CR39 is a type of plastic. Most manufacturers of discount eyeglasses offer their cheapest pairs with CR39 lenses. This is because CR39 is extremely cheap. The process of making them is easy and very affordable for optical companies. Believe it or not, CR39 is actually better optically than Polycarbonate. The difference is incredibly subtle, but it's there. However, many people choose other lens materials anyway because of the down sides of CR39. Because it is a type of plastic, CR39 is not very durable at all. If the lenses are stressed or bent in any way, or if the eyeglasses are twisted by accident, the chances of the lenses breaking or chipping are not terrible. CR39 also tends to be a bit heavier and thicker than a Polycarbonate lens of the same prescription would be. If you are looking into buying prescription sunglasses, CR39 might actually be a good idea. It tends to tint, and hold tint better than other lens materials. In fact, most non-prescription sunglasses are made with some type of plastic lenses.
Polycarbonate is comparable to an acrylic material, and it has many uses. It has actually been used to make bomb proof shelters. Polycarbonate is incredibly durable. It can practically be bent in half without breaking. Which is why it is the law for kids eyeglasses to be made only with Polycarbonate. Compared to other lens materials, it is also thinner and lighter. For the most part, wearing a pair of eyeglasses with Polycarbonate lenses does not weigh your face and head down, or give you headaches. Another great quality it possesses is scratch resistance. No matter what lens material you have, scratches are always possible. However, Polycarbonate is known to be very resistant to obtaining a permanent scratch.
Trivex Introduced in 2001 by PPG, as the only lens material other than polycarbonate to pass FDA Impact Resistance Test (@ 1mm CT), the High Velocity Impact Test, and meet ANSI Z87.1 '89 standards, Trivex has been slowly increasing in both popularity and availability. While Trivex has a slightly lower refractive index (1.53 compared to 1.58), it's specific gravity, lowest of all lens products, which makes it the lightest of any lens material available today. Like polycarbonate, Trivex also has inherent UV protection. However, unlike polycarbonate, Trivex is optically superior. Further distinguishig itself, Trivex is ideal for drill mounting. The tensile strength of Trivex makes it highly resistant to cracking around drill holes, so much in fact, Younger Optics guarantees its Trivex products (Trilogy) for life, against stress fractures and drill mount cracking.
Hi-Index lenses come in several indices of refraction. The higher the number the thinner your lenses will be. They come in a 1.60,1.67, and the thinnest of them all The 1.74. The High index lenses except for the 1.74 are good for drill type lens mountings as well. The 1.74 in higher prescriptions can significantly reduce thickness, magnification and minification.