Natural logarithm wavelength modulation spectroscopy[In-WMS]is demonstrated in this Letter.Unlike the conventional wavelength modulation spectroscopy[WMS]-2ftechnique,it is a linear method even for large absorbance,which is the core advantage of In-WMS.The treating method used in In-WMS is to take the natural logarithm of the transmitted intensity.In order to determine the proper demodulation phase,the η-seeking algorithm is introduced,which minimizes the absolute value of the first harmonic within the non-absorbing region.Subsequently,the second harmonic of the absorption signal is extracted by setting the demodulating phase as 2η.To illustrate the validity of In-WMS,it was applied to water vapor experimentally.The result shows that even if the absorbance[base-e]is between 1.60 and 6.26,the linearity between In-WMlS-2f and volume fraction is still established.For comparison,measurement with conventional WMS-2fwas also done,whose response no longer kept linearity.The ηvalues retrieved in continuous measurements and the residuals were shown so as to evaluate the performance of the η-seeking algorithm.Time consumed by this algorithm was roughly 0.28 s per measurement.As an alternative WMS strategy,In-WMS has a wide range of potential applications,especially where the absorbance is large or varies over a wide area.