Product Name: CaF2 Bi Concave Double-Concave Symmetry Lens Model NO.: CCL Coating: Polished Customized: Customized Certification: RoHS, ISO9001 Lenses Color: Clear Material: Glass Shape: Single-lens Transmittance: >95% Type: Concave Lens Usage: Lighting, Medical, Optical, Photography Transport Package: Plastic Box Specification: 1.0mm-700.0mm Origin: China HS Code: 9001909090 Product Description Diameter:1.0mm ~700.0mmScratch & Dig:80/50~20/10Spherical Surface Power:5 Fringes~1 FringesSpherical Surface Irregularity:λ/2~λ/8Center Error:5arc min ~ 30 arc SecBi-concave lenses have a negative focal length and are best used to diverge a converging beam. Similar to a plano-concave lens, a bi-concave lens can diverge a collimated beam to a virtual focus like in a Galilean-type beam expander. The focal length of each lens can be calculated using the following equation:f= -(R1*R2)/((n-1)*(R2-R1)).where n is the index of refraction and R1,R2 is the radius of curvature for each surface of the lens. They can be also coated with MgF2 to protect the surface or AR coated to increase the transmission.