Dual-band co-aperture infrared optical system design for irradiance measurement

Abstract

Irradiance is a basic parameter in radiation measurement and play a big role in the research of radiation source.Since infrared target simulator is difficult to precisely calibrate itself and the irradiance value of standard blackbody is infinitely small,besides,some other objective environment factors like dust,dirty spot,vapour,especially the temperature lay worse effect on common infrared system,so it's crucial to decrease energy deficiency and various aberrations throughout integrated elements of optical system to increase measurement precision. Therefore,in this paper, a relatively precise imaging system is designed to measure the irradiance of the simulator itself--the dual-band co-aperture infrared optical system,it can work well under bad conditions said above,particularly when the target isn't fill up with the FOV(field of view). Generally infrared optical system needs big clear aperture, as for the objective of this system,an improved Cassegrain optical system as the co-aperture can be used to receive middle-wave infrared(MWIR3~5μm) and long-wave infrared(LWIR8~12μm) from standard blackbody radiation.As we all know that Cassegrain system has a satisfying relatively bigger aperture and reflective system has no chromatic aberration problem, a proper obstruction ratio of second lens and a hole in the centre of primary lens of the original system must be changed reasonably .So the radiation with least energy deficiency and aberration can be received successfully now. The two beams depart from the hole of primary lens separated by a coated (reflect MWIR and transmit LWIR film or vice versa) beam splitter, then the two different wavelength waves can be divided into two different optical path and finally received by MWIR and LWIR detectors respectively.The design result shows that the distortions of system are both small and the curves of modulation transfer function (MTF) approach the diffraction limit simultaneously in MWIR( 3～5μm) and LWIR( 8～12μm). The dual-band co-aperture infrared optical system has these advantages with a compact structure,a relatively big f-number, less aberrations and energy deficiency. The standard infrared target simulator can be precisely calibrated .