The Comprehensive Guide to Eyeglass Lens Materials and Coatings
Selecting the right eyeglass lens material and coatings is a crucial decision that can significantly impact a patient’s visual experience, comfort, and overall satisfaction. As eye care professionals, we are tasked with navigating the vast array of lens options available in the market, each offering unique benefits and trade-offs. In this comprehensive guide, we will delve into the intricate world of lens materials and coatings, exploring their properties, advantages, and considerations to help you make informed recommendations for your patients.
Lens Material | Index of Refraction | Specific Gravity | ABBE Number | Thickness |
---|---|---|---|---|
CR-39 (Plastic) | 1.498 | 1.32 | 58 | 2.0 |
Crown Glass | 1.523 | 2.54 | 58 | 2.2 |
High Index Plastic | ||||
H.I. 52 | 1.52 | 1.22 | 51 | 1.5 |
Trivex | 1.53 | 1.10 | 43 | 1.0 |
H.I. 54 (Spectralite) | 1.54 | 1.21 | 47 | 1.5 |
H.I. 55 | 1.55 | 1.28 | 38 | 1.5 |
H.I. 56 | 1.56 | 1.41 | 39 | 1.5 |
H.I. 57 | 1.57 | 1.17 | 36 | 1.5 |
Polycarbonate | 1.586 | 1.20 | 30 | 1.0 |
H.I. 60 | 1.60 | 1.34 | 37 | 1.5 |
H.I. 66 | 1.66 | 1.35 | 32 | 1.5 |
H.I. 70 | 1.70 | 1.41 | 39 | 1.5 |
H.I. 74 | 1.74 | 1.46 | 33 | 1.5 |
High Index Glass | ||||
H.I. 60 | 1.60 | 2.73 | 42 | 2.2 |
H.I. 70 | 1.70 | 2.99 | 32 | 2.2 |
H.I. 80 (Flint) | 1.80 | 3.37 | 25 | 2.2 |
Some Learning Take-aways on Lens Material Vs Refraction Index Vs Specific Gravoty vs ABBE Number Vs Thickness:
- High-index materials generally have higher refractive indices, making them thinner and lighter, but they tend to have lower ABBE numbers, leading to more chromatic aberration.
- CR-39 (Plastic) and Crown Glass provide a balanced combination of high ABBE values (58) and standard refractive indices, offering good optical clarity but at the cost of thickness and weight.
- Polycarbonate and Trivex lenses are excellent for impact resistance and lightweight properties but suffer from lower ABBE numbers, causing potential visual distortion.
Understanding Lens Materials
Refractive Index
At the heart of lens selection lies the concept of refractive index, a measure of how much a material bends or refracts light as it passes through. The refractive index of a lens material is a crucial factor that determines its thickness, weight, and overall optical performance.
Lens materials can be broadly categorized into three main groups based on their refractive index:
- Normal Index Lenses: These lenses, with a refractive index ranging from 1.48 to 1.54, include standard plastic (CR-39) and crown glass. These lenses are thicker and heavier but affordable.
- Mid-Index Lenses: Polycarbonate and Trivex lenses fall into this category, offering thinner profiles, lightweight construction, and improved impact resistance. Polycarbonate lenses have a refractive index of 1.586, making them a popular choice for safety glasses and children’s eyewear.
- High-Index Lenses: With refractive indices of 1.60 and above, high-index lenses are designed to provide the thinnest and lightest lenses, especially useful for patients with high prescriptions.
V-Value or ABBE Number
ABBE number is a measure of the possible chromatic aberration in a lens material. Through the optical center of any lens, regardless of material, color distortion is virtually non-existent. As the eye turns behind a powered lens, prism is induced, causing dispersion. Depending on the V-value, more or less dispersion is induced. Materials with a higher ABBE value exhibit less chromatic aberration and visual distortion, making them a preferred choice for ensuring optical clarity.
Specific Gravity
The weight in grams of one cubic centimeter of any given material. The higher the specific gravity of the material from which a lens is made, the more it will weigh.
