Image Quality Improvement of Digital Holography by Multiple Wavelengths or Multiple Holograms

Digital holography (Yamaguchi & Zhang, 1997; Osten et al., 2002; Kreis, 2005; Ferraro et al., 2005; Frauel et al., 2006; Nomura & Imbe, 2010) is a useful technique for recording the fully complex field of a wavefront. In line with advances in imaging devices such as CCDs, digital holography is accessible. The digital holography has been used for lots of applications including encryption (Javidi & Nomura, 2000; Tajauerce & Javidi, 2000; Nomura et al., 2004) and three-dimensional object recognition (Poon & Kim, 1999; Tajauerce et al., 2001; Javidi & Kim, 2005). However, the quality of a reconstructed image of a digital hologram is not good, because it is suffering from speckles. Comparing a conventional film hologram, the extent of a digital hologram is small and the resolution of the hologram is low in single figure. This is why the reconstructed image has noticeable speckles. To reduce the speckles, some signal processing approaches have been proposed (Javidi et al., 2005; Do et al., 2005; Maycock et al., 2005; Kreis & Schluter, 2007). By use of wavelet decomposition, three-dimensional image fusion has been proposed and some speckles reduction has been achieved. Independent component analysis has been also applied to reduce speckles. Aperture synthesis approach is also proposed to enhance the resolution. Somewhat speckle noises have been reduced by the approaches. In this paper, two methods to reduce speckle of the reconstructed image are presented. One is the method based on the superposition of the reconstructed images with different wavelengths. Another is the method based on the multiple holograms. In Section 2, the characteristics of speckles are reviewed briefly. In Section 3, the former method is described. Some experimental results are shown to confirm the proposed method. In Section 4, the latter method is described with some experimental results for confirmation.