GB861259A - Improvements in and relating to grating monochromators - Google Patents

Abstract

861,259. Monochromators. PARSONS & CO. Ltd., Sir H. G. July 27, 1959 [Aug. 5, 1958), No. 25008/58. Class 97(1) The dispersive element of a "blazed" grating monochromator comprises a plane grating having a plurality of areas, each area being ruled with equi-spaced parallel grooves, the groove spacings for the separate areas being equal or simple multiples of one another with all the grooves parallel, and a different groove angle being associated with each area. Fig. 2 shows the grooves of such an echelette grating in transverse section, the longer "blazed" surface of the sawtooth groove being so inclined with respect to the incident beam as to produce spectral reflection in the direction of the desired first order diffraction maximum. The intensity drops to either side of the "blaze" wavelength and to extend the spectral range over which high intensity is obtained, an adjacent region of the grating is "blazed" for a different wavelength. The grating may be rotatable about an axis through its mounting, or about an external axis O as in Figs. 4a and 4b, different portions of the grating intercepting the incident beam BD on rotation of the grating to give wavelength selection with fairly even intensity. A particular wavelength will be present in various orders if the groove spacings in the different areas are in simple ratio. Alternatively, all groove spacings may be equal, varying only in blaze angle in the different areas. If the groove spacings in different areas are not exactly equal or in exact simple ratio, adjacent areas may be tilted slightly relative to one another to give diffraction maxima in the same direction for a wavelength intermediate to the blaze wavelengths associated with the two areas. For other wavelengths, the rotation of the grating will cause the energy contribution from one area to predominate, and the discrepancy is reduced.