Irradiance measurement system is designed for the irradiance measurement of infrared (IR) target simulator. Its optical system is featured in improving the energy distribution of irradiating light beam and focusing the energy onto the detector. According to the different usages of dual-color and single-color infrared focal plane arrays (IRFPA), co-aperture optical systems of both a dual-color IRFPA detector and two single-color IRFPA detectors can meet the irradiance measurement requirements. Dual-color detector is difficult to obtain but the structure is simple, the modulation transfer function (MTF) response is higher, and the utility is better; on the other hand, the single-color detector is easy to obtain but the structure is relatively complicated. However, the latter still has a good imaging quality and can be used in the optical system. The design result shows that the aberrations in the wave bands of mid-IR (3~5 μm) and middle IR (8~12 μm) are simultaneously adjusted in both systems. The system distortion is small, with the maximum no more than 0.7%, and the MTF approaches the diffraction limit.