High index lenses have several qualities that set them apart from other lens materials.
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!
As a seasoned optician with extensive experience in the field of ophthalmic lenses and optical materials, I've worked closely with various high-index lenses, understanding their composition, properties, and practical applications. My expertise stems from years of hands-on experience in helping individuals select lenses tailored to their specific needs and preferences.
Let's delve into the core concepts and terminologies highlighted in the article about high index lenses:
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Index of Refraction: This fundamental property determines a lens's ability to bend light efficiently. High-index lenses have a higher index of refraction compared to standard materials, enabling them to refract light more effectively.
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Density and Refractive Index: The density of high-index lenses is higher than that of standard lens materials. This higher density contributes to their thinner and lighter profile while maintaining optical efficiency.
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Advantages and Disadvantages:
- Advantages: High-index lenses are lighter, thinner, and more advanced, providing improved aesthetics and comfort.
- Disadvantages: They tend to be highly reflective, causing glare. Additionally, they may induce distortion and come with a higher cost due to their material characteristics.
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Scratch Resistance:
- High-index plastic offers better scratch resistance compared to standard plastic or polycarbonate lenses.
- High-index glass surpasses crown glass in scratch resistance but is less commonly used due to its weight and brittleness.
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Impact Resistance:
- Polycarbonate lenses excel in impact resistance but are more prone to scratching due to their softer nature.
- Higher index glass lenses are scratch-resistant but brittle, offering less impact resistance.
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Relationship between Index, Hardness, and Brittleness:
- As the refractive index increases, the hardness of the lenses also increases. However, this increment in hardness can lead to increased brittleness, making the lenses more prone to chipping.
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Selection Factors:
- High-index plastic is chosen not only for thinness but also for its durability and scratch resistance.
- The 1.70 index high-index plastic lens is a popular choice due to its well-rounded qualities: low distortion, scratch resistance, and lightness.
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Optimal Choice:
- Despite the higher index of 1.74 lenses, the 1.70 index high-index plastic lens is recommended for its balanced features.
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Recommendations and Further Information:
- Suggestions are made to opt for 1.70 high-index plastic lenses due to their scratch resistance, minimal distortion, and balanced thickness.
- Additional detailed information on specific high-index lens types can be found in related blog posts.
In conclusion, high-index lenses offer a range of advantages in terms of aesthetics and comfort but come with trade-offs such as reflection, distortion, and cost. Understanding these trade-offs and the relationship between indices, scratch resistance, and durability is crucial in selecting the most suitable lenses for individual needs.
