  Phase retrieval and the twin-image problem in digital in-line holographic
microscopy can be resolved by iterative reconstruction routines. However,
recovering the phase properties of an object in a hologram needs an object
plane to be chosen correctly for reconstruction. In this work, we present a
novel multi-wavelength Gerchberg-Saxton algorithm to determine the object plane
using single-shot holograms recorded with multiple wavelengths in an in-line
holographic microscope. For micro-sized objects, we verify the object
positioning capabilities of the method for various shapes and derive the phase
information using synthetic and experimental data. Experimentally, we built a
compact digital in-line holographic microscopy setup around a standard optical
microscope with a regular RGB-CCD camera and acquire holograms of
micro-spheres, E. coli and red blood cells, that are illuminated using three
lasers operating at 491 nm, 532 nm and 633 nm, respectively. We demonstrate
that our method provides accurate object plane detection and phase retrieval
under noisy conditions, e.g., using low-contrast holograms without background
normalization. This method allows for automatic positioning and phase retrieval
suitable for holographic particle velocimetry, and object tracking in
biophysical or colloidal research.
