IASI is an Infrared Atmospheric Sounding Interferometer devoted to the operational metrology and to atmospheric studies and is to be installed on-board the second ESA Polar Platform METOP-1. This paper is devoted to an overview of the optical activities of the B phase of the project and presents the optimization of the spectral division. The last developments aiming to increase the optical transmission of the main optical parts are described, they are based on multidielectric coatings and the goal was to improve the transmission in the higher spectral zone, beyond 14 micrometer. The Cold Optical Unit is located at the focal plane of the Michelson Interferometer. This sub-assembly is designed to divide the IR input flux into 3 spectral bands and to focus it on the 3 detector arrays of detection. It includes a spectral separation using 2 beam splitters, consisting of two dichroic plates, dividing the incoming flux into 3 spectral bands which are 3.4 to 5.0 micrometer, 5.0 to 8.26 micrometer and 8.26 micrometer to 15.5 micrometer. Each array stands behind an objective and a set of 4 microlenses. This unit defines the aperture and the field of view of the instrument and operates at 100 K with passive cooling. The principles of cold optics are presented. An overall design of the optics has been performed taking into account the space requirements. All critical items have been manufactured to check that they can meet the required performance (photoconductive and photovoltaic detectors, hybrid focal plane and packaging, microlenses, aspherical lenses, dichroic plates, field lens, antireflective coatings, cold links).