Design Of Large Zoom Ratio Medium Wave Infrared Continuous Zoom System

Because the infrared zoom optical system has strong anti-jamming ability,environmental adaptability and good concealment,so it can work all-weather and have other unique advantages,it has attracted more and more attention in military areas such as air defense early warning,battlefield tracking and detection,security,medical and other life fields.In addition,unlike the traditional fixed-focus infrared lens and shifttype infrared zoom lens,infrared continuous zoom lens can not only realize the search and detection of large field of view,but also realize the tracking and observation of small field of view without causing the loss of target.With the development of application scenarios,there are more and more requirements for infrared continuous zoom lens.It requires not only good imaging quality of the system,but also higher zoom ratio and larger zoom range.Miniaturization and lightweight make the design of the system more difficult.Based on this situation,this paper first reviews the research results of various infrared continuous zoom systems at home and abroad in recent years,analyses and compares the advantages and disadvantages of several zoom structures,and then combines the theoretical basis of zoom system,deduces the differential equation of continuous zoom system,and obtains the Gauss solution.By studying and comparing various design schemes and concepts,an improved two-group linkage zoom compensation structure is selected.Finally,a refrigerated medium-wave large zoom ratio continuous zoom optical system is designed.The zoom range of the system is 15mm~550mm,the working band is 3.7um~4.8um,the F number is fixed to 4,the field of view is 44.58°~1.28° and the detector used is 640×512 refrigerated staring focal plane detector with the pixel size of 15 um.The image quality is improved by introducing some aspheric surfaces.The detector's cold diaphragm is used as the aperture diaphragm of the system to achieve 100% cold diaphragm efficiency.At the same time,the secondary imaging technology is used to compress the aperture of the lens.The image quality analysis results show that the MTF curve is not less than 0.25 at Nyquist frequency in all focal lengths of the system,and it is close to the diffraction limit.The dot array scattered spot is within one pixel size,and the distortion is less than 5%.The image quality is excellent.Finally,the cold reflection analysis and tolerance analysis of the system are carried out.The defocusing caused by temperature change is eliminated by using the method of post-group focusing compensation.Aiming at the optimization of cam curve,this paper proposes an optimal design idea of cam curve.In the steeper section of the curve,the arc length(rotation angle)of cam curve is gradually increased by dividing the abscissa space with quadratic polynomial,and the arc length is evenly divided in the slower section of the curve.The cam curve of the system is optimized and analyzed.The results show that the cam curve can be significantly improved,the pressure angle of the curve can be reduced,the optimized curve is smooth without inflection point,the zoom is smooth,and the overall performance of the cam can be effectively improved.The final design system has good image quality,compact structure and meets the design requirements.