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.
Note that there are also gradient-index lenses which also have a cylindrical shape.
There, however, the two end faces are polished, and the mantle surface is not optically relevant.
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.
There are also half-rod lenses, which are rod lenses cut into two equal pieces.
There are two different definitions of focal length of a rod lens.
The effective focal length, which is the distance between a plane through the center of the lens and the beam waist (focus) of an initially collimated input beam, is given by the equation
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