New approaches to measuring lens aberration of optical exposure tools in semiconductor manufacturing environment are discussed. First, plural image quantities are measured and expressed by linear combinations of contributions from several Zernike aberration terms. Then, the linear problem is solved to find the best combination of each term to explain the experimental results. Using side-lobe intensity near octagonal aperture edges in an attenuated phase-shifting mask as a detector, third-order Coma and trifoil aberrations were successfully measured.