This paper presents analytical modelling for describing the speckle noise decorrelation in phase data from two- or multiple-wavelength digital holography. A novel expression for the modulus of the coherence factor is proposed for the case of two-wavelength speckle decorrelation from imaging roughness and surface shape through an optical system. The expression permits us to estimate the speckle decorrelation phase noise in surface shape measurements. The theoretical analysis is supported by realistic simulations including both the surface roughness and shape. The results demonstrate the very good agreement between the modulus of the coherence factor estimated with the simulation and the one calculated with theory.
Over the past two decades, laser beam melting has emerged as the leading metal additive manufacturing process to produce small and medium-sized structures. Due to the complex physical phenomena involved in the lasermaterial interaction, instabilities in the melt pool morphology affect the final quality of the structure and remain difficult to predict by simulation. Several monitoring approaches, based on the radiation of the melt process or on a secondary illumination source, have been developed to measure its length, width and height. Nevertheless, the final morphology of the part is influenced by the volumetric forces as well as by the capillary forces applied to the melt. Thus, the shape of the melt surface is of primary interest to control the stability of the melt. Due to its intrinsic heterogeneity, its motion on the powder bed and its own dynamics, the measurement of the melt pool surface shape requires a full-field "one-shot" acquisition with a short exposure time of a few microseconds. In this paper, we propose multi-wavelength digital holography for the in-situ investigation of the melt pool.
This paper presents a model for decorrelation noise due to surface shape in two-wavelength digital holography. Theory is supported by numerical simulations giving very good agreement.
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