Meteosat Third Generation is the next ESA Program of Earth Observation dedicated to provide the International Community, with an operational satellite system able to support accurate prediction of meteorological phenomena, and climate monitoring. The satellites will be operating from the Geostationary orbit using a 3 axes stabilized platform. The main instrument is called the Flexible Combined Imager (FCI), currently under development by Thales Alenia Space France, the PFM being integrated and tested in our Cannes facilities. This instrument will provide full images of the Earth every 10 minutes in 16 spectral channels between 0.44 and 13.3 µm, with a ground resolution ranging from 0.5 km to 2 km. The FCI is composed of a TMA telescope developed by OHB, followed by an IR objective lenses (called SSA) which collimate the beam and split it into several spectral groups. The output beams then reach a cryostat, in which the cold optics (CO-I) focalize them onto the IR detectors. The assembly of the Spectral Separation Assembly (SSA), the CO-I and the detectors is called the SSDA and has been specifically tested as a sub-system. This article will first recall the optical design of the FCI. Then we will describe the principle of the main optical tests both at telescope level and SSDA level. The test results will be presented and compared to the success criteria and specification. In particular the telescope WFE, focal length and line of sight have been proved to be compliant to the success criteria. Concerning the SSDA, we will present the results of MTF measurements, co-registration between spectral channels, polarization and spectral transmittance, which were also all found compliant to their respective success criterion. Stray-light for both TA and SSDA has been characterised on-ground in order to provide the initial input necessary for the development of correction algorithm. At the beginning 2020, these two successfully tested sub-systems were integrated together to build the FCI PFM, which is expected to undergo optical vacuum testing at the timeframe of September 2020.