Phase retrieval, i.e. the reconstruction of the complex field from intensity images by iterative search under constraints, is an important tool in Optics, in particular in lensless optical imaging where diffracted intensity patterns are recorded. But the usefulness of phase retrieval goes well beyond imaging applications, reaching into the field of optical manipulation. It will be shown that phase retrieval can become an enabling technique for a number of challenging problems in optical micromanipulation: In a multi-spot trap for several particles, for example, it allows one to measure the local forces acting individually on each trapped particle. Moreover, direct quantitative measurements of the optical torque acting on a particle of arbitrary shape are possible, inferred from intensity images and phase retrieval under suitable constraints.
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