Double concave lens, with a positive focal length, is commonly used for applications such as reducing beam size, reducing focal length, or enlarging images.To reduce spherical aberration, when using a double concave lens for collimated beam focusing, the beam should be incident on the lens surface;When used for point light source collimation, the beam is incident on both sides.Double concave lenses focus light into a point and are commonly used for aiming and focusing monochromatic light sources,When in use, place the concave surface facing the incident light.Double concave lenses coated with antireflection film, germanium, silicon or zinc selenide substrates are suitable for infrared applications, and fused silica is suitable for ultraviolet applications.