Determining intrinsic sensitivity and the role of multiple scattering in speckle metrology

Speckle patterns are a powerful tool for high-precision metrology because they enable remarkable performance in relatively simple setups. Nonetheless, researchers in this field follow rather distinct approaches owing to underappreciated general principles underlying speckle phenomena. For example, speckle can be produced from a simple scatterer or from more complex, multiple scattering geometries. In this Expert Recommendation, we propose a standardization of metrics to quantify intrinsic speckle sensitivity that enables direct comparison between all scattering geometries. Moreover, we provide a general criterion that allows one to predict where multiple scattering is truly advantageous for a given task. This standardization and criterion will catalyse progress in speckle metrology but will also translate to other domains of disordered optics which are undergoing rapid developments at present.