**6.1 Monochromator**

The basic components of a monochromator are shown in Figure 16. The disperser may be a grating or a prism. The limiting aperture serves to define the size of the radiation beam that passes through the monochromator, and limits the size and shape of the input beam that passes through the monochromator. The collimating optics transforms the diverging input beam from the input slit into a parallel beam of radiation incident upon the disperser. The focusing optics focuses the dispersed beam from the disperser onto the output plane at the position of the output slits. The optical and wavelength dispersion properties of a monochromator are best understood by noting that there are two kinds of optics in a monochromator. One kind forms images of the source (input slit) at the output plane (output slit), as shown by the dashed lines inside the monochromator of Figure 16. The optical magnification of the system is usually one, so that the image of the input slits produced at the output plane is the same size as the input slits. The second kind of optics disperses the radiation. For gratings and prisms, this dispersion is spatial with the beams of different wavelengths effectively propagating into different directions from the disperser. As a result, there are many images of the input slits produced at the output plane, one for each of the wavelengths present in the input radiation. Due to the finite width of the input slit and the image, and the continuous wavelength range of most radiation sources, the images of different adjacent wavelengths will partially overlap and there will be a range of wavelengths that are passed through the output slit. This effect gives rise to the bandwidth property of a monochromator.
