A bare plastic lens scratches and reflects. A hard coat (a cured lacquer) gives scratch resistance; an anti-reflective (AR) stack of vacuum-deposited metal oxides removes reflections and improves clarity and cosmetics; hydrophobic and oleophobic top layers make lenses easier to clean. Premium AR coatings bundle all three with a warranty.
UV400 means the lens blocks all light below 400 nm; polycarbonate, Trivex and most high-index materials block UV inherently, CR-39 needs a treatment. Blue-light filtering is done in the material or the coating. Mirror coatings and tints are applied to sunglass lenses; photochromic can be in-mass or a spin-coated layer.
Coating is done in vacuum chambers by lens makers and larger laboratories. It is the most capital-intensive step in the lab, which is why many labs send lenses out for AR. Coating equipment comes from Satisloh, Bühler Leybold, Schneider and a few others.