Visible Light And Medium/Long-Infrared Tri-Band Synchronized Co-Zoom Optical System Design

A tri-band continuous zoom optical system incorporates the characteristics of a tri-band system (e.g., round-the-clock detection as well as accurate and comprehensive access to target information) along with the advantages of a continuous zoom system (e.g., a wide detection range, rapid search, and continuous observation of the target). Considerably usually the visible and infrared band optical systems are designed separately and then combined into a multi-band zoom optical system with large system volume. Target search and focal length readjustment prior to further target observation are necessary because of external environment changes, such as target obscurity, camouflage, smoke interference, alternation of day and night, and changes in light path, resulting in a long change process and a slow system response;Therefore, the target that is being tracked and observed may be lost if it moves quickly.Furthermore, for different focal length range optical system,after the optical path has been converted to another band, it may be beyond the observation distance and the object cannot then be accurately tracked and measured.For the above problems, this paper outlines and evaluates a design that achieves continuous zoom for three bands (visible light, medium/long-infrared) in the same optical path using mechanical compensation methods.This greatly reduces the complexity of the system, realizing both system miniaturization and weight reduction,which has obvious advantages. Analysis is conducted to determine the differences between the visible and medium/ long-infrared light focal lengths and zoom ratios during zooming.Based on theoretical analysis of the zoom,a method is obtained to precisely and directly compensate for this difference.The focal lengths and zoom ratios for visible and medium/long-infrared light at any zoom location become similar with this method.The system zoom ratio is 7X, while the transfer function values for visible,mid-infrared,and long-infrared are >0.47 at 80 1p/mm spatial frequency,>0.36 at 201p/mm, and>0.32 at 201p/mm, respectively.The image quality meets the design requirements after athermalization of the optical system at -40 to +60℃ is achieved,and a zoom motion curve that is smooth and has no fold point is obtained. It can be achieved in a rapidly changing and complex environment without conversion optical path to direct, rapid, simultaneous observation the targets,this system yields synchronous observation, tracking and measurement on the target with three bands.