Focus detection in digital holography by cross-sectional images of propagating waves

Official URL: http://dx.doi.org/10.1117/12.2037255

In digital holography, computing a focused image of an object requires a prior knowledge of the distance of the object from the camera. When this distance is not known, it is necessary to repeat the image reconstruction at a range of distances followed by evaluation of each image with a sharpness metric to determine the in-focus distance of the object. Here, we present a method to find the focus distance by processing the image transverse to the object plane instead of the processing in the image plane as it is usually done. Since the reconstructed hologram image is spatially symmetric around the focus point along the propagation axis, simply finding the symmetry points in the image cross-section specifies the focus location, and no other sharpness metrics are necessary to use. Also with this method, it is possible to find the focus distances of multiple objects simultaneously, including the phase only objects without any staining. We will present the simulations and the experimental results obtained by a digital holographic microscope.