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The Ultraviolet Imager (UVI) is one of the four instruments onboard the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) satellite, a collaborative science mission between the Chinese Academy of Sciences (CAS) and the European Space Agency (ESA). The UVI will capture the terrestrial auroral images that depict the energy depositions in the solar wind-magnetosphere coupling system. The primary function of UVI is to image the entire auroral oval at characteristic wavelengths while effectively mitigating contamination from dayglow, achieving a spatial resolution of approximately 100 km or better. The co-axis four-mirror all-reflective optical system provides a circular field of view of 9.97°, enabling coverage of the entire polar region when the spacecraft’s geocentric distance exceeds 50,000 km. This capability allows UVI to continuously monitor the complete auroral oval for over 40 hours. UVI operates in the long wavelength range of the N2 Lyman-Birge-Hopfield (LBH) band, specifically between 160–180 nm. To achieve the required spectral response and significantly reject out-of-band stray light, multilayer coatings are applied to its mirrors. The detector utilized is an intensified charge-coupled device (ICCD), with photons emitted from the phosphor being coupled through a relay lens to the ICCD. By default, the CCD captures one frame (512 × 512 pixels) every two seconds; thus, thirty frames per minute are coadded to produce a single UVI image. This processing can be performed in orbit or on ground to compensate the possible satellite jitters. The detailed geometric and photometric calibration procedures for UVI are elaborated upon in the paper.