Diffraction efficiency

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.
If the diffracted power is designated with formula_1 and the incident power with formula_2 the efficiency formula_3 reads
Grating efficiency.
In the most common case – the diffraction efficiency of optical gratings (therefore also called "grating efficiency") – there are two possibilities to specify efficiency: