The arrangement of Fig. 4 advantageously allows flexibility in the positioning of SLMs 15 and can facilitate smaller angles α between different beams 12. The actual size of a SLM is often greater than the active area of the SLM. Fig. 4 shows that the apparent positions 15A’ and 15B’ can be located so that the active areas appear to be immediately next to one another even though the packaging 37 of SLMs 15A and 15B extends outside the active area 38.