Transmission (400 to 2500nm) and reflectance spectra (400 to 2200 nm) were recorded from polyethylene films (low-density PE, cryst. 60%, additive free), with a thickness ranging from 0.2 to 8.46 mm. The information depth, defined as the thickness of material that results in the loss of 50% of the light measured as reference, was calculated for both reflectance and transmission geometries. The resulting curves clearly show PE characteristics. For transmission, the information depth varies between ~ 100 mu m (at 2300 nm) and 830 mu m (at 1600 nm). For reflectance, the information depth varies between 0 and 2.5 mm. The reflectance geometry proves to be the better way of measuring through a PE film. The calculated information curve also shows that at PE bands of low to medium strength the contribution of the bulk is very influential. The greater the strength of the PE band, the lower the contribution of the bulk to the spectrum. In case of an inhomogeneous sample, the information depth will reflect the effect of the inhomogeneity on repeated measurements; the weaker the band, the smaller the effect of inhomogeneity will be.
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