Abstract: A method, and apparatus for performing the method, of manufacturing a blazed diffraction grating by making a photographic record of the interference fringes produced at the intersection of two coherent secondary light beams derived from a common coherent primary light beam, in which one of the two secondary light beams is compressed or expanded in at least one dimension before it is incident on the region where intersection with the other beam takes place, the angle of intersection between the two secondary beams being such that they are perfectly in register across a certain plane in the region of intersection, the interference fringes formed at the plane being recorded photographically by exposing a photo-sensitive material, preferably an etchable photo-resist, at this plane. Expansion can be achieved by passing one of the recording beams through a prism or by effecting the separation of the primary beam into two secondary beams by means of a blazed diffraction grating.