Research On Aerial Camera Image Inspection And Focusing System Based On Spatial Filtering Effect

The aerial camera is an optical imaging device mounted on the aircraft to shoot ground targets.It has the advantages of good mobility,strong purpose,and high resolution.It is essential for resource detection,terrain mapping,and military surveillance.With the continuous development of aviation technology,people’s requirements for the image quality of aerial cameras are also constantly improving.However,the aviation environment in actual flight is more complex.For example,the camera distance,ambient temperature,and atmospheric pressure will cause the imaging sensor of the aerial camera to deviate from the focal plane,resulting in defocusing.Real-time and accurate automatic focusing technology is critical in modern aerial photography to obtain high-resolution and high-definition images for aerial cameras.This paper expounds on the realization principle and development history of the automatic focus detection and adjustment technology of aerial cameras,aiming at the problem that the classical focusing algorithm is difficult to apply to aerial camera focusing,and the relative displacement between the image and the filter in the classical rectangular filter based on the spatial filtering effect,an image sharpness evaluation function based on the effect of multiple differential spatial filtering is proposed.It is embedded in the hardware platform of the automatic focus detection and adjustment system for verification.The main work done in this paper is:（1）The problem that the traditional image focusing method is not ideal for aerial cameras is analyzed.An image focusing method based on the effect of multiple differential spatial filtering is proposed.From the perspective of the optical system model and mathematical analysis,the spatial filtering velocimetry and spatial filtering effect are analyzed,and it analyzes the influence of the transmission function of the spatial filter.The related parameters of the simulated spatial filter on the high-frequency selectivity of the spatial filtering method and the focusing effect are analyzed.Aiming at the problem that the relative displacement between the rectangular filter and the image causes the output signal to show sinusoidal variation,which reduces the focus detection accuracy,a multiple differential filter is designed to eliminate the sinusoidal variation,and mathematical analysis is carried out.The specific implementation process of the multiple differential filtering is given..A static imaging experiment is designed.The collected images are processed by using the evaluation function based on air domain,transform domain,information entropy,statistical sharpness,and the multidifferential filtering focusing method in this paper.The focusing results show that the focusing accuracy of the image focusing method based on multiple differential filtering proposed in this paper is 22.75 μm,which is at least 16.71% higher than that of other experimental algorithms.It is smaller than the half focal depth of the optical system（76.80 μm）,which can be applied to the focal plane detection of aerial cameras.（2）The commonly used focus detection windows are introduced,including the central area method,the first-order moment method,etc.Finally,the window selection method based on multi-scale pulse cosine transform is selected,taking into account the accuracy and real-time performance of focus detection;The strategy was studied,the hill-climbing method was improved,and the problem was easy to fall into local extreme points was overcome.Finally,the search strategy was selected by combining the improved hill-climbing method and curve fitting.The system has good real-time performance and accuracy.（3）The hardware circuit of automatic focus detection and adjustment with DSP+FPGA as the core is designed,the overall structure block diagram of the hardware is given,and each module’s functions are introduced.Finally,a dynamic imaging experiment was carried out in the laboratory to verify the automatic focus detection and adjustment system’s accuracy,stability,and real-time performance.The experimental results show that the detection method of the multi-differential filter focus detection method proposed in this paper is effective.The focal accuracy is 41.16μm,which is smaller than the maximum allowable error of the optical system（76.8μm）.It has high real-time performance and does not need to take repeated pictures of the same scene.It is very suitable for automatic focus detection and adjustment of aerial cameras in complex aviation environments.