What Makes the Index “High?”
High index glasses contain lenses that have a refractive index that is generally above 1.60. If you are in the market for prescription glasses, you may be wondering what the difference is between 1.53, 1.60, 1.74, 1.80, and various other refractive indexes advertised online. Since most people shopping for glasses online are not optometrists, it pays to have a better understanding of refractive index, and what it means for your prescription lenses.
The “refractive index” of a lens is the efficiency with which it bends light. The higher the index, the thinner the lens can be to bend the same amount of light as a lower index lens. In other words, a 1.53 index lens may be 5mm thick for a given prescription, while that same prescription can be made in a high index 1.74 lens at a 2mm thickness (arbitrary numbers). So, with high index plastic, you can expect your lens to be thinner and lighter the higher the index is.
A good rule of thumb regarding what high index lenses cost is: the higher the index, the higher the price. Once you get above about 1.79, you can expect that high index lenses will be made of glass instead of plastic. This will increase the cost of the lenses even more, and it also makes the lenses heavier. The advantage is that 1.80 high index lenses are considered to be ultra high index lenses. If your main concern is to get the thinnest lens possible, it’s a good idea to take a look at lenses with an index above 1.74 (the cutoff between “high index” and “ultra high index”), but unless your prescription is very strong (+4.00 or -8.00 or stronger), it is very unlikely that you’ll notice a difference between 1.67 or 1.74 high index and 1.80 high index.
It is also useful to note that polycarbonate lenses (the standard material at many labs in America) are considered a “mid index”. These lenses have a refractive index of 1.58 (as opposed to plastic’s 1.48), so they are thinner than plastic but thicker than high index. It is also worth noting that polycarbonate is best known for being very impact resistant, making it the most common choice for safety glasses, but it is easier to scratch than plastic or high index plastic.
High index eyeglasses are some of the most attractive glasses you can get because of their thinness, so they are worth considering if you are concerned about aesthetics. It is, however, important to balance how high your refractive index is with how much your wallet can handle. If you are having a hard time deciding which high index lens to get, a good rule of thumb is that 1.70 is a standard high index, most people can’t tell the difference between 1.70 and higher indexes, and 1.70 is often reasonably priced compared to many other high index lenses.