Lens Materials and Scratch Resistance

High index lenses have several qualities that set them apart from other lens materials.

Lens Materials and Scratch Resistance

Their index of refraction, or their ability to bend light efficiently, is due to their density compared to “standard” lens materials. They are lighter, thinner, and more advanced than standard lens materials. They also have some downsides, such as their highly reflective nature (which causes glare), the distortion that they cause, and their higher cost. All of these qualities are due to the high density of the lens material. This higher density also generally makes high index lenses harder than standard lenses, making them more scratch resistant. This does not mean that they’re more impact-resistant; actually, the opposite is true. This hardness is often accompanied by brittleness and the possibility for chipping. Here are some useful things to know about lens materials and scratch resistance:

• High index plastic is more scratch-resistant than standard plastic or polycarbonate. Generally, the higher the index of the plastic, the more scratch-resistant it is.
• High index glass is more scratch-resistant than crown glass.
• Even the highest-index plastic is still less scratch resistant than any glass material. You trade glass’s unmatched scratch resistance for lightness and impact resistance when you choose plastic.
• Of all the plastics, polycarbonate is the least scratch-resistant but the most impact resistant.
• In general, as index increases, hardness increases, and lenses become more brittle. So, the highest index glass lenses are extremely brittle but thin and scratch-resistant. Conversely, polycarbonate, one of the lowest-index lenses, is extremely impact resistant and flexible but very soft and easy to scratch.
• Most high index plastics rate relatively the same for scratch resistance, and any high index plastic is going to be more scratch resistant than standard plastic or polycarbonate.

High index plastic is sometimes chosen not for its thinness, but specifically for its scratch resistance and durability. It is a good choice for a pair of glasses that you want to wear a long time. High index glass, on the other hand, is rarely chosen simply for its scratch resistance because of its weight and brittleness compared to high index plastic. The most common high index plastic lens chosen nowadays is 1.70 index because it is well-rounded: it has low distortion for high index, it is almost exactly as thin as 1.74 index, and it is scratch-resistant and light. If you are interested in high index plastic lenses, we strongly recommend you go with 1.70, even over 1.74. If you want more information on specific high index types, you will find it in one of our other blog posts. Thanks for reading!