Crown glass is a commonly used lens material made of sand, soda, and lime. It has a refractive index of 1.523 and is highly transparent. Plastic lenses such as CR-39 are made from allyl resin and are lighter weight but more prone to scratches than crown glass. Other materials discussed include polycarbonate, Trivex, and NXT Material 2, each having their own advantages and disadvantages in terms of properties like impact resistance, refractive index, Abbe value, and weight. Lens material properties like refractive index and Abbe value determine how light bends and disperses through the lens.

1. Lens Material
By- Rawal Singh
Optometrist

2. Crown Glass
• It is a mixture of sand , soda and lime
• Later on it gets addition of oxides for tint ,R.I
• Refractive index of 1.523
• Highly transparent material ,resist most
chemicals, does not discolor on aging.
• Specific gravity of 2.54 g/cm3
• Glass has the abbe value of 58.6/57

3. Refractive index of Crown glass
Refractive index Abbe Value Specific Gravity Manufacturer
1.523 58.6 2.54 Crown glass
1.523 57 2.41 Photography
1.60 42 2.38 Clear16
1.60 42.2 2.73 Photography16
1.706 31 2.99 High lite
1.805 25.4 3.39 High lite
1.805 25.4 5.18 SF-6

4. Specifications of Crown glass
• High index glass is made with titanium oxide
and glass mixture
• Titanium increases the R.I of glass but also
increased the chromatin dispersion and
density.
• Flint glass SF-6 is used for X-ray protection as
it was also a high index material but couldn’t
be tempered to meet FDA.

5. Advantages of crown glass
• More transparent
• Can not be scratched easily
• Higher indexes are available than plastic
• Higher abbe value so less chromatic
aberrations
• Does not get discolor in aging
• Cost effective

6. Disadvantages of Crown glass
• More weight
• Easily breakable
• Less choice PAL
• Less technology in ARC and photo chromatic
lenses

7. Plastic lenses
• It is an allyl resin
• CR-39 is a polymer that will not soften when
heated hence it is also known as
thermosetting plastic.
• SRC coating applied to both surface of lenses
but may be applied to front surface of semi
finished multifocal
• SRC on back surface can be applied on
multifocal after surfacing

8. Cont…..
• More SRC more difficult to tint
• SRC damaged by excess heat
• SRC coated lenses can not be over heated in salt
pan or air blower
• After prolonged exposure to cold air glass &
plastic lenses will fog when bought into a warm,
humid environment.
• Because plastic lenses have lower thermal
conductivity hence clear more slowly than glass

9. Cont….
• Plastic lenses are better in terms of fog for less
exposure activity in cold air than glass
• Anti fog spray cause condensation to form a
thin film rather than droplets on lens surface
with less effect on visual acuity.

10. Advantages of Plastic lenses
• Lighter in weight
• Higher impact resistant than crown glass
• Available in multi designs
• More choices in multifocal lenses
• Lighter in weight for high R.I also
• Easy to grind
• Can be tinted

11. Disadvantages of plastic lenses
• More prone to scratches
• Less choice in high R.I
• Prone to discolor in aging

12. Refractive indexes
Refractive index Abbe value Density Manufacturer
1.498/1.50 58 1.32 CR-39 PPG
1.530/1.53 45 1.10 CR-39 PPG
1.537/1.54 47 1.21 SPECTRALITE(SOLA)
1.549/1.55 38 1.24 EASYLITE
1.557/1.56 36 1.24 EVOCLEAR
1.586/1.59 31 1.20 POLYCARBONATE
1.595/1.60 36 1.36 THIN &LITE
1.660/1.66 32 1.35 HYPERINDEX
1.694/1.70 36 1.41 EYRY (HOYA)

13. Polycarbonate
• Used in industrial protective eyewear
• Can be surfaced to 1mm center thickness
• It is thermoplastic as it will be soften when
heated
• It is flexible and impact resistant
• Higher index lenses reflects light more hence ARC
is must
• Chromatic aberrations are more when pt looks
away from optic axis

14. Cont….
• Absorbs all UV rays (below380nm)

15. Advantages of polycarbonate
• Highly impact resistant
• 100% UV protection
• Thinner lenses
• Good for childeren

16. Disadvantages of polycarbonate
• More prone to scratches as compare to cr-39
• Lesser the abbe value so more chromatic
dispersion

17. Trivex
• Better abbe value than polycarbonate
• Similar impact resistant to polycarbonate
• 100% UV protection
• Thinner lenses
• Superior optics , impact resistant, ultra light
weight

18. NXT Material 2
• Light weigh material
• Strong and compatible with photo chromatic
pigments and with polarization.
• Helmet visors , airline cockpit door view ports,
• R.I 1.53 and density of a 1.11 an abbe value of
45 and is highly flexible.

19. Laminated lenses
• Lenses those are made from two or more
layers of material

20. Properties of ophthalmic lenses
• A beam of light fall on a lens surface, some of
light passes, some of light reflected, some of
light is absorbed by material
• Reflectance
• Absorption
• Transmittance
• Intensity of beam= R+A+T

21. Abbe value
• Degree to which light is dispersed while
entering a lens material.
• Dispersion is the amount that the material
spreads out the different wavelength of light
passing through it.
• Lower the abbe value greater the dispersion
and affects the peripheral optics.
• Abbe value cant be affected by any surface
technique as its material property.

22. Reflectance
• Light reflection at each of lens surface
• Result in loss of lens transparency
• R=100(n-1)2/(n+1)2 its calculated from the
refractive index of material.
• Higher the R.I greater the reflectance

23. Cont….
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

24. Absorption
• Percentage of light absorbed between front
and back surfaces of lens

25. Refractive index
• Ratio between velocity of light in air to the
velocity of light in the given medium and
denoted by n’
• N=velocity of light in air/velocity of light in the
medium
• It describes the ability to bend light
• Higher index more it is able to bend light

26. Specific gravity
• Physical density or weight of material in grams
per cubic centimeter

27. Impact resistance
• Dropping a 5/8 steel ball from 50inch onto the
lens
• Lens is place front side up on a neoprene
gasket.