Optical prism comprising two external parallel and reflecting surfaces, one internal interface, parallel to said surfaces and acting as light splitter; said prism furthermore comprising an entrance side forming an angle β with respect to said interface and at least two exit sides forming an angle γ with respect to said interface, said external surfaces being located at different distances from said interface, the lengths and the thicknesses of the prism are chosen such that at least one beam entering through said entrance side may cross said interface at least twice and exits through said exit sides.