Ruby Half Ball Lens is a hemispherical optical element processed with Ruby as the base material. It is a corundum (Al₂O₃) containing chromium, which is colored red due to chromium, and has the same physical and chemical properties as sapphire, but with differences in optical properties. The light transmission range covers the visible wavelengths, but the ultraviolet to infrared transmittance is lower than that of sapphire.

They are made into hemispherical structures by high-temperature melting, precision polishing, and coating techniques, and are often used in optical systems for high wear resistance or in extreme environments.

Common application areas include industrial sensing, used in barcode scanners, laser rangefinders and other equipment focusing lens group, resistant to high mechanical shock and dust environment; as a high-temperature furnace observation window or high-power laser protective cover, the use of its high-temperature resistance, but also can be applied to some of the high-end equipment (eg, microscope objective lenses) using the ruby lens to enhance durability