Lens materials




Optical crown glass (B270 - Soda/lime glass)edit

  • Refractive index (nd): 1.52288
  • Abbe value (Vd): 58.5
  • Density: 2.56 g/cm3 (the heaviest corrective lens material in common use, today)
  • UV cutoff: 320 nm

Glass lenses have become less common owing to the danger of shattering and their relatively high weight compared to CR-39 plastic lenses. They still remain in use for specialised circumstances, for example in extremely high prescriptions (currently, glass lenses can be manufactured up to a refractive index of 1.9) and in certain occupations where the hard surface of glass offers more protection from sparks or shards of material. If the highest Abbe value is desired, the only choices for common lens optical material are optical crown glass and CR-39.

Higher-quality optical-grade glass materials exist (e.g. Borosilicate crown glasses such as BK7 (nd=1.51680 / Vd=64.17 / D=2.51 g/cm3), which is commonly used in telescopes and binoculars, and fluorite crown glasses such as the best optical quality low dispersion glass currently in production, N-FK58 made by the German company Schott with the following characteristics (nd=1.456 / Vd=90.90 / D=3.65 g/cm3) and are commonly used in high-end camera lenses).

One must bear in mind that the human eye itself has an Abbe value Vd≈50.2 so the extremely expensive high-end optical glass types mentioned above would be of dubious value when used to make corrective lenses. Also, one would be very hard pressed to find a laboratory that would be willing to acquire or shape custom eyeglass lenses from these materials, considering that such an order would most likely consist of just two different lenses that are specific to the wearer. In general, Vd values above that of Crown Glass and CR-39 are of dubious value, except in combinations of extreme prescriptions, very large lens sizes that cover a good portion of the face, high wearer sensitivity to dispersion, and occupations that involve work with very high contrast elements (e.g. reading dark print on very bright white paper, construction involving contrast of (dark) building elements against a cloudy white sky, a workplace with recessed can or other concentrated small area lighting shining on very bright white surfaces, etc.).

Plasticedit

For CR-39:

  • Refractive index (nd): 1.498 (standard)
  • Abbe value (Vd): 59.3
  • Density: 1.31 g/cm3
  • UV cutoff: 355 nm

Plastic lenses are currently the most commonly prescribed lens, owing to their relative safety, low cost, ease of production, and high optical quality. The main drawbacks of many types of plastic lenses are the ease by which a lens can be scratched, and the limitations and costs of producing higher-index lenses. CR-39 lenses are an exception in that they have inherent scratch resistance.

Trivexedit

  • Refractive index (nd): 1.532
  • Abbe value (Vd): 43–45 (depending on licensing manufacturer)
  • Density: 1.1 g/cm3 (the lightest corrective lens material in common use)
  • UV cutoff: 394 nm

Trivex was developed in 2001 by PPG Industries for the military as transparent armor. With Hoya Corporation and Younger Optics PPG announced the availability of Trivex for the optical industry in 2001. Trivex is a urethane based pre-polymer. PPG named the material Trivex because of its three main performance properties, superior optics, ultra light weight, and extreme strength.

Trivex is a relative newcomer that possesses the UV-blocking properties and shatter resistance of polycarbonate while at the same time offering far superior optical quality (i.e., higher Abbe value) and a slightly lower density. Its lower refractive index of 1.532 vs. polycarbonate's 1.586 may result in slightly thicker lenses depending upon the prescription. Along with polycarbonate and the various high-index plastics, Trivex is a lab favorite for use in rimless frames, owing to the ease with which it can be drilled and its resistance to cracking around the drill holes. One other advantage that Trivex has over polycarbonate is that it can be tinted.citation needed

The material was invented by Edwin C. Slagel and patented in September 1998.

Polycarbonateedit

  • Refractive index (nd): 1.586
  • Abbe value (Vd): 30
  • Density: 1.2 g/cm3
  • UV cutoff: 385 nm

Polycarbonate is lighter weight than normal plastic. It blocks UV rays, is shatter resistant and is used in sports glasses and glasses for children and teenagers. Because polycarbonate is soft and will scratch easily, scratch resistant coating is typically applied after shaping and polishing the lens. Standard polycarbonate with an Abbe value of 30 is one of the worst materials optically, if chromatic aberration intolerance is of concern. Along with Trivex and the high-index plastics, polycarbonate is an excellent choice for rimless eyeglasses. Similar to the high-index plastics, polycarbonate has a very low Abbe value, which may be bothersome to individuals sensitive to chromatic aberrations.

