A triplet lens
is an optical
device consisting of three single lenses
. The triplet design is the simplest to give the required number of degrees of freedom
to allow the lens designer to overcome all Seidel aberrations
The term is used in two ways. The three lenses may be cemented together, as in the Steinheil triplet. A triplet may also be a design with three spaced glasses, e.g. the Cooke triplet.