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In the last two decades, Digital holography (DH) has been established as one of the most effective coherent imaging technologies and it has been widely used in different applications, as for example in 3D imaging and display, optical metrology and microscopy. However, due to the coherent nature of light sources, DH have to managing the issue of speckle noise, which often introduce remarkable degradation in holographic reconstructions. Speckle noise can be fixed in two main ways, i.e. reducing the coherence of light source (e.g. using Light Emitting Diodes) or implementing high performance de-speckling methods. This paper focuses on the second way by considering the recent advances in holographic image denoising. Currently, a universal denoising strategy for completely suppressing holographic noise has not yet been established. Therefore, the removal of speckle noise from holographic images represents a bottleneck for the optics and photonics scientific community. Here, the best-performing noise reduction approaches, that have been proposed so far, are presented, by paying particular attention to how such processing can affect highly demanded DH applications. Among them, hybrid solutions based on engineered holographic recording and suitable numerical processing are discussed and compared each other.
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