The optics metrology laboratory of Sincrotrone Trieste is operating some non-contact interferometers since 1992, in order to characterise slope errors, figure deviations and surface roughness for synchrotron radiation optics (SR) up to 1.4 metres in length, prior to their installation at the beamlines. During these years, prompted by the increasing needs of experimentalists, the requirements for FEL and SR optical components have become more and more severe. We will review here the history of our measurements during the last nine years, comparing the match between the given specifications and measured optical quality of the delivered items. We will also illustrate which has been the evolution of the main optical concepts, that has ultimately boosted the suppliers to develop machining and testing methods to a novel level of accuracy.