This paper presents the results of measurements of fluorescing cerium-doped yttrium aluminum garnet crystals after being irradiated by an accelerated electron beam with energy of around 4 MeV. The measurements were performed using the PHIL linear accelerator at LAL (France). We observe linear dependence of the crystal emission to the electron beam charge and the isotropy of the photon emission. We provide the calibration coefficients of the photon emission depending on the charge of the accelerated electron beam for two crystals originating from two different manufacturers.