A colour switch for selectively switching between colour bands for use in projection optics and in direct view optics has fewer and thinner layers than in the prior art, but exhibits equal or better colour purity and light efficiency. The filter has a first filter section (2-8) for selectively blocking green light and including a cholesteric filter (2), a quarter wave plate (4) and a liquid crystal switch (6). The filter has a second filter section (10-22) for selectively blocking blue and/or red light including a stack of retarder elements (10, 14, 16, 20) and liquid crystal switches (12, 18), preferably DHF switches. The parameters of the filter are determined by an optimisation process including minimising a cost function G with respect to the parameters of the second filter section. Polarising element (8) may be positioned in front of the stack, and of a type which does not absorb radiation. |