Calcium Fluoride (CaF2)
Bi-Concave Lenses

Bi-concave or Double-concave (DCV) 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. Bi-Concave lenses have an equal radius of curvature on both sides of the optical system, their focal lengths are negative, as well as the radii of curvature of the curved surfaces. Negative focal length causes collimated incident light to diverge, they are often used to diverge a convergent beam. Due to their features, bi-concave lenses are generally used to expand light in the Galilean-type beam expanders or increase effective focal length of a converging lens by using in pairs in existing systems such as light projection systems. They are also useful when it comes to imaging reduction. In optical systems, it is common to choose their optics carefully so that the aberrations introduced by the positive- and negative-focal-length lenses approximately cancel. These negative lenses are commonly used in telescopes, cameras, lasers or glasses to help magnification systems be more compact.