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 of grating.

If the diffracted power is designated with  and the incident power with  the efficiency  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:
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.

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Diffraction