. Objective: To evaluate the relative strength and shatter resistance of spectacle
lenses currently used in sunglasses and dress, sports, and industrial eyewear.
Design: Seven lenses that met the US American National Standards Institute
(ANSI) Z80 standards for dress glasses (made of high-index plastic, allyl resin
plastic, heat tempered glass, chemically tempered glass, and polycarbonate, and
with center thickness ranging from 1 mm to 2.2 mm) and 4 lenses that met ANSI
Z87 standards for industrial safety eyewear (allyl resin plastic, heat-tempered
glass, chemically tempered glass, and polycarbonate, all with 3.0-mm center
thickness) were tested for impact resistance to 5 projectiles (air gun pellets, golf
balls, tennis balls, lacrosse balls, and baseballs).

57. • Results: Based on 348 lens impacts, dress and industrial lenses
made from glass, allyl resin plastic, and high-index plastic
shattered at impact energies less than those expected.
Polycarbonate lenses demonstrated resistance to impact for all
tested.
• Conclusions: Under the test conditions of this study,
polycarbonate lenses demonstrated greater impact resistance
than other commonly used spectacle lenses that conform to
prevailing eyewear standards. These findings suggest that
current ANSI Z80 and ANSI Z87 standards should be
reevaluated.

58. Tints and coatings

59. • Lenses that are used for the specific purpose
of reducing the amount of transmitted light
are known as absorptive lenses.
• Absorptive Lenses can be categorized as :
– Tinted Lenses
– Polaroid lenses
– Photo chromatic Lenses
– Special Filters

61. How tinted lens are made?
• Plastic lenses are tinted by dipping them in a solution
containing the appropriate organic dye.
• To manufacture tinted glass, different oxides(concentration
<1%) are mixed with glass material.
• Glass Lenses with Surface Coatings lens may be tinted by
depositing a thin metallic oxide on the surface of the lens.

62. • iron = green
• Manganese = pink
• Cobalt = blue
• cerium = pinkish brown
• Nickel = brown
• Uranium = yellow
• Chromium = green
• Gold = red
• Silver = yellow
• Didymium = pink
• Vanadium = pale green

63. Prescribing Indications

64. Grey
• Neutral density filter.
• Absorbs all colors of the spectrum equally.
• Ideal for people who do not want relativity of
colors affected.
• Allows colors to be seen in their natural state
relative to one another.

65. Selection of Grey tints
• Light sensitive people / Anti-Fatigue
• Intolerant to color distortion
• Night Drivers
• Cool sunglass color
• Indoor use to reduce glare from fluorescent lamps,VDU and TV
screen
• Limitation- not best contrast enhancing colour, slight reduction in
VA.

66. Pink
• Mainly given to attenuate the light in a pleasing way.
Selection of pink tint
• For low light sensitive individuals
• Good eye cosmesis
• Color of choice in neurological problems(eg Migraine)
• Appreciated for indoor use
• Computer user, TV etc.
• Limitation- poor UV absorption, not suitable for outdoor use.

67. MIGRAINES & FL-41 TINTED LENSES
• FL-41 was first described in a research project that took place in
Birmingham, England in the early 90s. In this study, children with
migraine headaches wore FL-41 filtered spectacles.
• Result: The researchers found that wearing the FL-41 filter
improved the light sensitivity in these children, as well as lessened
the frequency and severity of their migraine headaches.
• They also have found that blepharospasm and migraine patients
suffer more from light sensitivity, vs. people who do not struggle
with these conditions. They have found that when these patients
have had their light sensitivity treated, their blepharospasm and
migraine symptoms have improved.

68. Blue
• Studies in psychology have shown that the color blue makes us feel more
comfortable and has a calming effect on our brains.
Indications
• Indoor tints for eye cosmesis
• Watching computer, TV, overhead projector etc.
• Prescription for foundry work,furnace,smelters,boiler tender etc
• For light sensitive individuals
• Blue eyes and fair skinned youngsters with large pupils and reading difficulties.
• Limitation of Blue tints- older individuals with media opacities( scattering
effect
• Not good UV and IR filter(welding application)

69. Purposes: To evaluate the optical performance of blue-light filtering
spectacle lenses and investigate whether a reduction in blue light
transmission affects visual performance and sleep quality.
Methods
• Experiment 1: The relative changes in phototoxicity, scotopic
sensitivity, and melatonin suppression of five blue-light filtering
plano spectacle lenses were calculated based on their spectral
transmittances measured by a spectrophotometer.
• Experiment 2: A pseudo-randomized controlled study was
conducted to evaluate the clinical performance of two blue-light
filtering spectacle lenses (BF: blue-filtering anti-reflection coating;
BT: brown-tinted) with a regular clear lens (AR) serving as a control.
• After one month of lens wear, subjective ratings of lens
performance were collected by questionnaire.

70. Result :
• theoretically reduced the calculated phototoxicity by 10.6%
to 23.6%.
• blue-light filters also decreased scotopic sensitivity by 2.4%
to 9.6%, and melatonin suppression by 5.8% to 15.0%, over
70% of the participants could not detect these optical
changes.
• No significant decrease in contrast sensitivity
Conclusion
• Blue-light filtering spectacle lenses can partially filter high-
energy short-wavelength light without substantially
degrading visual performance and sleep quality. These
lenses may serve as a supplementary option for protecting
the retina from potential blue-light hazard.

