In the analysis of the body component with spectroscopic techniques, transmission method and reflection method are two common approaches that used for the signal acquisition. Based on the position relationship between the optical fiber probe and the biological tissue, the reflection method can be further divided into two forms: contact measurement and non-contact measurement. The traditional contact measurement can easily realize the extraction of useful signals from deep tissue by adjusting the position between the light source and receiving fiber. However, study shows that the contact pressure and the heat transfer between the probe and tissue are known to cause the change of measurement conditions, which will further influence the stability of spectral data. By comparison, non-contact method is free from the influence of changes of pressure and temperature. Nonetheless, large amounts of surface reflectance that do not carry any useful information will be gathered into the fiber. The surface reflectance is so strong and concentrated that it would easily influence the dynamic range of the detector. Therefore, how to eliminate the impact of surface reflectance is the biggest problem the non-contact measurement faces. In view of this, a systemic study is conducted in this article and the cross- polarization method proposed by our group is investigated through both theoretical deduction and experimental analysis. First, the polarization state of surface reflection with different objects as experimental materials is studied. Results indicate that the polarization state of the surface reflectance is closely related to the surface roughness, the smaller the roughness is, the higher the polarization state will be. As a result, it can be concluded that the surface reflectance of Intralipid solution possesses with the polarization-maintaining property. Then, the extinction effect of the cross-polarization method was verified in Intralipid-20% solutions. Results showed that 97 percent of the surface reflections were eliminated effectively through this method. Finally, a comparative analysis on spectral data respectively obtained by contact and non-contact measurement was carried out. Result showed that there was a high coincidence between the two curves, which further illustrated the cross-polarization method can effectively eliminate the surface reflectance. Overall, this research explored the ability of cross polarization method to eliminate the surface reflectance, aiming at abolishing the effect caused by the variations of measurement conditions and further promoting the development of non-invasive sensing with NIR spectroscopy technique.
