High Index Lenses
High index lenses allow for your glasses to be thinner, lighter, and better looking. The index of an optical material refers to the refractive index of the material of which the lens is produced. As the index of refraction of a material increases, light rays are affected more strongly by the material. Therefore, it is possible to obtain the same effect while using less of the material. This is how high index lenses help to decrease the thickness and weight of your glasses.
Production of High Index Lenses
High index lenses are produced by combining monomers together in a chemical reaction to create long molecular chains. In the case of standard ophthalmic lens material, commonly known as CR-39 ophthalmic plastic, this reaction begins with only one monomer and the curing process is relatively quick. For high index lenses however, the process begins with more than one monomer, and can take a couple of days. Temperature control during this process must be very precise for the lenses to be cured correctly. The time and expense inherent in this process is the chief reason for their higher cost.
Types of High Index Lenses
High Index lenses come in different indices of refraction: polycarbonate, trivex, 1.60, 1.67, and 1.74 high index lenses are generally available. Generally, the higher your prescription, then the higher index of refraction you will need in your lenses to keep them thin and comfortable.
Other Factors to Consider
It is important to keep in mind that as the index of refraction of a lens increases, so does the amount of light it reflects. So, while anti-reflective coatings are important in the reduction of glare with any lens, they are especially important with high index lenses as the amount of reflected light is much higher with these lenses. Another thing to keep in mind, is that not everyone needs high index lenses. If your prescription is fairly mild, then you will probably be able to save some money by not getting high index lenses.