Abstract: An optical grating assembly comprises a piezo-electric substrate which supports on two opposite faces thereof respective metallic layers. One of these faces of the substrate also carries a grating either formed in that face, in the metal layer supported by that face, or in a resin layer carried by that face. Application of a variable voltage between the metal layers sets up an electric field of variable strength in the substrate and this results in the pitch of the grating being variable due to the piezo-electric nature of the substrate.

Abstract: Monochromator comprising a concave grating, the lines of which are situated at the intersection of the concave surface with a family of surfaces geometrically like the equiphasic surfaces obtained as loci of the luminous intensity maxima produced by the interference of two beams from two point sources C and D, an entry slit A and an exit slit B, wherein the polar coordinates of the points C and D and of the entry slit A and exit slit B in a system of polar coordinates having as its origin the apex of the grating and as reference axis the axis of the grating in a plane of reference passing through this axis are connected by the following relationships: ##EQU1## in which (.alpha., l.sub.A), (.beta., l .sub.B), (.beta., l.sub.C) and (.delta., l.sub.D) are the polar coordinates of A,B,C and D, respectively, R is the radius of curvature of the grating, .omega.=(.alpha.+.beta.) /2 and .phi.=(.alpha.-.beta.)/2.