An imaging pixel array with event detectors for detecting input light signals and activating the imaging pixel array based on a predetermined threshold intensity of an input light. Each event detector is preferably made of auxiliary photosensitive pixels with a thresholding circuit. The imaging pixel array is activated to perform signal integration if the input light signal is above the threshold intensity. A self-triggered X-ray sensor can be realized with such event detectors. A CMOS active pixel sensor can be used as the imaging pixel array with advantages.