In this paper, an opt-electronic method based on light-blocking theory of detecting impurity signals in ampoules was introduced. The signals of impurities contained in ampoules were investigated and analyzed. First of all, Butterworth digital filter, means filter, ideal low-pass filter and wavelet transform filter were used for the signal pre-processing, and the corresponding SNR(signal to noise ratio) were 13.98, 10.80, 9.18 and 14.89 dB, respectively. Experimental results show that the wavelet transform filter is the best choice. In addition, in order to improve the credibility of the test results, the methods of differential and the criterion of variance of section signal were proposed.

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