Eyeglass lenses are made from various materials, each with unique properties. For decades, the standard material for plastic lenses has been CR-39, an organic polymer that offered a lightweight alternative to glass. CR-39 is an abbreviation for Columbia Resin #39, reflecting its origin as the 39th compound created in a specific research project. This material continues to be a widely used option in the optical industry, especially for standard prescriptions.
The Chemical Origin and History of CR-39
CR-39 is chemically known as poly(allyl diglycol carbonate), derived from the monomer allyl diglycol carbonate (ADC). This material is a thermoset plastic, meaning that once it is molded and cured, its shape is permanently set and cannot be melted down and reformed. This chemical structure grants it particular stability and resistance to various chemicals.
The compound was first synthesized in 1940 by the Columbia Resins project. Its initial commercial application was in the aviation industry during World War II, used to produce lightweight fuel tanks for B-17 bomber aircraft.
The optical potential of CR-39 was realized after the war, and the Armorlite Lens Company began manufacturing the first eyeglass lenses in 1947. CR-39 was approximately half the weight of traditional crown glass lenses. This lighter weight and improved safety profile made it the dominant choice for prescription lenses, replacing glass as the industry standard.
Key Optical Characteristics
The performance of CR-39 lenses is defined by its refractive index and its Abbe value. The refractive index measures how efficiently the material bends light; CR-39 has a low index, typically 1.50, which is the lowest among common plastic lens materials. Because of this lower index, CR-39 lenses must be thicker than high-index materials to achieve the same corrective power.
The material’s primary optical advantage is its high Abbe value, approximately 58. The Abbe value indicates the amount of chromatic aberration, or color fringing, a lens produces. A higher Abbe value signifies lower light dispersion, which translates to superior optical clarity and less noticeable color distortion. This low dispersion is the primary technical reason CR-39 is still favored for low-to-moderate prescriptions.
Practical Advantages and Common Use Cases
The inherent properties of CR-39 translate into several distinct benefits for consumers. The high Abbe value ensures exceptional visual quality, providing minimal chromatic aberration and a clear, sharp view.
CR-39 offers a significant advantage in weight compared to the glass lenses it replaced, making it comfortable for prolonged wear. Although it is not the lightest plastic available today, its relatively low manufacturing cost makes it the most economical option for prescription eyewear.
The material is also prized for its ease of tinting, readily accepting various dyes for prescription sunglasses. Due to its balance of good optics and affordability, CR-39 is primarily used for standard, single-vision prescriptions that are not too strong.
Limitations and Alternatives
Despite its optical purity, CR-39 has limitations that are primarily related to its thickness and impact resistance. Its low refractive index means that for individuals with strong prescriptions, the lens edges can become noticeably thick and heavy. This increased thickness can be aesthetically undesirable and less comfortable, particularly in frames that require a thinner profile.
CR-39 is a thermoset material and is not rated for safety eyewear, as it offers lower impact resistance than some modern alternatives. While it is much less likely to shatter than glass, it can crack or break upon high impact, making it unsuitable for children’s eyewear or safety glasses. Furthermore, CR-39 does not inherently block ultraviolet (UV) radiation, requiring an added UV-blocking coating to provide full eye protection.
Modern alternatives address these limitations through different material science approaches. Polycarbonate lenses, introduced in the 1970s, have a higher index and far superior impact resistance, making them the standard for safety and children’s glasses. High-index plastic lenses, with refractive indices of 1.60 and higher, are specifically designed for strong prescriptions, offering a significantly thinner and lighter profile than CR-39. A newer material, Trivex, offers a combination of high impact resistance and a higher Abbe value than polycarbonate, striking a balance between safety and optical clarity.