An apparatus comprising a first cell (10b), said first cell comprising a plurality of first elements (34b), said first elements being controllable between a non-reflective state, in which electromagnetic radiation having a first polarization is reflected to a first extent, and a reflective state, in which said electromagnetic radiation having a first polarization is reflected to a second extent, said second extent being greater than said first extent. Said apparatus further comprises a second cell (10a), superimposed on the first cell, said second cell comprising a plurality of second elements (34a), said second elements being controllable between a non-reflective state, in which electromagnetic radiation having a second polarization is reflected to a third extent, and a reflective state, in which said electromagnetic radiation having a second polarization is reflected to a fourth extent, said fourth extent being greater than said third extent. Said first and second elements are arranged so that said first polarization is different from said second polarization.