Frequent geometries considered involve the interaction between two identical spheres of radius R where the effective radius becomes
In the case of interaction between a sphere of radius R and a planar surface, one has
The above two relations can be obtained as special cases of the expression for R given further above. For the situation of perpendicularly crossing cylinders as used in the surface forces apparatus, one has
where R and R are the curvature radii of the two cylinders involved.