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An algorithm based on synthesis of two frame images is proposed to expand the dynamic range of CMOS image sensors. By changing the exposure time, a long-exposure and a short-exposure frame are captured and digitally integrated into one high dynamic range (HDR) image with the method of weighting summation. The exposure time can be adjusted according to the scene. The weighting factors are self-adapting to prevent pixel saturation and eliminate image flicker. Using long exposure time 24ms and short exposure time 3ms, the dynamic range is increased by 18dB, peak signal noise ratio (PSNR) can reach to 19.1850, and signal to noise ratio (SNR) is 14.6527. Results show that the dynamic range gain is between 13dB and 32dB without SNR and PSNR reduced.