A Poincaré sphere geometric approach to the enpolarization/depolarization effects of dichroic devices/media is presented. The degree of polarization (DoP) of the emergent light is expressed as a function of two parameters: the DoP of the incident light and the dichroic strength of the device. The polar plots of this dependence give an intuitive grasp on how the alternation of the two effects depends on the competition between these parameters. The same approach can be transposed in an adapted terminology in various fields of physics, in all the problems whose underpinning mathematics is that of Lorentz transformation.