Abstract: The invention provides beam splitter for use in high power multi-wavelength laser holography. The beam splitter comprises a bi-refringent prism 12 through which passes the beam to be split the prism 12 splitting the beam into two separate beams having mutually orthogonal polarization planes. A half-wave plate 16,18 is arranged in the path of at least one of the separated beams to enable the planes of polarization of the separated beams to be realigned. Two further prisms 14, each arranged in the path of a respective one of the separated beams, serve to compensate for the dispersion of the wavelengths of the incident beam by the bi-refringent prism 12.