Abstract: First and second triangular prisms are bonded together at their bases with a semi-transparent reflecting material therebetween. A light beam from a laser is incident on the upstream face of the first prism and refracted thereby so as to be incident on the semi-transparent reflecting material at a predetermined angle to the longitudinal axis thereof. One component of the beam is reflected from the semi-transparent material and refracted by the first prism so as to emerge therefrom parallel to the axis of the semi-transparent material. Another component of the beam passes through the semi-transparent material and is refracted by the second prism so as to emerge therefrom parallel to the axis of the semi-transparent material. A lens converges the two beam components to produce an interference pattern.