The photon arrival time monitor (PAM), based on X-ray–induced optical reflectivity or transmission changes in solid samples, will be implemented and served to precisely measure the X-ray arrival time and time jitter at the European XFEL facility. The spectral encoding method applies a broadband chirped supercontinuum as a probe, resulting in a wavelength-to-time mapping because different spectral components arrive at different times. One could foresee to record the arrival time for each shot in the burst with the combination of spectral encoding and a fast-readout 1D detector. This document describes the conceptual design, physical requirements, and technical design and considerations for a PAM prototype.