The conditions for preparing evaporated Al films of highest reflectance for mirrors and gratings to be used in the vacuum ultraviolet are summarized, and an account is given of aging studies. The preparation of bloom-free, one-micron thick coatings for grating blanks is described, and the effect of thin overcoatings on the efficiency of ruled gratings is discussed. Data are presented on the reflectance and aging of other film materials, such as Pt, ZnS, and Al2O3, which are more suitable for use at wavelengths below 1200 A. A discussion is given of the preparation and the reflecting properties of various multilayer film combinations suitable for reducing the reflectance of mirrors and gratings in the visible and the near ultraviolet while at the same time preserving a high reflectance in the extreme ultraviolet. A 250-A thick layer of MgF2 evaporated onto an Al coating immediately after its deposition was found both to raise the reflectance to about 80% at wavelengths down to 1200 A, and to retard aging.
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