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 $${\displaystyle \eta ={\frac {P}{P_{0}}}\ .}$$

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]

• The absolute efficiency is defined as above and relates the power diffracted into a particular order to the incident power.
• The relative efficiency relates the power diffracted into a particular order to the power that would be reflected by a mirror of the same coating as the grating, therefore attributing to inevitable reflection losses at the grating but not caused by inefficient diffraction itself.

• Christopher Palmer (2020). "Efficiency Characteristics of Diffraction Gratings". Diffraction Grating Handbook (8th edition). MKS Newport. Retrieved 13 March 2020.