Lens Thickness
In the U.S., governmental standards are specified for minimum lens thickness. Even though lenses can technically be made thinner than indicated in the table above, it is against government regulation to do so in safety lenses.
Lens Material Characteristics and Considerations
Crown Glass Lenses
Crown glass lenses were historically a staple in the optical industry, offering consistency in power and strength depending on the lens’s curve and thicknesses. It remains suitable for most lens prescriptions up to +/- 2.00 diopters, but stronger prescriptions require a lens too heavy and thick for the average frame. Crown glass is also a fragile material and requires chemical treatment in order to meet the safety standards set by newer lens options. For these reasons, crown glass is not nearly as popular in the U.S. as it once was.
CR-39 Plastic
CR-39 was the first plastic material used for ophthalmic lenses and is often just referred to as “plastic” with no other identifying numbers or phrases. It is still a popular lens material for a wide variety of single vision and multifocal designs. Plastic lenses are approximately 50% lighter than crown glass lenses of comparable size and shape, resulting in significantly greater comfort for the wearer. Since the surface of a plastic lens is softer than its glass counterpart, it does tend to scratch more easily. However, the many excellent scratch-resistant coatings available today have generally solved this problem. Plastic lenses may be tinted virtually any color desired. Plastic lenses do not automatically come with UV protection so a coating to protect against UV must be added.
Polycarbonate Lenses
A higher index of refraction combined with a low specific gravity make lenses made of this material thin and lightweight. However, its relatively lower ABBE value means that there is a greater tendency for white light to disperse into its component colors, producing more chromatic aberration than there is in most other lens materials. Polycarbonate lenses absorb UV radiation and do not need a UV coating. These lenses are extremely impact-resistant, making them especially suitable for sports, industrial safety applications and children’s eyewear. Although highly impact-resistant, the surfaces of polycarbonate lenses are soft and scratch very easily.
Trivex Lenses
Trivex lenses are highly impact-resistant and are recommended for use in situations where safety is needed, such as sports, certain occupations, and for children. The ABBE value of Trivex is 43, creating a clearer lens. Trivex can also be easily tinted, and blocks UVA and UVB so therefore, needs no UV coating.
High-Index Plastic Lenses
High-index lens materials result in lenses that are thinner, which makes them more attractive and lighter, and thus more comfortable. Most patients with mild prescriptions don’t particularly benefit from high-index materials. These materials tend to have worse ABBE values and can be more brittle. High-index plastic materials are often divided into categories of mid-index, 1.54 to 1.57, and high-index, 1.59 to 1.74.
Lens Coatings: Enhancing Visual Experience
Anti-Reflective (AR) Coatings
Anti-reflective coatings dramatically reduce glare and light reflection, improving visual clarity and contrast, especially in high-index lenses. AR coatings also enhance the appearance of the lenses, making them almost invisible. For patients who spend a lot of time in front of screens or driving at night, AR coatings are particularly beneficial.
Scratch-Resistant Coatings
Scratch-resistant coatings are essential for materials like CR-39 plastic, which are prone to scratches. These coatings protect the lens surface, ensuring longevity and maintaining optical clarity over time.
UV-Blocking Coatings
While some lens materials like polycarbonate and Trivex offer built-in UV protection, others, such as CR-39, require additional coatings to block harmful ultraviolet rays. Prolonged exposure to UV rays can lead to eye diseases like cataracts, making UV-blocking coatings an important feature for protecting eye health.
Additional Lens Treatments
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- Anti-fog coatings: Prevent lenses from fogging in humid or moisture-rich environments
- Photochromic: Adapt to changing light conditions, providing comfort in varying environments
- Tinted: Many tints of all colors are available to offer sunglasses or fashion options
- Polarized: Sunglass option that reduces glare to provide optimal visual clarity and safer driving
Key points from OTI to keep in mind as an optician
The selection of the right eyeglass lens material and coatings is a nuanced process that requires a deep understanding of the available options. By mastering the principles of refractive index, ABBE value, and the diverse range of lens materials and coatings, you can provide your patients with personalized solutions that enhance both visual performance and comfort. Below is a cheat sheet/image you can