Spectrometric imaging of polarization colors and its application in forensic fiber analysis

A hyperspectral imaging instrument analyzing fibers between crossed polarizers spectrometrically is presented. The principle of operation is introduced and illustrated briefly by the theory of polarization and birefingence and calculations based on Stokes parameters and Müller matrices. Afterward, the developed instrumentation and results are detailed. Results obtained by the calculations and developed instrumentation indicate that the retardance of birefringent materials can be calculated over a high range and with a high accuracy. In addition, the spatial resolution of the instrument suffices to analyze different regions within a fiber individually. This allows the selection of a region at the center of an analyzed fiber that is shown to provide an optimal signal. The developed software enables analysis of the polarization color and the "normal", i.e., absorptive color of the sample individually. These features make possible a preliminary identification of fibers, without isolation of the fiber from a microscope slide. The instrument forms a powerful new approach to automated analyses in forensic fiber evidence, as it can establish analyses of morphology, color, and identity of a set of samples in a high-throughput, automated, and objective way.