High-index plastics (thiourethanes)edit

  • Refractive index (nd): 1.600–1.740
  • Abbe value (Vd): 42–32 (higher indexes generally result in lower Abbe values)
  • Density: 1.3–1.5 (g/cm3)
  • UV cutoff: 380–400 nm

High-index plastics allow for thinner lenses. The lenses may not be lighter, however, due to the increase in density vs. mid- and normal index materials. A disadvantage is that high-index plastic lenses suffer from a much higher level of chromatic aberrations, which can be seen from their lower Abbe value. Aside from thinness of the lens, another advantage of high-index plastics is their strength and shatter resistance, although not as shatter resistant as polycarbonate. This makes them particularly suitable for rimless eyeglasses.

These high-refractive-index plastics are typically thiourethanes, with the sulfur atoms in the polymer being responsible for the high refractive index. The sulfur content can be up to 60 percent by weight for an n=1.74 material.

Ophthalmic material property tablesedit

Plastic
Material Index
(Nd) 		Abbe
(Vd) 		Specific
Gravity 		UVB UVA Reflected
lighta 		Minimum
thickness
typ/min (mm) 		Notes
CR-39 Hard Resin 1.49 59 1.31 g/cm3 100% 90% 7.97% ?/2.0
Essilor Ormix 1.6 41 1.3 g/cm3 100% 100%
Hoya EYRY 1.70 36 1.41 g/cm3 100% 100% 13.44% ?/1.5
MR-6 1.6 Plastic 1.6 36 1.34 g/cm3 100% 100% 10.57%
MR-7 1.665 Plastic 1.665 32 1.35 g/cm3 100% 100% ?/1.2 Daemyung Optical (Ramia)
MR-7 1.67 Plastic 1.67 32 1.35 g/cm3 100% 100% 12.26%
MR-8 1.6 Plastic 1.6 41 1.30 g/cm3 100% 100% 10.43%
MR-10 1.67 Plastic 1.67 32 1.37 g/cm3 100% 100% 12.34%
MR-20 1.6 Plastic 1.60 42 1.30 g/cm3 100% 100%
MR-174 1.74 Plastic 1.74 33 1.47 g/cm3 100% 100% 14.36% Hyperindex 174 (Optima)
Nikon 4 Plastic NL4 1.67 32 1.35 g/cm3 100% 100%
Nikon 5 Plastic NL5 1.74 33 1.46 g/cm3 100% 100%
Polycarbonate 1.586 30 1.20 g/cm3 100% 100% 10.27% ?/1.0 Tegra (Vision-Ease) Airwear (Essilor)
PPG Trivex (Average) 1.53 44 1.11 g/cm3 100% 100% 8.70% ?/1.0 PPG, Augen, HOYA, Thai Optical, X-cel, Younger
SOLA Finalite 1.60 42 1.22 g/cm3 100% 100% 10.65%
SOLA Spectralite 1.54 47 1.21 g/cm3 100% 98% 8.96% (also Vision 3456 (Kodak)?)
Tokai 1.76 30 1.49 g/cm3 100% 100%
Glass
Material Index
(Nd) 		Abbe
(Vd) 		Specific
Gravity 		UVB UVA Reflected
lighta 		Minimum
thickness
typ/min (mm) 		Notes
1.6 Glass 1.604 40 2.62 g/cm3 100% 61% 10.68% VisionEase, X-Cel
1.7 Glass 1.706 30 2.93 g/cm3 100% 76% 13.47% X-Cel, VisionEase, Phillips
1.8 Glass 1.800 25 3.37 g/cm3 100% 81% 16.47% X-Cell, Phillips, VisionEase, Zhong Chuan Optical (China)
1.9 Glass 1.893 31 4.02 g/cm3 100% 76% 18.85% Zeiss, Zhong Chuan Optical (China)b
Crown Glass 1.525 59 2.54 g/cm3 79% 20% 8.59%
PhotoGray Extra 1.523 57 2.41 g/cm3 100% 97% 8.59%

Indices of refraction for a range of materials can be found in the list of refractive indices.

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