Serengeti Nuvola Sunglasses, Satin Tortoise Serengeti Polar phd 2.0 lens is built with NXT technology and made from trivex material, which is lighter than polycarbonate and offers a superior optical quality Using only the highest-quality Injected nylon TR90 for superior durability, these frames are exceptionally light, extremely flexible and comfortable With superior Serengeti Polarization, the fatigue, discomfort, squinting and impaired vision associated with all forms of glare simply go away. Polarization available in select models only. Darkening in bright sunlight and lightening in overcast conditions. Serengeti photochromic technology allows your eyes to adapt to any light Serengeti spectral control technology filters specific light wavelengths, sharpening the way you see color contract and improving visual definition