Distinguishing between the two most common modern lens materials—standard plastic (CR-39) and polycarbonate—is necessary for understanding your eyewear’s durability, safety features, and performance. Both are synthetic plastics, but their fundamental compositions lead to significant physical differences in how they feel and function. Knowing your lens type is important for making informed decisions about frame choices and proper care. While visual inspection is usually insufficient, a combination of simple home tests and document checks can reveal the material identity.
Fundamental Differences in Material Composition
The core difference between these two lens types lies in their chemical structure and manufacturing process. Polycarbonate is a thermoplastic polymer molded using heat, known for its inherent toughness and flexibility. It is widely used in safety applications because it is up to 10 times more impact-resistant than standard plastic, making it virtually shatterproof.
Standard plastic, known commercially as CR-39, is a thermoset resin that is cast and cured, resulting in a harder and more rigid material. Polycarbonate possesses a higher refractive index (typically 1.59) compared to CR-39 (1.50). This higher index allows the polycarbonate lens to be made thinner than a CR-39 lens for the same prescription. While polycarbonate is lighter per volume than CR-39, its high index is the primary reason it results in a lighter finished product, especially for stronger prescriptions.
Practical Home Identification Methods
One immediate, non-destructive way to check the material is the tap test, which uses the acoustic properties of the lens. Gently tap the lens surface with a small, hard object, such as a metal ring or a fingernail. Standard CR-39, being a harder resin, produces a sharper, high-pitched “tonk,” similar to tapping glass. Polycarbonate, a softer, more flexible thermoplastic, yields a duller, lower-frequency “thud” because it absorbs vibration.
The weight test compares the overall heft of the eyeglasses. For an identical frame and prescription, polycarbonate lenses are often noticeably lighter than CR-39 because the material is less dense. This test is most reliable when comparing high-prescription lenses, where the weight difference is significant. Low prescription lenses will not show as pronounced a difference, making the test less conclusive.
A thickness check involves looking closely at the lens edge, especially with a stronger prescription. Due to polycarbonate’s higher refractive index, the edge thickness is significantly reduced compared to the same prescription made in CR-39. If your lens is visibly thin despite a high prescription, this strongly indicates polycarbonate material. This difference is most apparent in semi-rimless or rimless frames where the edge is exposed.
Utilizing External Documentation and Visual Cues
The most definitive method for identifying your lens material is to consult the original purchase documentation, such as the receipt or the lens envelope provided by the optician. Lens material is always specified on these records, often listed as “Poly” or “PC” for polycarbonate, or “CR-39” or “Plastic” for standard material. This documentation provides a conclusive answer without physical testing.
For safety glasses, manufacturers often include tiny, permanent markings near the edge to indicate impact resistance standards. Under bright light or slight magnification, look for an etched code. The letter ‘F’ indicates low-energy impact resistance, characteristic of polycarbonate. Standard plastic lenses designed for safety may carry an ‘S’ marking, indicating a lower “extra strong” impact rating. These microscopic etchings are usually located near the temple hinge or the top edge.
A final visual cue involves observing the light refraction at the edge of the lens. Due to its higher refractive index, polycarbonate can sometimes exhibit a slightly different light distortion pattern than CR-39, though this is challenging for the untrained eye. A more practical visual indicator is the presence of a faint, laser-etched manufacturer’s logo or a tiny material code on the lens.