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Dielectric multilayer reflectors that are non-polarizing are an important class of optical device and have numerous applications within optical fibres, dielectric waveguides and light-emitting diodes. Here, we report analyses of a biological non-polarizing optical mechanism found in the broadband guanine-cytoplasm ‘silver’ multilayer reflectors of three species of fish. Present in the fish stratum argenteum are two populations of birefringent guanine crystal, with their optical axes either parallel to the long axis of the crystal or perpendicular to the plane of the crystal, respectively. This arrangement neutralizes the polarization of reflection as a result of the different interfacial Brewster's angles of each population. The fish reflective mechanism is distinct from existing non-polarizing mirror designs^,,, in that, importantly, there is no refractive index contrast between the low-index layers in the reflector and the external …