A diffractive/refractive system with a relative aperture of f/4.0, the EFL of 150mm at 3.7-4.8μm is designed. A
diffractive optical element (DOE) is fabricated by means of diamond turning on a conic substrate of the Germanium
lens in this system. The characteristics of the diffractive optical element are analyzed in the software of Diffsys. And the
zone radius of DOE and step height are detected by profilometry and result is produced. Test results of DOE are
coincided with the design figures and the DOE has tiny surface error and high diffractive efficiency. Result of
Modulation Transfer Function (MTF) for the system is tested by Ealing and the tested value is closely approximate to
diffractive limit. The DOE has better behaviour of chromatic aberration and athermalization.
With the principle of mechanical passive athermalization, a method of making the dual field-of-view (DFOV) switching
zoom system for passive athermalization is presented. The long effective focal length (EFL) and short EFL have the
same focus shift values of temperature by optical material combination of switching groups. So the long EFL and short
EFL of this system achieve the best temperature compensation simultaneously by moving the compensated lens with the
same distance, and the system has the best images and parfocality in a large working temperature.
A DFOV switching zoom system is designed. It has a relative aperture of f/4.0, 100% cold shield efficiency, the EFL of
180mm/60mm at 3.7-4.8μm. The movement of compensated lens can be achieved with four layers of aluminum/titanium
materials. Compared with the MTF of a normal switching zoom system without athermalization, this system needn't
move the compensated lens repeatedly to obtain the best images from -30°C to 70°C and enhances the performance of
target tracking and recognition.
In an infrared zoom system, it is difficult to obtain the best thermal compensation for all effective focal length (EFL) simultaneously by moving a single lens group. According to the principle of optical passive athermalization, the equations of focal length, achromatization and athermalization of both long and short EFL are established respectively. By analyzing the thermal aberration value relations between long EFL and short EFL, the thermal aberration values of the switching groups for short EFL athermalization are calculated. Firstly, the athermalization of long EFL is designed. Then through reasonable optical materials matching of the switching groups, the short EFL achieves athermalization as well.
In this paper, a re-imaging switching zoom system is designed. It has a relative aperture of f/4.0, 100% cold shield efficiency, the EFL of 180mm/30mm at 3.7-4.8μm. The long EFL includes four refractive elements and one hybrid refractive/diffractive element. The switching groups of short EFL have two types, one is composed of four refractive elements, and the other is composed of two refractive elements and one hybrid refractive/diffractive element. Both of the short EFL achieve athermalization. With the aluminum materials of system structures, the zoom system achieves optical passive athermalization. It has the diffraction limited image quality and stable image plane from -30°C to 70°C.