Lenses for flashlights and other lighting units have been provided in a variety of forms, generally having in common a shape that is symmetrical about an axis along which the light is directed, i.e., the optical axis. Several such lenses have included a hole in a rear end of the lens adjacent a light source. Within the hole, the light source may be adjusted in position along the optical axis. Typically, adjusting the light source's position relative to the rear hole of the lens results in a varying light beam emerging from a front face of the lens. However, prior art lenses were limited in their capacity to combine a maximum intensity for a spot beam with a substantial uniformity for a wide beam.
Prior art lenses were also typically provided with a central convex lens surface on a front face combined with at least one additional convex surface where the light was either received into the lens, reflected within the lens, or emitted from the lens. The additional convex surface in the prior art may have been deemed necessary for a proper focusing of light from the source into a beam. Prior art lenses were alternatively provided with light-receiving, reflecting, and emitting surfaces that were flat as viewed in cross-section. Such flat surfaces were also likely deemed necessary for light-focusing or manufacturing purposes.