This paper describes a model of IR absorption based on the line-by-line integral. It briefly introduces Beer-Lambert law. Non-Dispersive Infra-Red (NDIR) technology uses a broadband light source which can not meet the condition of the monochromatic light. Thus the line-by-line integral method is used to establish a spectral absorption model with a certain bandwidth. Then using Matlab programs, the specific method of the line-by-line integral is proposed. By analyzing the transmission ratios of the methane 3.31um band under different conditions, the relationships between the pressures and the spectral transmittance are obtained, and the applicable line shape function is determined in methane detection. Through the comparison of the results calculated by the model with measurements, the accuracy of the model is verified. Using this infrared spectral absorption model, a series of transmission ratios are calculated after infrared light passes through different concentrations of the methane.
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