LumiBrite is a proprietary luminous material developed and used by Seiko in its watches. It is applied to the hands, hour markers, and sometimes the bezel of a watch to make them glow in the dark, ensuring legibility in low-light conditions.
In simple terms, it's Seiko's version of "glow-in-the-dark" paint for watch dials, and it's known for being exceptionally bright and long-lasting.
