Strehl ratio formula

[elhombre45625]

Hello FredVanGoor,
I would like to know the origin of your Strehl ratio formula.
In the definition I found on the Strehl ratio, it looks like:

Where function I[] stands for the maximum of fourier transform (= intensity at focus) and E(x, y), the spatial electric field in near-field.
I have tested both on a dataset, and the values are (very) different.
Can you clarify the conditions where the Strehl ratio formula you give can be applied?
Thanks!