[en] The design of the grating instruments for use with a large light source, such as an electron storage ring, tokamak and others, is discussed. An analytical expression of spot diagrams is a powerful tool in the design of spectroscopic instruments, but it loses power when the number of optical elements increases. This difficulty can be overcome by tracing ray through the whole optical trains. Ray tracing formulas are introduced to make the analytical representation of spot diagrams. The design of a monochromator is to determine the optimum mounting parameters for a given type of grating and a given scanning mode or to determine the optimum values of both the mounting and the ruling parameters under given conditions. A merit function to determine the parameters was defined and analyzed. The parameters can be determined by minimizing the merit function. For the design of a simple system consisting of two concave mirrors or a concave mirror and a concave grating, the analytical expression for the conditions of focussing and coma elimination is useful. In the region of wavelength smaller than 50 nm, any grating presently available has a diminishingly small reflectance at normal incidence, while such gratings lose much of their high resolution capability when used at grazing incidence to gain high reflectance. To overcome this difficulty, it will be necessary to devise new methods which will simultaneously accomplish both high resolution and reasonably high reflectance. (Kato, T.)