Infrared-stimulated luminescence from oxygen vacancies in CaO

2.1-eV $F$-center luminescence has been excited in thermochemically reduced CaO at 78 K using low-intensity photons with energy $E<\mathrm{}2.4\mathrm{}$ eV, i.e., less than the energy of the main $F$-center absorption. Thermoluminescence measurements indicate that electrons trapped at substitutional ${\mathrm{H}}^{2-}$ ions are involved in the energy transfer leading to the luminescence.