Due to the extremely small process window in the 32nm feature generation and beyond, it is necessary to implement active techniques that can expand the process window and robustness of the imaging against various kinds of imaging parameters. Source & Mask Optimization (SMO) 1 is a promising candidate for such techniques. Although many applications of SMO are expected, tolerancing and specifications for aggressively customized illuminators have not been discussed yet. In this paper we are going to study tolerancing of a freeform pupilgram which is a solution of SMO. We propose Zernike intensity/distortion modulation method to express pupilgram errors. This method may be effective for tolerancing analysis and defining the specifications for freeform illumination. Furthermore, this method is can be applied to OPE matching of free form illumination source.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.