Hybrid refractive/diffractive optical system design for light and compact uncooled longwave infrared imager

Compares with traditional optics, the difractive optical element(DOE) has unique property of minus dispersion. The special state can be used in the optical system to improve performance, lighten weight and reduce volume effectively. In the paper, an infrared optical system with DOE for LWIR thermal imager is proposed. The primary optical parameters of thermal imager are wavelength range 8.0-12.0 mu m, effective focal length (EFL) 150 mm, f/numbe 1.0 and field of view 8.58 degrees. The system uses uncooled infrared detector with 320x240 pixels and 45 mu m pixel size. The f/number matches the sensitivity range of detector array. The infrared optical system is designed by CODE-V optical design software. It is consisted of two lens, the materials of the two lenses are Germanium. The DOE is fabricated on the convex of the first lens and it can be fabricated by diamond turning technology. The imaging quafity of the optical system approached to diffraction limit. The value of modulation transfer function (MTF) at Nyquist frequency(11lp/mm) is great than 0.78.