Chapter 39 Polarized Light for Detection of Pathological Changes Within Biological Tissues Ani Stoilova, Dimana Nazarova, Blaga Blagoeva, Velichka Strijkova, and Plamen Petkov Abstract Here we present two techniques using polarized light for detection of abnormal morphological changes in tissue samples. A Zeta-20 polarized optical microscope has been used to visualize human lung histologic specimens with pathological changes caused by anthracosis, tuberculosis, inﬂuenzal pneumonia, pulmonary infarction and malignant tumour formation as well as tissue sample of normal lung. The images have been taken and reconstructed in accordance to the method reported by Jacques and Lee (Lasers in surgery: advanced characterization, therapeutics, and systems VIII: polarized video imaging of skin. In: SPIE. BiOS ’98 international biomedical optics symposium, vol 3245, San Jose, CA, USA, 1998. https://doi.org/10.1117/12.312307). Tissue polarimetry has been also used to study the above-mentioned biological samples. The experimental set-up working in reﬂec- tance geometry consisted of a DPSS LASER with wavelength of 635 nm, which passes through linear polarizer, quarter wave-plate and focusing lens before reaching the biological sample at illumination angle of 45  . Basic polarimetric parameters such as azimuth, angle of ellipticity, degree of polarization and illuminating power were measured and used to evaluate the change in the polarisation state of the incident light after interaction with the biological samples. Keywords Polarized optical microscopy · Tissue polarimetry A. Stoilova (*) Department of Physics, University of Chemical Technology and Metallurgy, Soﬁa, Bulgaria Institute of Optical Materials and Technologies, Bulgarian Academy of Science, Soﬁa, Bulgaria D. Nazarova · B. Blagoeva · V. Strijkova Institute of Optical Materials and Technologies, Bulgarian Academy of Science, Soﬁa, Bulgaria P. Petkov Department of Physics, University of Chemical Technology and Metallurgy, Soﬁa, Bulgaria © Springer Nature B.V. 2020 P. Petkov et al. (eds.), Nanoscience and Nanotechnology in Security and Protection against CBRN Threats, NATO Science for Peace and Security Series B: Physics and Biophysics, https://doi.org/10.1007/978-94-024-2018-0_39 477