| Diffraction phase microscopy is a kind of high-precision phase imaging technology,which integrates the imaging characteristics of label-free,non-contact,full-field quantitative measurement of quantitative phase imaging technology,and can accurately obtain the phase information of the measured microscopic samples.Meanwhile,the DPM utilizes diffraction grating,4f lens system,filter and camera to form a compact common path Mach-Zehnder interferometer.Compared with the traditional non-common path Mach-Zehnder interferometer,the object light in the optical path passes through the same optical elements as the reference light,which improves the stability of the system and obviously reduces the time noise.Moreover,the optical path system satisfied the recording condition of off-axis holography,so the reconstruction of object phase image can be realized only by capturing single frame hologram.That is conducive to the measurement of the dynamic characteristics of the sample.Diffraction phase imaging was originally used lasers as lighting sources.Because of the high coherence of laser,it leads to the generation of laser speckle and reduces the quality of the image.when the white light illumination diffraction phase imaging system is used,the coherence length is very short,which can avoid the coherent noise caused by the coherence of the light source,and has higher spatial and temporal sensitivity than the laser diffraction phase imaging system.Therefore,white light diffraction phase imaging technology is widely used in cell imaging,nondestructive testing and material characterization.Although white light diffraction phase imaging technology has high temporal and spatial sensitivity,the white light diffraction phase imaging system uses an extended white light source to illuminate the light wave.The illuminated light wave can’t be a complete planar light wave.its spatial coherence area is generally smaller than the measured field of view,which makes the halo effect dependent on the structure of the object appear in the phase image and hinders the accurate measurement of the sample phase information.At present,there are halo effect effects in phase imaging techniques using low coherent light source illumination,such as Zernike phase contrast imaging,SLIM phase imaging,white light illumination light intensity transmission equation phase imaging.Therefore,eliminating the halo effect is an urgent problem for imaging systems using low coherence light source illumination.In this paper,the elimination method of halo effect in white light diffraction phase imaging is studied:The paper first discusses the basic principles of white light diffraction phase imaging technology,including the optical path structure of the imaging system,the recording process of holograms,the characteristics of spectrum distribution,the digital reproduction method and the unwrapping algorithm of phase expansion.The basic theoretical knowledge of these imaging provides support for the subsequent research work.Then,on the basis of imaging theory,the cause of halo effect in diffraction phase imaging is analyzed.In order to eliminate the halo effect,the halo effect can be eliminated by optimizing the experimental device or numerical processing.In this paper,two numerical processing methods are proposed,namely,regular iterative algorithm and Hilbert transform method.The regular iterative algorithm is to obtain the accurate phase information of the sample through multiple iterations by establishing the mathematical model of the white light diffraction phase imaging system and using the prior condition regularization constraints of the sample to obtain the halo-free phase image.The Hilbert transform method does not require any prior knowledge of the sample.Simply the derivative of the measured phase image is used as Hilbert transform,and the correct high frequency data and low frequency data in the transformed image and the original measured image are accurately mixed to effectively eliminate the halo effect and retain the high resolution of the phase imaging of the white light system.Compared with the iterative algorithm,it reduces the workload and improves the computing speed.Finally,based on the above research work,a white light diffraction phase imaging device is built,and the spatiotemporal noise of the device is tested,which proves that the device has very high imaging spatiotemporal sensitivity.Based on this device,a large number of polystyrene microspheres and human hemoglobin cell samples were measured.In this paper,the phase image is processed by the iterative algorithm and the Hilbert transform.The phase image is compared with the measured phase image.Experimental results show that both methods can remove halo effect well.However,the regular iterative algorithm is more complex and time-consuming,which is not conducive to the measurement of the dynamic characteristics of the sample,while the Hilbert transform method eliminates the halo effect more quickly. |
