Wavefront Flatness or Curvature . Plane gratings should ideally diffract plane wavefronts when illuminated by collimated incident light. Concave gratings should ideally diffract spherical wavefronts that converge toward wavelength-specific foci. It is important to specify that grating wavefront testing be done in the diffraction order of use if possible, not in zero order, since the latter technique does not measure the effect of the groove pattern on the diffracted wavefronts. Groove Spacing or Frequency .
Alignment refers to the angle between the groove direction and an edge of the grating substrate.
Diffraction Efficiency . Occasionally efficiency specifications at more than one wavelength are called out. Either relative or absolute diffraction efficiency should be specified. Absolute efficiency is specified as the percentage of the power incident on the grating that is diffracted into a particular order by the grating. In some cases, the bandwidth of the exit slit in the spectrometer used to measure the grating efficiency may need to be called out as well.