There are two types of curves that can be used to predict diffraction efficiency: an absolute efficiency curve and a relative efficiency curve. Absolute efficiency curves plot the amount of light that will be diffracted into a specified order (as a percentage of the incident monochromatic radiation on the grating) for any given wavelength. All curves in our catalog are for absolute efficiency in the first order using aluminum coated gratings. The curves also represent an average of the S and P-states of polarization. For gratings coated in gold, a 15-20% increase in absolute efficiency can be expected from 700nm to 1100nm.
The ratio between the absolute efficiency and the amount of light reflected by a plane mirror with the same coating as the grating is the relative efficiency. Thus relative efficiency curves consider the performance of the coating used on the grating. Relative efficiency does not account for the strengths and weaknesses of specific coating types.