Plano-Concave lenses have negative focal lengths and can be used to diverge collimated beams. In this case, the curved surface of the lens should face the source to minimize spherical aberration. In addition, they can be employed to offset the effects of spherical aberration caused by other lenses in an optical system.
The focal length of each lens can be calculated using a simplified thick lens equation:
f= R/(n-1),
where n is the index of refraction and R is the radius of curvature of the lens surface.
They can be also coated with MgF2 to protect the surface or AR coated to increase the transmission.