Relative Efficiency

Also referred to as groove efficiency of diffraction gratings, it expresses the energy diffracted into the required order as compared to the efficiency of the coating itself at the same wavelength. For example, a grating that yields 40% diffraction into a specific order at 500nm compared to a mirror with the same coating that reflects 80% of light at 500nm would have a relative efficiency of 50%. Theoretically, the relative efficiency for a grating can appear very high, yet, the actual piece could yield very low diffraction into the desired order if the coating reflectivity is low at the wavelength region of interest.