Flare is a critical impact on extreme ultraviolet (EUV) lithography. Flare can be calculated by integrating flare point spread function (PSF) within the bright field. Flare PSF is defined as (1-TIS)δ(r)+PSF sc (r); where TIS, total integrated scatter, is traditonally defined as integration of PSF SC to infinity, and r is distance on wafer. PSF SC is traditionally derived from power spectral density (PSD) of surface roughness of mirrors of optics. However, the amount of scatter light depends on mirror PSDs, while a portion of scatter light having a larger scatter angle cannot reach wafer; this means there is energy loss in optics. Hence TIS should be defined as total amount of as-scattered light, while PSF SC should be defined as amount of light reaching wafer for use to calculate image intensity. We then introduced two PSFs: PSF SC and PSF SC0 . PSF SC0 is directly derived from mirror PSDs and used to calculate TIS. PSF SC is derived based on amount of light reaching wafer taking obscuration inside optics into account. We also applied other considerations: release of approximation in domain conversion from PSD to PSF, and scatter extinct effect by multilayer. Using these considerations we can calculate flare behaviors which agree well with experiments.