分析了投影光刻系统中部分相干成像的过程，结果表明部分相干成像的像强是相干成像的像强对光源上每一点互相干强度的权重和。得出了近年来改进照明条件提高分辨力的物理本质：部分相干成像系统是一个空间频率可变的系统，频率的调制是通过照明函数的变化来实现的。提出了进一步提高光刻分辨力的新途径。 The process of partial coherent imaging in the projection lithography system is analyzed. The results show that the image coherency of the partially coherent image is the sum of the coherence intensities of coherent images and the mutual coherence intensities of each point on the light source. In recent years, the physical essence of improving the resolution of lighting condition is obtained. Partially coherent imaging system is a system with variable spatial frequency. The modulation of frequency is realized by the change of illumination function. Proposed a new way to further improve the resolution of lithography.