Hodgkinson, JaneMasiyano, DacksonTatam, Ralph P.2014-04-152014-04-152013-02-01J. Hodgkinson, D. Masiyano, and R.P. Tatam. Using integrating spheres with wavelength modulation spectroscopy: effect of pathlength distribution on 2nd harmonic signals. Applied Physics B, February 2013, Volume 110, Issue 2, pp223-2310946-2171http://dx.doi.org/10.1007/s00340-012-5166-7http://dspace.lib.cranfield.ac.uk/handle/1826/8389We have studied the effect on 2nd harmonic wavelength modulation spectroscopy of the use of integrating spheres as multipass gas cells. The gas lineshape becomes distorted at high concentrations, as a consequence of the exponential pathlength distribution of the sphere, introducing nonlinearity beyond that expected from the Beer-Lambert law. We have modelled this numerically for methane absorption at 1.651μm, with gas concentrations in the range of 0-2.5%vol in air. The results of this model compare well with experimental measurements. The nonlinearity for the 2f WMS measurements is larger than that for direct scan measurements; if this additional effect were not accounted for, the resulting error would be approximately 20% of the reading at a concentration of 2.5 %vol methane.en-UKThe original publication is available at www.springerlink.comArticle