Calcium Fluoride (CaF2)
Plano-Concave Lenses

Plano-concave lenses are negative lenses which are thicker at the edge than in the middle, when light passes through them, it diverges and the focus point is virtual. Their focal lengths are negative, as well as the radius of curvature of the curved surfaces. Given their negative spherical aberration, plano-concave lenses can be employed to balance out spherical aberrations caused by other lenses in an optical system. Plano-concave lenses are useful for diverging a collimated beam and collimating a convergent beam, they are used to expand light beams and to increase focal lengths in the existing optical systems. These negative lenses are commonly used in telescopes, cameras, lasers or glasses to help magnification systems be more compact.

Plano-concave lenses perform well when the object and image are at absolute conjugate ratios, greater than 5:1 or less than 1:5. In this case, it is possible to reduce spherical aberration, coma, and distortion. Similarly to with plano-convex lenses, to achieve maximum efficiency the curved surface should face the largest object distance or the infinite conjugate to minimize spherical aberration (except when used with high-energy lasers where this should be reversed to eliminate the possibility of a virtual focus).

Due to its high transmission from 0.18 µm to 8.0 μm, CaF2 exhibits a low refractive index varying from 1.35 to 1.51 and is commonly used for applications requiring high transmission in the infrared and ultraviolet spectral ranges, it has a refractive index of 1.428 at 1.064 µm. Calcium fluoride is also fairly chemically inert and offers superior hardness compared to its barium fluoride, and magnesium fluoride cousins. Paralight Optics offers Calcium Fluoride (CaF2) plano-concave lenses with antireflection coatings for the 2 µm to 5 µm wavelength range deposited on both surfaces. This coating greatly reduces the surface reflectivity of the substrate, yields an average transmission in excess of 97% over the entire AR coating range. Check the following Graphs for your references.