Research On The Influence Of Spatial Frequency Of The Reference Pattern On Imaging Characteristics In Ghost Imaging

Ghost imaging(GI),also known as correlated imaging,quantum imaging and so on.Different from traditional optical imaging,it reconstructs the information of an object by using the fluctuation correlation of intensity between two light paths.GI has the characteristics of non-locality and high resolution,which has attracted much attention.Through the efforts of researchers in the past 20 years,great progress has been made in GI.The theory of GI is becoming more and more perfect,and the quality of GI has also been greatly improved,which laid the foundation for the application of ghost imaging in practice.Digital image processing has been very mature since 1950 s.These methods in digital image processing can extract image features,denoise and so on.In recent years,many researchers have begun to introduce some methods of image processing into GI to extract the features of target objects,thus achieve the goal of target recognition.However,it is usually used to process the images that have already been obtained,and seldom used in the imaging process.Because the image quality of ghost imaging is inferior to that of traditional optical imaging,the feature of target object obtained by processing ghost imaging is not obvious.In digital image processing,because some features of image are more obvious and easy to process in frequency domain,image can be converted from spatial domain to frequency domain by some methods,such as Fourier transform.Spatial frequency filtering is a commonly used method for image processing in frequency domain,which includes low-pass frequency filtering and high-pass frequency filtering.Ghost imaging based on low-spatial-frequency and high-spatial-frequency of the reference beam combines ghost imaging with frequency,which can obtain low-frequency(general outline)and high-frequency(edge,detail)images of the target object directly during the imaging process without obtaining an image at first.It can reduce the superposition of noise in ghost imaging process,and improve the quality of low-frequency and high-frequency ghost imaging of target objects.In this paper,We will discuss the possibility that the contours or details of images can be obtained by using the low-spatial-frequency or high-spatial-frequency parts of the reference beam in ghost imaging,and explore the influence of the threshold selection which is used to divide the reference pattern into low-frequency andhigh-frequency parts.First,this paper introduces digital frequency filtering into GI,and the experiment of ghost imaging based on different frequencies of reference light has been completed.We found that the image of the low frequency(or high frequency)part of the imaging object can be obtained by correlating the low-frequency(or high-frequency)part of the reference pattern with the object beam.Second,the influence of the threshold selection which is used to divide the reference pattern into low-frequency and high-frequency parts is quite different.For simple binary objects,with the increase of the threshold,the signal-to-noise ratio(SNR)of low-frequency ghost imaging increases firstly and then decreases,while SNR of high-frequency ghost imaging decreases gradually.For complex grayscale objects,with the increase of the threshold,the signal-to-noise ratio(SNR)of low-frequency ghost imaging increases gradually,while SNR of high-frequency ghost imaging increases firstly and then decreases.Third,to further improve imaging quality,the method of subtracting background before correlation calculation is also presented.Experimental results have shown that this method can reduce the noise and improve SNR.