| Infrared system has great advantage over visible system. Zoom system not only can search target at wide field but also rack target at narrow field, it solves the problem of losing rapid moving target in a short period when the system's field shift between dual field-of-view and multi-field-of-view. With the increasing demands in military, infrared zoom system has achieved great development.This article summarizes both international and domestic development of infrared zoom system and analizes the Gaussian theory of zoom system, including compensation method and Gaussian solutions for positive /negative group compensation. According to this, the article made a comparison between the two compensation methods. Cold reflection of the system which has a cooling-detector should be considered. The article analizes the reason of cold reflection and gives two parameters named YNI and I/Ibar to weigh intensity of cold reflection. The article also introduces ways to correct cold reflection. Military equipment is used in field, its performance is affected by temperature, especially this will cause defocus, so athermalization design must be taken into consideration.According to application requirements, a mid-wave optical system with a zoom range of 20:1 has been developed. The final design is a zoom system operating in 3.7~4.8μm, with a F number of 4.0, EFL of 330~16.5mm and FOV of±18.22°~±0.94. The image quality is good and the MTF at 16lp/mm in all focus position is over 0.5. The infrared zoom system was also put through narcissus analysis after the image quality was improved. Comparison between three athermalization methods demonstrates that the mechanical active athermalization method is suitable for this zoom system. By small axial mobile of the fourth lens, the image plane is stable and image quality is good in a wide temperature range of -40~40℃from narrow field to wide field. Finally, cam curve which is monotonic and smooth was given, and the curve shows that the cam is easy to process. |
PDF Full Text Request