Speckle interferometry is an optical metrology technique for characterizing rough surfaces. In one application, the
deformation of a specimen under a load may be determined by comparing measurements before and after the load is
applied. Owing to the surface roughness, however, the results are impaired by phase singularities, leading to a strong
noise in the measurement results. Usually, filtering and smoothing operations are performed to reduce the noise.
However, these procedures also affect the underlying systematic phase and are therefore disadvantageous. Instead, we
examine incoherent averaging, a physical procedure, to reduce the number of phase singularities in the first place. We
tailor the spatial coherence of the light using extended light sources of continuous or multipoint shape, achieving
smoother phase distributions. The mechanism behind the reduction process involves subtle effects like enhancing phase
singularity correlations in the fields before and after the deformation takes place.