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VIHI (Visible and Infrared Hyperspectral Imager) is one of the three optical heads in the SIMBIO-SYS experiment onboard BepiColombo mission (the other two being STC, Stereo Camera, and HRIC, High Resolution Image Camera). The payload is designed to scan the Hermean surface from a polar orbit with the three channels to map the physical, morphological, tectonic and compositional properties of the planet. The main scientific objectives of the experiment are the study of the surface geology and stratigraphy, the surface composition, the regolith properties, the crustal differentiation, impact and volcanic processes. To fulfill these objectives the VIHI experiment uses a high performance optical layout (Schmidt telescope and spectrometer in Littrow configuration) which allows to investigate the 400-2200 nm spectral range with 256 spectral channels (6.25 nm/band sampling). The instrumental mapping capabilities are possible thanks to an IFOV of 250 degrad corresponding to a spatial scale of about 100 m/pixel at Periherm and 375 m at Apoherm. The instrument operates in pushbroom configuration, sampling the surface of Mercury with a field of view FOV of 64 times 0.25 mrad. The main technical challenges of this experiment are the focal plane design (HgCdTe thinned to improved the efficiency at visible wavelengths), the short dwell time (from about 40 msec at equator to about 100 msec at poles), thermal control, mechanical miniaturization, radiation hardening, data rate and compression. A description of the Internal Calibration Unit (ICU) concept and functionalities is given.