Design of compact dual-field lens for visible light

To make observing and sighting system realize detecting and recognizing function with simple and compact structure, optical compensation method is selected to design compact dual-field lens working at visible light. Firstly, camera was selected and focal length was calculated according to specifications of filed angle and operating range, F number was calculated according to pixel size and minimal illumination of the selected camera, the characters of dual-field optical system were analyzed. Secondly, conventional fixed stop position scheme and switchable stop position scheme were compared, it was concluded that the latter realized total optical length, maximal effective aperture, zoom stroke compressed and larger relative aperture, better image quality. Finally, compact dual-field lens working at visible light composed of 11 lenses, with total optical length 150 mm, maximal effective aperture Φ42 mm, zoom stroke 35.97 mm was designed by switchable stop position scheme. The lens’s parameters were: short focal length was 32 mm, F number was 2.3, met the requirements that horizontal field angle was larger than 12° and detecting range was farther than 5 km; long focal length was 126 mm, F number was 3, met the requirements that horizontal field angle was larger than 3°, distortion was less than 0.5%, and recognizing range was farther than 5 km. Designing results show that for short focal length 32 mm and long focal length 126 mm, MTF curves of all fields get higher than 0.45, RMS spot diagrams of all fields are less than or close to 4 μm, image quality is good enough. Tolerance analysis showed, at 135 cycles/mm, the probability that MTF curves of all fields were higher than 0.3 was 90% above.