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Before all, I also posted a question to maths forum, but I am convienced that this is the forum I was more like looking for.

Firstly, hello to this forum, secondly I want to apologize if this is not a place I would like to post a question like that, maybe in another thread ?; ) Anyways, I am a student and I am having problems with phase retrieval. I want to get a phase profile from a single measurement of a laser beam profile. I measured the intensity and I would like to extract a phase. The intensity is a 124x124 pixel grid from a sensor.

To do that I implemented a Gerchberg-Saxton algorithm which is an error reduction algorithm (I find it best to look at the paper from J.R. Fineup: Phase retrieval algorithms: a comparison) and well it gives me some results but the algorithm says that after some steps I should satisfy function's and fourier's constraints but I have no idea what that means. From the text I can't make a clear answer for that question so maybe anybody knows here ? It says that the values must be non-negative, but the values of modulus, imaginary part of the estimate of the phase ? I have no idea.

Maybe you have another ideas how to tackle problem of the phase retrieval?

Thank you very much : ) If something is not clear, ask and I will try to make my question clearer.