All the major synchrotron radiation facilities around the world have recently started upgrade projects to go towards the
4th generation of x-ray sources, in the direction of fully "Diffraction Limited Storage Rings" (DLSRs) in order to
produce photon beams with better quality. Several Free Electron Lasers (FELs), also providing diffraction limited beam,
are operating and increasing their performances, while other ones are almost ready to be operational. To fully exploit the
ultimate source properties of these next-generation light sources, the quality requirements for x-ray optics have
significantly increased, especially for reflective optics like mirrors. To maintain the coherence of the beam, such optical
components will need to have shape accuracies in the nanometer regime over macroscopic length scales up to 1 meter. If
we consider the ratio between these two parameters, we can quantify how challenging is not only the manufacturing
process but also the characterization and measurement of such optics. We will outline such challenge taking some
experience from the example case of European XFEL.
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