Suppression Of Noise In Digital Holography

The principle of digital holography is similar to the holography. It makes holography a great development as combining digital image processing and optical holography. With the improvement of digital holographic imaging device and computer’s performance, digital holography has been successfully applied in many fields. The reconstruct of digital hologram includes zero-order image, original image and conjugate image because of interference in digital holographic recording. The zero-order image that occupies most of the energy in the reconstruct plane seriously affects the quality of the reconstructed image. At the same time,the laser after scattering by rough object surface is randomly superimposed in the space. The superposition results in speckle noise that affects the quality of the reconstructed image. The speckle noise is a multiplicative noise that has a great impact on details of image.In this paper, a problem on suppression of noise in the digital holography is researched.There is zero-order image and speckle noise in the reconstructed image of reflective off-axis digital holography. The traditional method on suppression of zero-order image includes digital holography subtracting the average intensity, spatial local mean, Laplace method, frequency domain method and wavelet transform method. This paper proposes a method based on Gaussian Kernel function to suppress the zero-order image. The proposed method takes 0.61 second to suppress the zero-order image in the computer that CPU processing frequency is 2.8GHz and RAM is 2GB. The experimental results show that comparing with digital holography subtracting the average intensity, Laplace method, frequency domain method and wavelet transform method, the proposed method has a better effect in the suppression of zero-order image and less 0.4 second than spatial local mean. The principle of mean and median filtering, statistical filters and wavelet threshold filtering is introduced for speckle noise in this paper and their advantages and disadvantages are analyzed. This paper uses a non-local mean filter to suppress speckle noise. As the non-local mean filtering can not keep details, an improvement is proposed by an adaptive filtering. And Speckle suppression index(SSI) and edge preservation index (EPI) are introduced to evaluate the image after noise reduction. The experimental results show that the speckle suppression index of improved non-local mean filtering result is 0.7863, and edge preservation index was 0.1671. The method based on Gaussian Kernel function to suppress the zero-order image has a better effect than the traditional method. Compared with traditional methods, the improved non-local mean filtering can not only effectively suppress the speckle noise but also can preserve the details of image. Finally, digital holography is applied to measure marine microorganisms. The in-line recording is used for record hologram and convolution is used for reconstructing the image. In this paper, the recorded marine holograms are post-processed.The holography can achieve accurate shape in the microscopic application about marine microorganisms.