  Phase and amplitude spatial light modulators (SLMs) capable of both binary
and multi-level modulation are widely available and offer a wide range of
technologies to choose from for holographic applications. While the replay
fields generated with multi-level phase-only SLMs are of a significantly higher
quality than those generated by equivalent binary phase-only SLMs, evidence is
presented in this letter that this improvement is not as marked for amplitude
SLMs, where multi-level devices offer only a small benefit over their binary
counterparts. Heuristic and numerical justifications for this are discussed and
conclusions drawn.
