Programmable Point-source Digital In-line Holography Using Digital Micro-mirror Devices

Abstract

Digital holographic imaging techniques allow fast retrieval of three-dimensional (3-D) amplitude and phase information of an object volume through numerical reconstruction of a two-dimensional (2-D) hologram. Digital holography consists of digital sampling of a hologram on an array of charged-coupled device detectors (CCD), and digital reconstruction of the object field through a numerical algorithm [Sch94, SJ02, Yar03]. The recording process encodes 3-D information of an object into the form of interference fringes on a twodimensional recording screen. These fringes usually contain high spatial frequencies that represent the mixing between the scattered object field and coherent reference wave. A reconstruction process is performed on the recorded hologram to recover the object wave. This numerical acquisition method eliminates the need for any chemical processing of the hologram and mechanical refocusing of the reconstructed image that is commonly required in the traditional holographic imaging system [CMD99, DJL99, SPISSSW97]. This process has opened new frontiers in digital holography with emerging applications in research and industry [Kre05, SJ05]. Typically, common digital holography recording set-ups include off-axis and in-line configurations [SJ94]. Digital in-line holography (DIH) represents the simplest realization of the digital holography (DH), allowing for rapid acquisition of hologram images without the use of lenses. Recently, DIH with a spherical reference field has emerged as an attractive tool in 3D imaging of biological objects and micro-spheres without the use of lenses [GXJKJK06a, RGMNS06, XJMK02a, XJMK02b]. Basically, the configuration consists of a coherent light source – a pinhole – to generate a spherical reference field. An array of charge-coupled device detectors (CCD) provides digital sampling of a hologram. However, the characteristics and parameters of these components can affect the overall performance of the system. Among factors that limit the performance of a digital in-line holographic microscope (DIHM) are the size and spatial location of the pinhole used. These affect the resolution, obtainable field of view (FOV) and object illumination angle (which determines the projection view in the reconstructed image volume.) In [GPO08], the effect of the pinhole size on the spatial coherence of the reference beam in a DIHM system was studied. The results showed that a reduction in the coherence of the light, due to increase in the size of the pinhole used, leads to broadening of the impulse response of the system. Consequently, this limits the obtainable resolution in the reconstructed image. Other resolution-limiting