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Joint transform correlator (JTC) is quite useful for pattern recognition in many fields, which can realize automatic
real-time recognition of target in cluttered background with high precision. For military application, JTC can also be
applied for thermo target recognition especially at night. To make JTC recognize thermo targets, an infrared telephoto
lens is designed in this paper. Long focal length and short tube length are required for this usage. So the structure of a
positive lens group and a negative lens group are adopted. Besides, the effective focal length and relative aperture should
be large enough to ensure the distant targets can be detected with adequate illumination. In this paper, the working
waveband of adopted infrared CCD detector is 8-12μm. According to Nyquist law, the characteristic frequency of the
system is 14lp/mm.
The optional materials are very few for infrared optical systems, in which only several kinds of materials such as
Germanium, ZnSe, ZnS are commonly used. Various aberrations are not easy to be corrected. So it is very difficult to
design a good infrared optical system. Besides, doublet or triplet should be avoided to be used in infrared optical system
considering possible cracking for different thermal expansion coefficients of different infrared materials. The original
configuration is composed of three lenses. After optimization, the image quality can get limit diffraction. The root mean
square (RMS) radii of three fields are 6.754μm, 7.301μm and 12.158μm respectively. They are all less than the Airy spot
diameter 48.8μm. Wavefront aberration at 0.707 field of view (FOV) is only 0.1wavelength. After adjusting the radius to
surface templates, setting tolerances and giving element drawings, this system has been fabricated successfully. Optical
experimental results of infrared target recognition using JTC are given in this paper. The correlation peaks can be
detected and located easily, which confirms the good image quality of the designed infrared telephoto lens.
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