Fig. 1. Schematic of the multi-resolution wavelet denoising

Fig. 2. Transmission spectra after processing by various threshold methods. (a) Original signal; (b) noise signal; (c) transmission spectrum after processing by hard threshold method; (d) transmission spectrum after processing by soft threshold method; (e) transmission spectrum after processing by index eclectic method; (f) transmission spectrum after processing by high-order approximation method

Fig. 3. SNR as a function of decomposition depth for different wavelet families. (a) db family; (b) sym family; (c) bior family; (d) rbio family

Fig. 4. Schematic of experimental setup for expired gas diagnosis

Fig. 5. Transmission spectra of indoor air without (a) and with (b) expired gas, as well as the corresponding results after wavelet denoising

Fig. 6. Real-time concentration of different components of indoor air and expired gases from different volunteers. (a) H2O; (b) N2O and CO