The first prism (11) has a base (11a) and two sides (11b,11c). The second prism (12) has a base (12a) and two sides (12b,c). Two surfaces of the two prisms are next to each other and have a polarising beam-splitting coating (14). The base of the second prism is next to the first side of the first prism. The third prism (13) has a base (13a) and two sides (13b,c) so that its base is next to the second side of the first prism. Beam-splitting coatings are positioned on adjacent surfaces (18,19) of the first and second prism or of the first and third prisms.
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