On the equation used to calculate lens surface radius of curvature based on the number of Newton’s ring

Newton’s ring is the most used method to test surface radius of curvature of a lens. An approximate equation is currently widely used by lens fabrication shops and in literatures to calculate the error of surface radius of curvature based on the number of exhibited Newton’s ring. It is shown in this paper that this approximate equation contains significant error and should be replaced by two other simple equations derived in a recently published paper.