Polycarbonate lenses are growing in popularity because they are the thinnest and lightest in weight. They are also the most impact resistant lenses available today, and are commonly used for children's glasses, sports glasses and safety glasses. If, however, you do not require the safety aspects of polycarbonate lenses, a high index material is possibly a better choice.
For people with a higher prescription, High Index materials allow thinner, lighter, more attractive lens without compromising optical quality.
CR-39 plastic is the most widely used material for eyeglass lenses. It's lighter in weight than glass, but not as scratch resistant.
A scratch resistant coating is highly recommended for plastic lenses.
At one time, glass was the only choice for corrective lenses. Glass provides good scratch resistance and can accommodate a wide range of prescriptions. However, glass is heavy and can be uncomfortable to wear. It's also easier to break or crack. If you're active you may want to select another lens material.
Photochromic lenses have a variable tint that changes with the amount of available light. In bright sunlight, they darken; indoors, they become clearer. They are available in both glass and plastic.
Anti Reflective coating
AR coatings reduce reflectance through the process of "optical interference" which counteracts reflected light waves. As a result, reflected light is reduced while light transmission is increased.
For CR-39 lenses, approximately 8% of transmitted light is lost to reflectance on the lens surfaces - 4% on the front surface, and 4% on the back surface. Only about 92% of light entering the lens is transmitted to the lens wearer's eyes. A high index material with a refractive index of 1.6 is transmitting only about 90% of incident light, with a 10% lost to reflectance. When coated on both front and back surfaces of a lens, an AR coating is designed to increase the percentage of transmitted light for both CR-39 and high index lenses to approximately 99%.
Scratch Protection Coating
Light weight hard resin lenses can be more easily scratched than glass lenses. Special coatings have been developed to help protect lenses from normal scratching. The modest additional cost for such scratch coating is a prudent investment.
The ultraviolet rays in sunlight pose potentially devastating harm to the eyes. Special treatment for hard resin lenses completely blocks hazardous UV light. Lenses like polycarbonate and high index include built-in UV protection.