A description is given of a simple method for converting a conventional monochromator to a double or multiple monochromator having much higher resolving power and giving almost complete elimination of scattered radiation. To construct a double monochromator, radiation which normally passes through the exit slit is reflected back through the dispersing system by a pair of mirrors at right angles to each other, and finally passes through the exit slit. A chopper is arranged to interrupt the radiation reflected from these mirrors, and the signal from the detector is amplified by an amplifier tuned to the same frequency as the chopper. Thus only that radiation which has passed twice through the dispersing system produces an output signal. The factors governing the design of multiple monochromators are discussed.
A. WALSH, "Multiple Monochromators. I. Design of Multiple Monochromators," J. Opt. Soc. Am. 42, 94-94 (1952)