Lens Type: Photochromic Base Curve: 4 SPIRIT lenses are two times lighter than mineral , Eco Nylon is made from Grilamid BTR XE 4010, the polymer’s scientific name, is a sustainable substitute for TR90. It is extracted from Castor Plant but offers the same characteristics as TR90 Nylon (durable, flexible, lightweight, can bend, less likely to break) without requiring the use of petrol. lenses which make them the perfect lens for outdoor activities, providing an ultimate comfort.