Study On Refrigeration Structure Design And Noise Analysis Of Weak Image Device

Weak imaging device--image intensifier is a kind of electro optical device that can amplify a weak optical image signal to meet the sensitivity requirements of human eyes or electronic observation systems. After more than half a century of development, it has evolved from the first generation to the forth generation. Besides the area of night vision, it is also applied to many other areas like security, medical care, scientific research, and so on.In this thesis, the principal of image intensifier in high speed gated image intensifier system was first explained. As a key device in imaging system, the noise performance of image intensifier will impose much negative impacts on whole system. There are lots of factors to decrease the image quality in it, like thermal noise of photocathode, Quantum Fluctuation in photocathode and micro-channel plate (MCP), and the granularity noise in fluorescent screen, etc.. It has been found that the thermal noise of photocathode is the major cause of background brightness of fluorescent screen according to the available researches in past. So, our work focuses on how to reduce the thermal noise of image intensifier.Normally, the thermal noise can be avoided by decreasing the temperature of device. However, the refrigeration capacity should be high to cool the whole device. So, in this thesis, we adopted semiconductor refrigeration combined with water circulation for photocathode refrigeration to reduce noise of image intensifier. One container was designed where image intensifier was placed inside. The container can integrate the semiconductor refrigeration, water circulation; anti-condensation/ anti-condensation image transparent windows in one structure to make sure that image intensifier can work stably, securely and reliably. By using semiconductor coolers and ice-water mixture circulation, the temperature of photocathode can be reduced to -39℃at best. With the same time, the temperature of photocathode can reach -20℃by using semiconductor coolers and room-temperature water circulation.Two experiments were made to evaluate the noise performance of the image intensifier depending on the temperature. The first experiment used a photomultiplier to measure the brightness of fluorescent screen under different temperatures. The experiment results showed that the background noise decreased exponentially as the temperature reduced. The average background noise decreased by 23dB when temperature reduced from 23℃to -37℃. The EBI(equivalent background illuminance) doubling constant was 5.79℃, which was close to the result in published literatures. The second experiment used a cooled CCD camera to take the picture of a standard resolution target, which was transferred by image intensifier to CCD camera. It has been measured when refrigeration temperature was set at 20℃and -15℃, respectively. The image contrast with refrigeration temperature of -15℃was much better than that with refrigeration temperature of 20℃. By comparing the line profile of images, we can also find that the Michelson Contrast increased by 10.9% between the image with refrigeration temperature of -15℃and with refrigeration temperature of 20℃.By the way, it has been found that the noise can be decreased to 16dB with semiconductor refrigeration combined with room temperature water circulation. It shows that our image system has good noise depression ability even it works with room temperature water circulation. It is quite useful for some practical applications.