GLASS LENSES

The oldest material used for spectacle lenses. Excellent optics properties, scratch resistant but approximately 33 percent heavier than traditional plastic lenses (CR39) and the least safe for impact resistance. Glass lenses are used less frequently with the exception for the "photogrey" lenses. The photogrey lenses are sun sensitive and change color. They are available in glass only. There is a plastic lens that changes color called "transitions plus" but it does not get as dark as the photogrey. However, in the Hi-Index lenses there is a newer version called "Transitions III" that works very well.

Traditional plastic lenses (called CR 39 for the batch number of the original mixture created) are much lighter than glass lenses and safer in case of breakage. They can be tinted almost any color and darkness. They are more easily scratched although the current lenses are much more resistant than the original ones. There is a scratch resistant coating that can be applied which greatly reduces scratches. (In our office they come with a one year warranty against scratching and if they do scratch we will replace them free of charge one time).

HI-INDEX LENSES

These are newer plastic materials that are denser (higher index) than traditional CR39 above. The end result is that the lenses can be made thinner and lighter. They can still be tinted, they absorb the UV light and come with a scratch coating warranty automatically (in our office). The most common high index lenses currently are the 1.56 and 1.60 although newer materials go even higher. The higher the number the higher the index, CR 39 is 1.498 index. There is a new process used on some Hi-Index lenses called Transitions III that allows the lens to darken outside and lighten inside. The ranges of light to dark is very good with this new lens.

POLYCARBONATE LENSES

The polycarbonate lens is one particular high index lens that was originally designed as a safety lens. It is about the most impact resistant lens material available and should be used for safety glasses. It is typically a less expensive option for those that desire a higher index on a budget. Personally, we prefer the optics of the newer materials over the polycarbonate lenses but highly use it for children's lenses and safety lenses.

ASPHERIC LENSES

The aspheric lens is actually a lens design that can be used in most lens materials. It is a way of shaping the lens to create a flatter, thinner and less distorted lens than traditional designs. In general it achieves this by changing the radius of curvature the further from the center of the lens. Combined with a 1.54 high index (most common) it creates an excellent design for cosmetic and optical purposes.

Example of Sola's Spectralite lens (Hi Index Aspheric) compared to CR39. .

Last update 2/15/05