Meniscus lens is a lens which has two spherical curved surfaces, convex on one side and concave on the other side.
It is thicker at the center than at the edges. The lens provides a smaller beam diameter in order to reduce the beam waste and spherical aberration. When a meniscus lens is combined with another lens, the focal length is shortened and the numerical aperture of the system is increased. This reduces the image distortion and increases image resolution.
There are two types of meniscus lenses: positive meniscus lenses and negative meniscus lenses.
Positive lenses are used in applications that require small focal number, which is less than 2.5. For focal numbers ranging between 2.5 and 10, meniscus lens can minimize spot size and enhance image quality. The concave side of the positive lens has a smaller curvature, and the convex side has a greater curvature.
Negative meniscus lenses are used for applications using infrared materials with high refractive index. These lenses are used as an alternative to other negative lenses with a greater curvature on the concave side and a smaller curvature on the convex side. They are used in beam expanding applications that require minimal spherical aberration.