Phosphorescence is a phenomenon in which a substance absorbs some light and over some time emits it with a forbidden transition. These transitions have very low probability and are therefore fairly rare. Phosphorescence produces a small but a long persistent light flux. In this master thesis I focus on the problem of measuring light flux, the efficiency of absorption and the measurement of the maximum of absorbed energy. Based on the measured central wavelength I calculate the density of active states. I carry out measurements on two samples of phosphorescent objects. For all experiments I use a simple measuring method, based on a digital camera, which is appropriate for measuring small light fluxes. This thesis can also be useful for teaching physics in school because the phenomenon of phosphorescence can be explained as storing of light energy. Furthermore, the measuring method can be applied to other experiments with light which are relevant for school. As an example I measure the light flux from the last quarter of Moon and from the almost full Moon, as well as the light energy which the flash emits. I compare these measurements with theoretical values. Based on these comparisons I analyse advantages and possible disadvantages of using the digital camera as a measuring instrument.