71. Brown
• Pleasing warm cosmetic colour
• Offers better contrast than green or grey
• Reduces glare and UV absorption.
• Ideal tint for daytime driving
• Better option in hazy condition
• Limitation of Brown tint- tends to distort color a little more than
other tint.
• It evenly transmit the whole visible spectrum thus allowing color to
be perceive in neutral state which also improves depth perception.

72. Green
• Focus in the area of the eye with the greatest sensitivity to color in
daylight.
• Good sunglass tint with enhanced contrast.
• Good intermediate filter to use when grey or yellow does not seem
right.
Indications
• Sunglass color in tropical climates
• Filters UV as well as IR radiations
• Real protection in bright sunlight
• Enhancing contrast and true color perception

73. Orange
• Essence of brown tint
• Reduces the transmission of blue and green
wavelengths
• Increase contrast
• Ideal support for cycling, mountain tours, skiing
etc.

74. Red
• Allows excellent depth perception in dim light
• Strongly blocks the transmission of blue and green wavelengths with sharp
contrast
• Indication- good for cycling / racing, hunting and skiing, can be helpful for
hue discrimination in color deficiency.
• Red tint for protanopes and green for deuteranopes.

75. Amber
• Color vary from very light straw to deep reddish brown
• Improves night vision with light enhancing effect
• Improves VA and depth perception
• Reduces the glare from front headlights
(Night driving lenses-Yellow or Amber tints with ARC )
Indications
• In low light conditions like Golf,Shooting,Fishing,Skiing etc
• Light management in cloudy and rainy days
• Brightening the darkness to see more detail with reduced eyestrain.

76. Yellow
• Designed as contrast filters
• Concentrates light in the area of spectrum to which the eye is most sensitive
• Better for light insensitive individual
Indications
• Provides contrast in hazy conditions, eg snow, cloudy and foggy weather
• For shooters and hunters
• Limitation- distorts color perception.

77. Violet
• The least chosen tint of all
• It is photoactive as ultraviolet
• Can irritate eyes
• Can cause eye damage.

78. Conditions for selecting tints
• If all colors are chosen( no color preference)
– prescribe Brown tints which provides UV
protection with contrast enhancement
• If all colors are rejected
– prescribe neutral grey tints which has least color
distortion with cool sunglass effect.

79. summary

80. Ophthalmic Lens Coatings

81. Anti-reflection Coating (ARC)
• Thin, clear single layer or multilayer to reduce surface
reflection from both surfaces of spectacle lenses
• AR coated lenses transmit 98 percent to 99.5 percent
of light to the eye.

82. Conditions of selecting of ARC lenses
• Individuals working in computers and TV for long hours.
• Beneficial in night driving/reduces glare
• High lenses Rx in myopes and hyperopes
• High index lenses
• Useful while working microscopes and binoculars
• Good eye cosmesis, improved VA

83. Scratch Resistant Coating / HRC -HMC
• A silicon dip coating process applied to the front and back surfaces
of the lens.
• Useful for individual who wants long lasting/durable lenses
protecting tints and ARC.
• Protect lenses from scratches Adds durability and a bit ARC
properties 92-96%
• Applied in both surfaces of lens
• May or may not be tintable
• Front or Back or Both.
• Applied to
– plastic, trivex and
– polycarbonate

84. Hydrophobic coat (Silicon)
• Causes water and moisture to bead and run off the
lens
• Resists smudges, dirt and fingerprint
• Increases durability of lens surface
• Makes easy cleaning of ARC lenses

85. Indication of Hydrophobic coating
• Sports
• Kitchen
• Certain factory and lab
• Durability
• Easy cleaning process.

86. Anti fog coating
• Aka anti mist coating
• Fog due to body heat and environmental factors
• Blurry vision due to sweat
• On back surface of lens
• Compatible with hard coating.

87. Ultraviolet coating
• Done by bathing in UV solution before colouration. Infused with
organic dyes and metallic oxide pigments, which absorbs and
reflects harmful rays.
– Responsible for Pterygium, Pinguecula, Snow blindness, cancers of eyelids
and ocular adnexa.
– UVA – Lens and Retina. UVB – Cells of the cornea and lids including
conjunctiva.
• It has a very light yellow hue
• Beneficial for individual doing outdoor activities. Skiers,
Astronomers, Mountaineers.
• Best quality lens coating allows only 2% UV rays.

88. Mirror coating
• Ultra thin coat of reflected molecules which deflects light
in bright conditions.
• A highly reflective, purely cosmetic, flash coating.
• Reduces light transmission-by reflection.
• Reflects UV and infrared rays, prevents glare.
• Used in combination with a tinted lens.

89. Indication of mirror coating
People involving sports:
• Cricket
• Water sports
• Land sports
• Snow sports
• General sports
• Sun wear etc.

90. Polarized Coating
 Molecules of polarizing agents placed in specific order to
block specific wavelength of light which is perceived as a
form of glare
 Eliminates glare of polarizing light coming out off horizontal
surfaces like water surface and road surfaces
(allows vertical rays to pass through lens)
 Water sports, Fishing, Sailing
 Limitations: LCDs, VDUs, ATM Machines

91. Smudge Resistant Coatings
• Hydrophobic top coat enable
lens to stay cleaner longer
and remain easy to clean
over time.
• Oil, dirt, fingerprints do not
sticl to the surface of the
lens.
• Improves durability

92. Edge Coating
• Applied to rimless or half rimless fittings.
• The coating provides a cushion between the lens and an
eyeglass frame and inhibits the penetration of moisture
and tints commonly used to fashion tint ophthalmic
lenses.
• Helps the edge appear like the colour of the
frame.
• Mainly for high minus lenses

93. Thank you