A dual-band step-zoom infrared optical system comprising only four lenses is designed by virtue of the special properties of harmonic diffractive element and the zoom theory. The result shows that the system has a cold shield efficiency of 100%
the MTF value is over 0.7 at 18lp/mm for the waveband of 3.7μm~4.3μm and the value is over 0.5 at 18lp/mm for the waveband of 8.7μm~11μm. The optical performance approximates the diffraction limit.