Single-domain nematic liquid crystal (LC) devices based on either the polarization-rotation or the birefringent effect or both have been investigated for reflective spatial light modulators (SLMs) which use a polarized beam splitter to separate the input light beam from its orthogonal output beam. We have evaluated each LC mode in terms of its contrast ratio, brightness, operating voltage, and its tolerance in cell-gap non-uniformity. We have studied the hybrid-aligned, the 0 degree, 45 degrees, and 63.6 degrees twist nematic LC modes which can be operated in either normally-white (NW) or normally-black (NB). We have also investigated the mixed twist nematic (MTN) and self- compensated twist nematic for NW and tilted homeotropic mode for NB. 2D simulations have also been carried out for one of the NB modes implemented inactive-matrix-driven reflective SLMs to elucidate the effect of fringe-field which tends to reduce mainly the brightness for the NB modes and contrast ratio for the NW modes. The locations of the disclination lines depend on the driving method and display pattern. To improve the brightness, we have also studied polarization- independent LC phase gratings using patterned alignment for reflective SLMs. The basic equations for the diffracted and non-diffracted intensities for both the reverse-twist and orthogonal-twist two-domain LC phase gratings have been derived. The device parameters, the operating voltage, and the optical diffraction efficiency are given for various cases with a twist angle equal to or less than 90 degrees.