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In the last three decades, the terahertz science has achieved great development, possibility to be utilized in a wide variety of fields such as biology, quality control, and security. Especially terahertz time-domain spectroscopy is very useful for nondestructive measurement of materials. Many excellent researchers have approached the commercialization of terahertz technology. However, in the ambient atmosphere, not in the laboratory environment, the signal is distorted due to absorption of water vapor. As a result, unintentional noise is generated in the absorption line, resulting in a great difficulty in spectroscopy. We propose an algorithm that selects a frequency band with no signal distortion and performs spectroscopy. It has an advantage that it can apply algorithms without complex calculation and additional optical components. We measure terahertz time-domain signals of several samples within nitrogen filled chamber, and then the nitrogen was removed to measure the signal in a high relativity humidity environment. We extract optical parameters from the obtained signals. The algorithm is verified by comparison of experimental results and literature value. The values of absorption peaks of samples from our algorithm show good agreement with a literature value. We compared these values and conclude that the thickness was measured and we can check the peak of the absorption line. We find that the proposed algorithm can extract optical parameters even in a high relativity humidity environment.
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