Partial spatial coherence illumination in digital holographic microscopy: quantitative analysis of the resulting noise reduction

Imaging applications needing illumination with sufficiently high power density often request coherent light such as provided by laser or super-luminescent diodes. However the very high spatial coherence of those sources can generate coherent speckle noise and multiple reflection effects that may degrade the resulting image quality. In order to overcome such issues, we have shown that using partial spatial coherence illumination in interferometric digital holographic microscopy (DHM) greatly improves the image quality. Two models are here proposed to quantitatively assess the noise reduction as a function of both the spatial coherence, and the distance between the noise source and the recorded plane. We emphasize that these approaches may be useful in numerous imaging situations not restricted to DHM systems. The first developed model uses the discretization of the field of view in the plane of the noise source. This model is more intuitive but encounters some limitations. The second model, based on a continuous approach, corroborates the discrete model and extends it when necessary. Experimental validation of both models has been performed with a DHM, whose illumination has an adjustable spatial coherence. The noise was generated using a microscope slide with deagglomerated particles. The relative standard deviation of fluctuations due to noise is shown to be inversely proportional to the product D vertical bar d vertical bar when this quantity is high, where D is the diameter of a pupil leading the spatial coherence and d is the defocus distance of the noise source. The continuous model is applicable in any case.