Expressible-depth control method in digital holographic display

Hologram is a recording in a two or three-dimensional medium. It is a form of interference pattern between welldefined coherence reference beam and the light with same wavelength arriving from an object. When the hologram is illuminated by the reference beam, the hologram reconstructs desirable wave-fronts by modulation of reference beam. The coherence of reference beam is important for recording the hologram since it determines if the diffraction pattern is made or not. Incoherent light source does not form an interference pattern. For this reason, most of the holograms are reconstructed with coherent light like a laser. However, coherent light also derives speckle noise that makes the reconstruction image unclear. So, it is meaningful to find the suitable amount of partial coherence for hologram reconstruction and receives lots of interests for a long time. But, up to my knowledge, there is no experiment that adjusts spatial coherence quantitatively. In this paper, we invent an optical method to control the amount of spatial coherence of the light source by using a digital micro mirror device (DMD). Here, the DMD takes a role of an adjustable spatial filter because the number of on-state pixels of DMD changes the amount of spatial coherence. As a result, we verify the relation between the spatial coherence and the expressible depth of reconstruction image and find the optimal amount of coherence of LED for our holographic reconstruction experiment.