A rod lens is a cylindrical lens which has the geometrical form of a cylinder and has a polished mantle, while the two flat end surfaces may be ground. It is analogous to a ball lens for focusing in both directions.

Rod lenses can be used, for example, for collimating a divergent beam in one dimension (see Figure 1) or for focusing light to a line. There are also imaging applications. Note, however, that a rod lens exhibits substantial spherical aberrations when light propagation is not restricted to a small fraction of its cross-section.

Rod lenses are available with different optical materials, mostly with glasses like fused silica and BK7, but also with crystalline materials like YAG or silicon, e.g. for infrared optics.

Rod lenses can have diameters of a few millimeters, but there are also cylindrical microlenses with substantially smaller diameters. Small-diameter rod lenses are frequently used as fast-axis collimators for diode bars.