can be achieved on a target slm the different quantised phase retardation levels may need not have a regular distribution the number of quantisation levels may be set at two for example for an slm producing phase retardations of 0 or u03c0 at each pixel in some preferred embodiments the quantiser is configured to separately quantise real and imaginary components of the holographic sub frame data to generate a pair of holographic sub frames each with two or more phase retardation levels for the output buffer fig6 shows an example of such a system it can be shown that for discretely pixellated fields the real and imaginary components of the complex holographic sub frame data are uncorrelated which is why it is valid to treat the real and imaginary components independently and produce two uncorrelated holographic sub frames in the ospr approach we have described above subframe holograms are generated independently and thus exhibit independent noise in control terms this is an open loop system however one might expect that better results could be obtained if instead the generation process for each subframe took into account the noise generated by the previous subframes in order to cancel it out effectively u201c feeding back.