Optical Metrology—On the Inspection of Ultra-Precise FEL-Optics

Dedicated instrumentation is required to inspect the quality of optical elements like mirrors or gratings used in grazing incidence condition at FEL-beamlines. Comparing the quality of optical elements available with the advent of 3rd generation storage rings two decades ago to the state of the art of today an improvement by one order of magnitude in terms of figure error has been achieved. These achievements were mainly possible by improvements on deterministic surface finishing as well as by new developments on the field of metrology. This chapter will give an overview and description of the state of the art ex-situ metrology applied to measure synchrotron optical components.