Abstract: Process for continuous determination of the optical layer thickness of coatings, which are applied on both sides of the spherical surfaces of concave convex lenses having different spherical radii R1 and R2. In this process a ray of light is beamed eccentrically during the coating process at each concave convex lens, and the reflection or transmission at the convex spherical surface and at the concave spherical surface is continuously measured with photodiodes, and the respective optical layer thickness is determined from the functional relationship between the reflection or the transmission and the optical layer thickness.