Design of refractive/diffractive dual FOV optical system for uncooled LWIR thermal imager

Because of the large negative dispersive characteristic of diffractive optical elements, they are applied to infrared dual-FOV systems. The fundamental principle of chromatic aberration for diffractive optical elements is analyzed according to Fourier optics. The characteristic of refractive lens and diffractive optical lens is compared. A sample design of a optical system with zoom ratio 4∶1 which can be used in uncooled infrared thermal imager is presented. The manner of zoom is accomplished by exchanging two lenses into the wide FOV system configuration. The binary surface and aspheric surface is used to improve the image quality. The design result shows that when spatial frequency is 11lp/mm and the short focus is 40mm, the MTF value of each FOV is bigger then 0.6; when the long focus of 160mm, the MTF value of each FOV is bigger then 0.7; and both the wide-FOV and narrow-FOV have high image quality.