Image formation in digital holography

In this study we investigate the imaging mechanism of digital holography. The imaging process is separated into three steps: hologram recording, phase retrieval, and object field reconstruction. For hologram recording, the average effect due to the sensor pixel aperture and the role of the physical reference beam are addressed particularly. The average effect of pixel aperture is equivalent to a low pass filter, which acts on the interference term between the object field and the reference wavefront. An optimal physical reference beam is then to minimize the bandwidth of the interference term so that more object information can pass through the filter. For the reconstruction of object field, emphasis is paid on the correspondence between the underlying physical process and the discrete system represented by the reconstruction algorithms. The implication of sampling theory on each reconstruction algorithm is discussed in detail. The sampling requirement imposes a limitation only on the maximum extension of object field. Our analysis indicates that the achievable spatial resolution by digital holography is determined by the recording numerical aperture and wavelength of light, the same as the conventional microscopy. The independent analysis of each part illumines the way to optimize the system performance.