Based on ISO 10110 standard, one of the key parameters in optical drawings is the radius of curvature of lens surfaces. It is specified on the first line of the surfaces column in the table of parameters. You can click here to review.
The lens optical properties are defined by its surface shapes. Being usually spherical, this means that the lens surface is part of the sphere surface.
In the above picture, you can see that the lens surface shape coincides with the sphere surface shape. The sphere radius equals the lens surface radius of curvature. In optical drawings, optical engineers specify this value for each lens surface.
In short, the lens surface radius of curvature is the radius of the sphere which forms this surface. The radius of curvature value is the sphere radius.
Check the parameters table in this article. You can see a mark near the radius curvature value. You can find two types of marks in lens optical drawings. These indicate what kind of lens surface to produce– Convex (CX) or Concave (CC).
The radius of curvature is a basic lens parameter. It defines the lens focal power i.e. its effective focal length. Errors made in calculating this value can have a substantial negative influence on your optical system.