The influence of extended light source on spectral reconstruction in the FTIR (fourier transform infrared spectrometer) based on step mirrors was discussed in the present paper. The relationship between coherent intensity and solid angle was calculated. It was found that the coherent length became shorter and the spectrum resolving power became lower as a result of solid angle produced by the extended light source. The spectrum resolving power of system could reach the value as the theoretic design if the solid angle of extended light source is smaller than 0.001. The radiance will reduce if the size of extended light source is shortened. Therefore, a suitable size of extended light source can be chosen, considering the requirement of SNR in the optical design.%分析了基于多级微反射镜的傅里叶变换红外光谱仪(FTIR-FOurier transform infrared spectrometer)中扩展光源对光谱复原的影响,计算了干涉强度与光源立体角之间的关系.分析表明,扩展光源存在的立体角导致系统的相干长度变短,光谱分辨率降低.当扩展光源的立体角小于0.001时,系统的分辨率接近理论设计值.减小光源尺寸会降低系统的光通量.因此,需要在设计中结合系统信噪比的要求,选取合适尺寸的扩展光源.
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