Study On Midwave Infrared Continuous Zoom Optical System With A Large Zoom Ratio

Infrared systems operating in midwave infrared are used in many applications.In zoom systems, the wide field of view is used for observing a large scene area forpossible targets of interest, and the narrow field of view is used for close-upidentification of the target of interest. Large zoom ratio means the zoom system has agreater range of observation and a higher accuracy of identification, and so there is asignificant need to design an infrared zoom system with a large zoom ratio.In order to achieve a large zoom ratio, the mechanically compensated zoom typeof two sets of two components linked together is presented in this paper. Comparedwith the other two mechanically compensated zoom type, we can found that this typeis suitable to design the zoom system with a large zoom ratio. With the mechanicallycompensated zoom type of two sets of two components linked together, we designedthe ideal optical system.By primary aberration theory, original structure of the actual optical system wasobtained. Then, the actual optical system was designed by computer optimization.Focal length range of the system is-10mm~-300mm; spectral band is3.7um~-4.8um;F∕number is2. The actual zoom system consists of eight elements, made from siliconand germanium. There are four aspheric surfaces utilized for compactness in order toachieve the desired optical performance. The zoom curves are smooth and themoving distances are short.Root mean square radius of the spot is less than the size of pixel of the detector over the full zoom range. The modulation transfer function value of the system in thecutoff frequency is more than0.29over the entire zoom range. The zoom system hashigh image quality. Tolerance analysis of the optical system shows that the system issuitable for manufacturing.The zoom optical system work with a cooled detector, and the detector would seeitself, it is named narcissus effect. To solve this problem, a method of quantitativeanalysis of the narcissus effect is presented in this paper. Then, we calculated theintensity of narcissus effect over the entire zoom range. There is a larger intensity ofnarcissus effect in the short focal length position, and in the long focal lengthposition, the intensity of narcissus effect is small.There are different amounts of distortion in different focal length positions. Tosolve this problem, a radial distortion model for zoom optical system is presented inthis paper, and it is prepared for distortion correction.