How low does the emittance of a storage ring have to be to make a fully coherent x-ray source?
Full spatial coherence is highly sought after because as a light source approaches that level of perfection, new x-ray probes and techniques become possible such as coherent diffraction imaging (CDI), x-ray photon correlation spectroscopy (XPCS), as well as enabling efficient focusing by Fresnel lenses to create diffraction limited beam-waists. One of the simplest ways to start thinking about coherence is to realize that the x-ray beam produced by a single electron passing through an undulator is completely coherent.