Whereas flat crystals are used in sequence spectrometers， multichannel spectrometers principally employ

curved crystals.

The curvature of the crystals is selected in such a way that by applying slit optics the X-ray entrance slit is

focused by the curved crystal onto the exit slit. This allows higher intensities in a space-saving geometric

arrangement.

Different types of crystal curvature are used for focusing. The most commonly used are the curvatures

that follow a logarithmic spiral (Fig. 20a) and the Johansson curvature (including grinding) (Fig. 20b).