The characteristic of a lens material is typically defined by 3 material properties:
1. Abbe Value - Clarity of the material
2. Refractive Index - Thickness of the material
3. Specific Gravity - Weight of the material

Abbe Value - The Abbe Value, in general, refers to the "Optical Clarity" of a lens. It is a number that describes the amount of chromatic aberration (tendency of a meterial to separate light based on varying wavelength) of an ophthalmic lens material. The higher the Abbe Value of a lens, the lower chromatic aberration. The lower the chromatic aberration, the better the clarity.

Refractive Index - The Refractive Index (or index of refraction) of a medium is a measure for how much the speed of light is reduced inside the medium. In other words, it is a measure of the bending (refraction) of a ray of light on entering a medium. A lens material with a higher Index of Refraction will bend more light and will require less of a curve to achieve a specific Rx. Surfacing with less curve for a given Rx will help keep the lens thinner. Higher index lenses typically have a higher specific gravity, due to the increased material density.

Specific Gravity - Specific Gravity is the ratio of density of a lens material to the density of water. The higher the specific gravity of a lens material, the higher its density will be. So for a given power, the lower the specific gravity the lighter the lens will be.

Material, Plastic	Index	ABBE	Specific Gravity
Transitions Plastic	1.50	58	1.27
CR-39 Hard Resin	1.50	58	1.32
Trivex	1.53	44	1.11
Spectralite	1.54	47	1.21
Ormex	1.56	36	1.23
Polycarbonate	1.59	30	1.20
Hi-Index 1.60	1.60	42	1.22
Hi-Index 1.67	1.67	32	1.35
Hyperview 1.66	1.66	32	1.37
Hi-Index 1.70	1.70	39	1.41
Hi-Index 1.74	1.73	33	1.47
Crown Glass	1.523	59	2.54

Lens Materials

• Glass - Is the historic choice of ophthalmic lenses, is the most stable, scratch resistant material, and provides the clearest optics of all lens materials. However, since glass is more brittle, less impact resistant (unsafe in case of accidents) and heavier than other lens materials, it is no longer the choice of many eye glass wearers, especially patients with higher prescriptions. Over the years, the variety of glass lens styles has also greatly diminished.
• Plastic - Also known as hard resin or CR39 is approximately 50% lighter than glass and comes in the widest variety of lens styles of any material. CR39 is far less scratch resistant than glass and therefore benefits greatly with a quality scratch resistant coating. Approximately 60% of lenses sold in the US today are made from hard resin. Hard Resin lens materials do not include UV protection and hence require a UV coating for protection from harmful ultra violet light.
• Polycarbonate - Polycarbonate is one of the thinnest and lightest materials available. Polycarbonate offers very high impact resistance and a refractive index of 1.59. It is usually preferred over conventional hard resin because of its impact resistance and lighter weight. Polycarbonate material blocks both UVA and UVB and offers availability of most lens styles. Due to the material characteristics Polycarbonate has a lower Abbe value.
• Trivex - Trivex is a newly developed polymer that combines the impact resistance of polycarbonate, exceptional optical clarity with an Abbe Value of 45, and a specific gravity of 1.11 (the lightest material available). Trivex has excellent tensile strength making it an ideal choice for rimless frames.
• Mid-Index - Generally considered to be any index above Trivex (1.53) but below polycarbonate (1.59). Mid-index lenses are generally considered in the same family. Product characteristics and lens style availability vary depending on manufacturer.
  • 1.54
  • 1.55/1.56
• Hi-Index (generally considered any index 1.60 or above) - Hi-Index materials provide flatter surfaces, reduced magnification or minification and help maintain a thinner lens profile. Hi-Index materials, however, typically have a lower Abbe Value and could potentially affect patients who are sensitive to chromatic aberration. In general, the higher the index of refraction the thinner the lens. Although Hi-Index may provide a thinner lens due to its high density, it may not be the lightest. Product characteristics and lens style availability vary depending on manufacturer.
  • 1.60
  • 1.66/1.67 - the difference in 1.66 and 1.67 is in the way the index of refraction is measured, not in the material itself.
  • 1.70/ 1.71