A Triplet Lens is an optical device consisting of three single lenses. The Triplet design is the simplest way to achieve the required number of degrees of freedom and to allow the lens designer to overcome all Seidel aberrations.
The Triplet configuration is used in two ways. The three lenses may be cemented together, as in the Steinheil triplet. Or a triplet may be designed with three spaced glasses, e.g. the Cooke triplet. The former has the advantage of higher optical throughput due to fewer air-glass interfaces, but the latter provides greater flexibility in aberration control, as the internal surfaces are not confined to have the same radii of curvature.
Typically a symmetrical optics design, these lenses are excellent when used at 1 to 1 magnification and have a wider field of view than cemented doublets. They also find wide range applications in optical relay systems.