ABSTRACT
Echelle gratings, or simply echelles, are defined as coarse, but precisely ruled gratings used only at high angles of diffraction and in high spectral orders. Typical groove frequencies are 316 gr/mm or less, with 20 gr/mm a rough lower limit, and angles of use that vary from 63° to 78°, but occasionally go as low as 40°. While normally used in reflection there are special applications where they can be used in transmission. Rarely are spectral orders used below 10, but the upper limit may reach 600, although 100 is more common.
