Another characteristic of lens you need to be concerned with is the raw material used. Each material has its own properties which affect the wearer’s eyesight efficiency.  We can categorize lens raw material into two main groups, crown glass or mineral (glass lens) and hard resin (plastic lens). Two technical values are used to measure lens properties as follows:

• Abbe Number – this number is used to indicate lens clearness, the value range is from 20 to 60. The higher abbe number means that more light can pass through the lens. Raw materials that have a low abbe number may have chromatic dispersion* (various colors rings around the lens). A lens should have an abbe number higher than 30.
• Index of reflection or Index – this value is used to indicate the thinness of the lens. A thinner lens has a higher value of index. But hi-index lens may also have chromatic dispersion*.

*Anti-Reflection Coating is a process to coat lens surface with chemical substance. The coating will reduce chromatic dispersion, glare and reflection.

Mineral has an abbe number = 58 and index = 1.523. Although it’s fragile and heavy mineral has a durable surface and is line resistant.  Mineral also has a higher specific gravity than a plastic lens, as a result, it is thinner compared to a plastic lens which has the same lens power. The color of mineral lens is determined by the manufacturer.

Hard Resin or standard plastic lens (Columbia Resin 39 or CR39) has an abbe number = 58 and index = 1.498. CR39 is the most popular and the cheapest hard resin lens. A plastic lens is light and not easy to crack or break. We can color or discolor a plastic lens and can apply several coatings or add a UV protection component.

Hi-Index Lens - Both mineral and hard resin have hi-index lens versions. A higher index value makes this lens thinner than the standard mineral or plastic lens. The lens with an index equal to 1.6 or more is called a hi-index lens. Hi-index lenses are made of different raw materials with a special design to make the lens thinner. Of course, the hi-index lens is more attractive, lighter, and more expensive.

Photo or Transition Lens – This lens will almost immediately become darker in bright light (UV ray exposure) and return to clear when indoors. This kind of lens can be either chromatic cells coating on top layer of lens surface or have chromatic cells in the lens content. Both mineral and plastic lenses are available. This lens is suitable for the wearer who has to stay both indoor and outdoor, and has the benefit that no sunglasses are needed. Please note that it takes about 15 minutes to transit from dark color to clear, so it may not be good to use the photo sentitive lens when driving a convertible car because a problem occurs when wearer drives into tunnels or dark environment.

High-Impact Resistance Lens – This lens is developed from the Trivex material which is a high impact resistance material. It has an abbe number = 44 which brings about clear sight with no chromatic aberration. It’s quite thin with 1.53 index value. Trivex is the lightest lens material in the market with specific gravity at only 1.11 g/cc. The lens is durable, not easy to break or crack, and does not have any sharp pieces when broken. Its sturdy construction has made this lens suitable for rimless frames especially for the 4 – 8 holes rimless frame design and does not crack when screwing frame into the lens. It’s also called safety lens which is suitable for children or adults who have active life style or engaging in sport.