In optics, diffraction efficiency is the performance of diffractive optical elements – especially diffraction gratings – in terms of power throughput.
It's a measure of how much optical power is diffracted into a designated direction compared to the power incident onto the diffractive element of grating.
If the diffracted power is designated with P and the incident power with P0, the efficiency η reads
η =
In the most common case – the diffraction efficiency of optical gratings (therefore also called grating efficiency) – there are two possibilities to specify efficiency:[1][2]