FYI they are totally different "singlets", a singlet exciton refers to the spin state of the particle (as opposed to a triplet exciton, which has a different to

FYI they are totally different "singlets", a singlet exciton refers to the spin state of the particle (as opposed to a triplet exciton, which has a different total spin). A singlet lens is just a lens with a single simple component.