The user can also set the grating efficiency for any given order and at any given wavelength.  During the raytrace, the grating efficiency rays with wavelengths inside the user set range are determined by linear interpolation.  The grating efficiencies for rays with wavelengths outside the range of the user set wavelengths are set to the efficiency of the closest wavelength with a defined efficiency.  So, if only one wavelength is defined with a diffraction efficiency, then all the wavelengths will have that same efficiency.