Spin coating

Optical glossary/Coatings and filters

Applying the photochromic layer onto the lens surface at even thickness, so color depth stays uniform at any prescription. In-mass photochromics carry the chemistry within the material instead — a difference that shows mainly at stronger prescriptions, where thickness varies more across the lens.

In more depth

In spin coating, a liquid photochromic layer is applied to the front of the lens and spun so it spreads to an even thickness, then cured. Because the active layer is the same thickness everywhere, the lens darkens to the same depth at its center and its edge.

In an in-mass lens the photochromic chemistry is mixed through the material itself, so darker color follows thicker material. On a strong minus lens, which is thick at the edge and thin in the middle, that can show as a darker rim and a lighter center.