CaF₂ Plano-Concave Spherical Lenses

Thorlabs' CaF₂ plano-concave lenses diverge a collimated beam from a virtual focus and are commonly used in Galilean-type beam expanders. Plano-concave lenses have a negative focal length and negative spherical aberration, which can be used to balance out aberrations of other lenses in the system.