An ultraviolet imaging system was designed for quantitative characterization of micro-and nano-structures. A beam splitter inserted in the detection path reflected the signal from sample to a pinhole on the imaging plane. The sample was scanned to obtain the sample profile. The DUV light from apparatus was utilized to reduce the diffraction limit size and enhance resolution; and laser interferometer was used to trace the line width to international SI unit. CCD image was used to auto-focus the sample. Sequence images were captured along axial direction and the focus sharpness was determined by focus evaluation function. Algorithms were compared and wavelet algorithm for critical dimension auto-focused in used the ultraviolet measurement system. For wavelet, decomposition level, wavelet vanishing moments and other parameters for UV imaging system was determined.