Research On Technology Of Automatic Focusing Based On Image Processing For Aerial Infrared Camera

The infrared-imaging technology has increasingly matured due to both the urgent needs of the military and the booming development in the field of aerospace after World War II.It has the characteristics of passive-working mode,anti-jamming capability,target recognition ability and all-day work future. It has been applied to military surveillance,monitoring and guidance by the developed countries.Currently,the technology of the aerial infrared remote sensing imaging has been widely used in agriculture,forestry,water resource and environmental monitoring.In order to image with large field of view and high-resolution,infrared camera develop towards to longer focal lengh and larger diameter.The following problem is complex high-altitude environment such as temperature,pressure,shock,vibration and other factors make the focal plane of a certain shift along the optical axis resulting in blurred images with low resolution, affecting the image quality seriously.In order to overcome the harsh effects,automatic focusing system must be designed timely to correct the FPA(Focal plane array)position.There are various auto-focusing methods,mainly including measuring the distance method,phase detection method,program control method,image-processing method and optoelectronic autocollimation method.Finally,this study select the automatic focusing method based on the image-processing by analysising the advantages and disadvantages of these methods.This method based on the infrared images directly,thus it can ensure the reliability.To find a function with future of high-sensitivity,short calculation,good SNR,this study does research to find ideal function in spatial domain,frequency domain and the information entropy and then test the performance of functions using the infrared image data obtained in different experimental scenario,finally come up with a function called Brenner which has a good performace. In addition to the nonuniformity correction and bind correction for the infrared image,image median filtering is also necessary to reduce the jitter of the curve.Then, the function is used to roughly focusing with large step at 100 um to quickly find the coarse focal plane and later it is used to fine focusing near the coarse focal plane in a narrow step at10 um to find the focal plane position.Comparing with the current image-processing platforms, the design has been realized on the auto-focusing hardware platform based on FPGA,which can meet therequirements of calculation and real-time capability. Based on medium-wave640×512 focal plane array cooled detector, the Cassegrain catadioptric optical system with focal length 300 mm, F/2 was set up,on which the design has achieved the functional verification and provided solutions for practical applications with engineering significance.