Ball lenses are sphere-shaped, light refracting devices used to improved signal coupling between fibers, emitters, and detectors. They are typically made of glass or another transparent material. Light is reflected within the lens due to the curved its curved surface, and the ratio d/D represents the collimated light input beam diameter to the ball lens diameter. Focal length is determined by the refractive index of the material and the lens diameter.
The material of lens construction largely dictates the type of light with which it is compatible; ultraviolet, infrared, or visible. Antireflective coatings greatly reduce insertion loss. Ball lenses are more common and less expensive than GRIN lenses, though the latter has remained more popular in critical instruments.