Which incident polarization do I need for phase-only modulation?

Our SLMs are based on liquid crystals, which are anisotropic materials. Therefore, for phase-only modulation you need linear polarization parallel to the director vector of the liquid crystal molecules.

For most of HOLOEYE LCOS SLMs the required incident polarization is parallel to the long axis of the display. For few SLM versions it is parallel to the short axis. Please refer to the SLM manual at the section of the SLM version you use.