The main features of the crescent lens are: one side is convex and one side is concave. It can be either a converging lens or a divergent lens.

The main feature is that the focal length is positive, one side is a plane, and one side is a convex surface. In the optical system, the main functions are beam expansion, imaging, beam collimation, focus alignment, etc.

A concave lens is composed of a plane and a concave sphere that can produce negative spherical differences, which is often used in the system to expand the light beam or increase the focal length to balance the deviation of other lenses.

It does not change the magnification of the system. Protection window usually used in electronic sensors or external environment.

Germanium single crystal (Ge) has high hardness, good thermal conductivity, insoluble in water, through the band of 2 μ m-14 μ m, it is a very commonly used infrared optical material.

The double-convex lens features a positive focal length, consisting of two convex surfaces with equal radii; commonly used for imaging relay systems and for imaging objects at a limited conjugate distance.

Double-concave lens is composed of two concave surfaces with equal side radius of curvature, which is often used for lens beam expansion, projection, and expanding the focal length of the optical system.

Two planes have angles; two non-parallel planes can avoid the interference effect (Etalon effect) reflected by the front and rear surfaces of the high parallelism window.