Treatments applied to the surfaces of lens elements to reduce flaws in an image
Coatings on a lens reduce reflections from the lens surface and therefore increase light transmission. Coatings also help to eliminate wayward light reflecting inside the lens—which causes flaws such as flare and ghosting—thus increasing image brilliance and contrast.
While all internal lens elements may be coated, the coating of the front lens element is most critical to the performance of the lens assembly as a whole. Coatings were initially applied as single layers (single coating) and later as multiple layers (multi-coating).
The colours of reflections seen in this front element indicate what kind of coating it has:
bright white reflections from the glass
bright reflections of blue and amber
reflections of many colors, especially green and purple