Liquid crystal displays (LCD) have found widespread application as spatial light modulators (SLM). Recently, we demonstrated that to obtain phase-only modulation or amplitude-only modulation regimes with present LCDs we have to insert the LCD between not only two linear polarizers but also between two birefringent wave plates. In this work we study the tolerance of these modulation regimes to variations in the orientation of the external polarization devices, but especially we focus on the tolerance to variations in the retardance of the wave plates. Specifically, we demonstrate that the amplitude-only regime is very sensitive to any deviation in the retardance. To obtain a complete and accurate characterization of the retardance value we propose to use a combination of three different polarimetric techniques easily available for any laboratory. We analyse their accuracy and determine the usefulness of the three techniques. © 2005 Taylor & Francis Ltd.