Serengeti Sunglasses are a longstanding favorite of drivers and pilots. Serengeti's trademark Spectral Control® System gives each lens the ability to sense and adjust to different lighting conditions. This Serengeti photochromic lens technology virtually eliminates eyestrain and fatigue.
PhD is a name Serengeti gives to lenses that darken in bright sunlight. These are medium darkness in the car and darken outside.
NXT is a brand name for Trivex. This lens material was developed as an improvement over polycarbonate by the US Military.