Two-dimensional Lensless Ghost Imaging With True Thermal Light

Correlated imaging, also called "Ghost" imaging, is a novel imaging technique which utilizes second-order intensity correlation of light field to recover the information of objects, which is different from traditional imaging. Its feature is that the light from the source is divided into two beams, one of them passes through the imaging lens to illuminate the object and all the light from the object is collected by a bucket detector without any spatial resolution. The other beam will be collected by a spatial resolving detector. Then the image of the object will be obtained by coincidence measurement of two signals from the two detectors. The first correlated imaging experiment was performed by two-photon entangled light produced by spontaneous parametric down-conversion. Later it was found that correlated imaging can be also performed with thermal light. And it was also found that thermal light source behaves as a conjugate mirror can be used to perform lensless correlated imaging. In order to improve the imaging quality, many ghost imaging techniques have been proposed such as computational ghost imaging, differential ghost imaging, and compressed ghost imaging. Most of ghost imaging experiments were performed with the pseudothermal light source instead of true thermal light due to the technical problem. However, until now ghost image obtained with true thermal light (include sunlight) have been cross-sectional or one-dimensional(1D) with low visibilities.With the goal of performing 2D correlated imaging with the sunlight, which is a most common naturally occurring light source, and aiming at the problem existing in the true thermal correlated imaging, an electrodeless discharge lamp with a higher light intensity compared with the hollow cathode lamp used before is employed as the light source to perform a two-dimensional lensless ghost imaging with true thermal light. However, it has low visibilities as before. The main problem encountered with true thermal light is that its coherence time is much shorter than the resolution time of the detection system. Therefore, under this condition the relationship between the true and measured values of temporal second-order correlation function at the moment of zero time delay is studied. It is found that their relationship is linear. Thus, under the given experimental condition the resolution time of detection system and coherent time are measured by a temporal HBT experiment. And then we can calculate the true value of the second-order correlation function of ghost imaging according to the measured value by which means the visibility of the ghost image can be dramatically enhanced. Since sunlight GI has similar problems with detection, such a method should also be applicable to obtain better visibility.