Negative cylindrical lenses act as plano-concave spherical lenses, except on only one axis. These lenses are used in applications that require one dimensional shaping of a light source.
A typical application is to use a single cylindrical lens to turn a collimated laser into a line generator. Pairs of cylindrical lenses may be used to anamorphically shape images. To minimize the introduction of aberration, the curved surface of the lens should face the source when used to diverge a beam.