The physical process of partially coherent image formation in the projection lithography has been analyzed. It has been proved that the image intensity distribution in a partially coherent imagery system is the weighted sum of the intensities of the result from coherent imagery with respect to each point in the source. The physical mechanism of resolution enhancement by improving illumination condition has also been investigated. The results show that a partially coherent imaging system is practically a variable spatial frequency system. Frequency modulation can be realized by adjusting illumination function. New approach to resolution enhancement is given.