In the previous article, we discussed “What makes a good pair of sunglasses?”. In this article, let’s talk about “Are polycarbonate lenses good?”.
Polycarbonate is PC. PC is a common plastic product material in life, such as water cup lids, car lampshades, medical parts, optical lenses, etc. It has the advantages of super toughness, but it is easy to wear and tear. The frame often uses PC materials generally because it is not easy to break. Regarding the material of glasses, they used to be glass lenses, but now most of them are resin. Polycarbonate is suitable for sports glasses, functional glasses, and children's glasses. Because PC is harder than resin, it is not easy to break and cause injury. The limitation is that it cannot be used for dyeing polarized light.
The specific gravity of PC is 1.20g/cm3. The specific gravity of CR-39 is 1.32 g/cm3. The specific gravity of MK-55-C (medium fold) is 1.28g/cm3. The specific gravity of the glass is 2.61g/cm3. The comparison shows that for the lenses of the same specifications and geometric dimensions, the PC lens has the smallest specific gravity, which further reduces the weight of the lens.(https://www.koalaeye.com/collections/blue-light-blocking-glasses)
Using the characteristics of excellent impact resistance, the lens can be designed to be thinner. In addition, the refractive index of Polycarbonate materials can also be found to explain the thinner lens. The refractive index of CR-39 (ADC) is 1.499, the refractive index of NK-55-C (center folding) is 1.550, and the refractive index of PC is 1.586. The higher the refractive index of the material, the thinner the lens can be made. Conversely, the lower the refractive index of the material made of the thicker the lens. Therefore, PC myopia lenses have a relatively thin edge thickness compared to CR-39 and center-fold lenses.
Polycarbonate has extremely excellent impact resistance and is known as the 'King of Plastics'. It can be widely used in the manufacture of aviation windows, bulletproof 'glasses', riot masks, and shields. The impact strength of PC is as high as 87/kg/cm2, which exceeds that of cast zinc and cast aluminum, and is 12 times that of CR-39. Spectacle lenses made of PC are the only spectacle lenses that cannot be broken when they are stepped on hard on the concrete floor. So far, in terms of safety, PC lenses are unparalleled.
Absorbing Ultraviolet Radiation
Modern medicine has confirmed that ultraviolet radiation is the main cause of eye cataracts. Therefore, the requirements for the absorption of ultraviolet rays in spectacle lenses are becoming more and more clear. General optical resin spectacle lenses, the material itself also has the ability to partially absorb ultraviolet rays. However, if you want to effectively prevent the penetration of ultraviolet rays, you must add a certain amount of ultraviolet absorbers. PC myopia lenses are more effective in blocking ultraviolet rays.
Good Weathering Resistance
Polycarbonate is one of the engineering plastics with excellent weather resistance. According to the data of outdoor natural aging experiment, after 3 years of outdoor, its tensile strength hardly changed, and its haze and yellow index increased slightly.
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