Bickel et al. investigated the polarization effect on the scattering of biological scatterers in 1976, but only since 1991 has polarized light started to attract a great deal of attention in the field of biomedical optical imaging. It has been demonstrated that polarization methods can distinguish less-scattered photons from diffusive ones, improve image quality, obtain better surface features, and provide information on the properties of tissues.
In a polarization imaging system, the light from the light source is first conditioned by the polarization elements, such as the polarizer and the retarder, then it illuminates the object under investigation. The polarized light interacts with the object through reflection, scattering, and absorption. Part of the light, either the backscattered light with same wavelength or the fluorescence light with a different wavelength, is collected by the collection optics and delivered to the sensor. One or more polarization elements are typically placed in the detection path to select the light with the desired polarization state.
The fundamentals of polarized light and the interactions of polarized light with tissues will be discussed in Secs. 6.1 and 6.2. Sections 6.3 and 6.4 will summarize the polarization imaging systems for biomedical optical imaging and discuss polarization elements, and Sec. 6.5 will discuss optical design for polarization imaging systems.
6.1 Basics of Polarized Light
Light can be described as an electromagnetic wave. The electric field E of a monochromatic electromagnetic wave can be presented as the vector sum of two electrical fields in the x and y planes, which are perpendicular to each other (as shown in Fig. 6.1):