Study On Noise Performance Of Low-Light Level Image Intensifier

In recent years, the low-light level night vision technology is widely used in military, scientific research, industry and other fields. The low-light level night vision system can detect weak optical images that can't be identified by human eyes, and play a "light amplifier" role. The image intensifier is an important component of low-light level night vision system, and its background noise, signal-to-noise ratio, resolution, spectral response, gain, and other critical characteristics determine the overall performance and price of night vision system. Noise performance of image intensifier, which restricts the line of sight and observation results of night vision system, is an important factor affecting the imaging process. Solving noise problems can improve the performance of low-light level night vision system, and provide basis for development of novel photoelectronic devices.Background noise and signal-to-noise ratio are important parameters of image intensifier noise performance. This thesis focuses on improving the noise performance of image intensifier. Relative exploration and research have been made in this thesis: 1) Study on background noise: According to the source of image intensifier noise and general ideas of study on image intensifier noise performance, a new method has been put forward in this thesis. Image intensifier background noise has been improved by inhibiting the photocathode field emission noise in this method. As the field emission noise depends on photocathode voltage and temperature, we study on relationships among photocathode voltage, temperature and background noise, especially the effect of photocathode voltage on image intensifier background noise in different temperature conditions. 2) Study on signal-to-noise ratio: According to the signal-to-noise ratio theory of linear system, noise factor of low-light level image intensifier has been analyzed. Since the system noise factor is relative to photocathode voltage, we focus our study on the relationship between system noise factor and photocathode voltage to improve the output signal-to-noise ratio of image intensifier.Optimization of noise characteristic is the basis for designing high performance image intensifier. From the analysis and calculation, which are based on the background noise and signal-to-noise ratio theory, it's known that: (1) In case of constant temperature, the background noise increases exponentially as the photocathode voltage grows. With the increasing of photocathode voltage, the higher the temperature is, the greater the slope of background noise growing curve is. (2) In case of constant photocathode voltage, background noise increases exponentially as the temperature grows. With the increasing of temperature, the higher the photocathode voltage is, the greater the slope of background noise growing curve is. (3) With the increasing of photocathode voltage, the system noise factor of image intensifier increases, and tends to be saturated.Two sets of experimental system have been built based on second generation image intensifier to verify the theoretical analysis. The first experimental system is used for testing the effect of photocathode voltage and temperature on background noise. The second experimental system is used for testing the effect of photocathode voltage on signal-to-noise ratio. Form the experimental results, it's known that: (1) When the temperature is -1.4℃, 22.5℃or 54.1℃, the image intensifier background noise increases 15dB exponentially as the photocathode voltage changes from-20V to-240V. When the temperature is 54.1℃, the increasing trend of background noise reaches maximum. (2) When the photocathode voltage is -160V,-200V or -240V, the image intensifier background noise increases 18dB exponentially as the temperature rises from -12.7℃to 24.7℃. When the photocathode voltage is -240V, the increasing trend of background noise reaches maximum. (3) As photocathode voltage changes from -20V to -240V, the image intensifier output signal-to-noise ratio increases monotonically from 4.352 and begins to be saturated at -160V, eventually reaches the saturation point 8.34.It's obtained that the experimental results match with theoretical analysis, and the optimal photocathode voltage is 160V or so. The theoretical analysis and experimental results can provide a method for improving the noise performance of image intensifier...