A portable X-ray imaging technique involves conducting a quick response to a target outdoors, which is of great importance for active diagnosis and prompt treatment. However, realizing portable X-ray imaging in terms of equipment miniaturization, image visualization, and the crosstalk of external light signals remains a challenge. Herein, we demonstrate the feasibility of utilizing a lead-free perovskite scintillator (Cs2ZrCl6: Lu3+) for exploring portable X-ray imaging under sunlight without the need for complex imaging devices. Our experiments verify that the ultrabright radioluminescence (347 lm m(-2), similar to 20 times greater than that of the commercial single-crystal scintillator Bi4Ge3O12) originates from the effective activation of triplet excitons (dark state) of the as-developed scintillator, which is unresponsive to UV and visible photons. Moreover, brightening of dark-state excitons is accelerated via the manipulation of the introduced lanthanide ions. These results highlight the importance of harvesting and taking advantage of engineering dark-state excitons, which is incentive for future research in low-cost X-ray detection for portable, fast-response radiography.