Measurements are presented of the optical properties and strength of quantum counter action in Er³⁺-doped SrF₂ which demonstrate the validity of a simple rate equation theory of quantum counter action. This analysis is used to assess the quantum counter as a potential narrow band infra-red detector. An extension of the theory to the case of modulated input radiation is shown to provide a method for measuring lifetimes of excited states which may not be accessible by conventional techniques, e.g. non-fluorescent levels. Some results obtained by this method are reported.