# Applications of Mueller Matrix Polarimetry to Biological and Agricultural Diagnostics: A Review
## 2 Principles of Mueller Matrix Analysis
### Mueller Matrix Fluorimetry  
Fluorimetry is a highly sensitive method based on measuring the intensity of fluorescence, that is, the ability of a substance to emit light when absorbing external electromagnetic radiation, mostly at a wavelength longer than that of the absorbed radiation. This method has a high sensitivity. However, this leads to the main disadvantage of the method; because of the high sensitivity, fluorimetry detects not only the substance of interest but also all possible impurities that cause background radiation. Consequently, cleaning requirements increase, and sample preparation becomes more difficult. It is also worth noting that not all substances are fluorophores; thus, fluorimetry has certain limits of applicability.  
The sources of fluorescence excitation in substances can be different, but, as a rule, ultraviolet radiation sources (UV, 10-400 nm) are used. According to the method of isolating fluorescent radiation from the incident radiation, two main groups of fluorometric devices are distinguished: filter fluorometers using optical filters and spectrofluorometers using monochromators. The detectors used in fluorimeters can be either single-channel (the intensity of only one wavelength is measured) or multichannel (the intensity of all wavelengths is measured simultaneously); in schemes with multichannel detectors, filters and monochromators can be excluded.  
The principles underlying the use of fluorescence polarization to study protein-protein and protein-DNA interactions are outlined in [91]. The polarization properties of fluorescence contain potentially valuable information on the chemical composition, molecular structure and orientation and local environment of fluorescent elements. A method that combines both fluorimetry and polarimetry can be appropriately designated as fluoripalorimetry. The method is a very powerful diagnostic tool that is quite simple and inexpensive.  
It is worth noting that hybrid supramolecular materials have been studied using a combined method that combines spectral fluorometry and MMP [92]. Spectroscopic fluorescence MMP, together with inverse analysis, made it possible to determine the organization and orientation of achiral fluorescent dye molecules around a supramolecular nanotube and, thus, confirmed its promise for the characterization of complex hybrid materials.  
A typical layout of a polarimetric fluorescence spectrometer involving MM measurement is shown in Figure 5.  
Note that, when using a wavelength tunable source to excite fluorescence, all polarimetric elements making up the PSG and PSA must be achromatic. Such an instrument can be termed an MM spectrofluorimeter.  
MM fluorescence spectroscopy has been used to identify precancerous tissue areas [50; 51].  
Figure 5: Typical layout of MM fluorescence spectrometer.  
From the above, we can conclude that, within its applicability limits, which are quite wide, fluoripolarimetry is a no less powerful method than spectropolarimetry, since it is based on a similar principle, that is, the measurement of the spectral dependences of the MM elements through the use of polarimetric elements.