# Recent Progress on Aberration Compensation and Coherent Noise Suppression in Digital Holography
## 5 Coherent Noise Suppression
In addition, the incoherent DH has attracted much attention in recent years [91; 92; 93; 94; 95; 96; 97; 98; 99], allowing a higher signal-to-noise ratio as compared with the laser source. The incoherent DH includes two kinds of techniques, i.e., optical scanning holography (OSH) [91; 92; 93; 94] and Fresnel incoherent correlation holography (FINCH) [95; 96; 97; 98]. OSH is based on scanning imaging, thus it is relatively complicated and slow but has no hard limitation on the recording size and pixel pitch. FINCH is based on the single-path and self-interference interferometer, hence it is simple and suitable for dynamic holograms. However, there is a large bias buildup for complicated objects in FINCH due to the self-interference holographic technique. The second kind of method is based on temporal integration by multiplexing holograms. The multiple holograms with uncorrelated noise patterns are recorded by different optical approaches. These approaches utilize the averaging effect to reduce the coherent noise. There are many ways of implementation, such as the different illumination angles proposed by [PERSON] et al. [100] in 2007, the different wavelengths presented by [PERSON] et al. [33] in 2008, the different polarizations adopted by [PERSON] et al. [35] in 2010 and [PERSON] et al. [101] in 2011, and the moving diffusers presented by [PERSON] et al. [102] in 2006 and [PERSON] et al. [103] in 2010. Moreover, the implementation was also realized by laterally shifting an object, proposed by [PERSON] [104] in 2011. The method of shifting the camera was demonstrated by [PERSON] et al. [105] in 2006 and [PERSON] et al. [106] in 2013, respectively. In 2016, [PERSON] et al. [107] utilized the slight rotation of an object to perform the coherent noise suppression. However, this kind requires a complex optical arrangement, which limits the number of recorded holograms with different noise. In addition, real-time measurement fails as a result of the longer capture time. The third kind of method is based on digital processing with a single hologram. For instance, [PERSON] et al. [31] adopted median and mean filtering in 2005. [PERSON] et al. [108] employed wavelet transformation process in 2008. [PERSON] et al. [109] reduced the coherent noise by means of the transformation of the multiplicative noise into an additive one in 2010. Despite the fact that typical digital processing methods only record one hologram, the imaging resolution was adversely affected due to the loss of object information. In 2007, [PERSON] et al. [110] proposed the discrete Fourier filtering method, in which the speckle was suppressed by shifting a bandpass filter of spectrum. The method has advantages over conventional median and mean filtering methods, but it may lose the resolution and in-axis optical arrangement is its constraints. Furthermore, [PERSON] et al. [111] divided a recorded hologram into several sub-holograms in 2012. Each sub-hologram was reconstructed individually, and the speckle-reduced image can be obtained by averaging them. In 2016, [PERSON] et al. [112] reconstructed an image downsampled randomly by application of the compressive sensing algorithm. By repeating this procedure several times with different random downsamplings, the multiple reconstructed images with different speckle were averaged. It was demonstrated that the speckle noise was reduced drastically. Compared with the first two kinds of speckle reduction methods, the digital processing methods only utilize one hologram to realize the noise suppression, instead of manual operation or recording multiple holograms. However, their image resolution is affected due to loss of sample information. The methods based on shifting a binary mask have been studied to suppress the coherent noise. In 2007, [PERSON] et al. [110] applied the discrete analog of the conventional Fourier filtering to DH. The proposed discrete Fourier filtering was to filter the Fourier plane date in the discrete Fourier plane.