Tham khảo tài liệu 'new developments in biomedical engineering 2011 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 352 New Developments in Biomedical Engineering Fig. 5. Filtering principles of light propagating inside a biological tissue. Superficial and deep regions are marked as 1 and 2 respectively. Registration of the co- and cross-linear polarizer output channels allows the determination of the degree of polarization DOP which is defined as DOP Iii - IL III I 7 where I and p are the mean intensity of the co- and cross-polarized speckle patterns. Subtracting the cross-polarized pattern from the co-polarized pattern suppresses the volume scattering. Spectral filtering Demos et al. 2000 is based on the spectral dependence of skin attenuation coefficients Salomatina et al. 2006 . Shorter wavelengths are attenuated more heavily in a scattering medium and yield a higher output of scattered light than longer wavelengths. Therefore region 1 for the blue light is expected to be shallower than the red light and we should thus use the blue laser for skin roughness measurements Tchvialeva et al. 2008 . In another study Tchvialeva et al. 2009 we adopted the above filtering techniques for speckle roughness estimation of the skin. However our experiment showed that the filtered signals still contained sufficient volume-scattered signals and overestimated the skin roughness. Therefore we formulate a mathematical correction to further adjust the speckle contrasts to their surface reflection values. Speckle contrast correction The idea of speckle contrast correction for eliminating the remaining volume scattering was inspired by the experimental evidence arising from the co-polarized contrast vs. DOP as Skin Roughness Assessment 353 shown in Figure 6 Tchvialeva et al. 2009 . There is a strong correlation between the copolarized contrast and DOP r p . Fig. 6. The linear fit of the experimental points for co-polarized contrast vs. DOP. We assume at least as a first approximation that this linear relation is valid for the entire range of DOP from 0 to 1. We also know that .
