Decay in Brightness of Infra-Red Sensitive Phosphors

An equation is derived for the decay in brightness of infra-red (IR) sensitive phosphors under constant IR stimulation. The derivation is based on a second-order process, but takes into account the absorption of the stimulating IR and the emitted visible light. The theoretical curves obtained differ markedly from the simple second-order decay curve, especially when the IR absorption is greater than the visible absorption. The equation involves three arbitrary constants which depend on the quantum efficiency, the number of active centers, and the absorption coefficients of the phosphor for visible and infra-red radiation. The equation is shown to fit the decay curve of a strontium selenide phosphor for an effective decrease in brightness of ${10}^5$. It is shown that theories involving a distribution of electron traps, with or without interaction, lead to conclusions that do not agree with the experimental results. Some experimental evidence is offered indicating that there is no change in the trap distribution as this phosphor is exhausted. The apparent deepening of the traps as the decay process continues can be attributed to the absorption of the infra-red.