Achieving accurate low k1 imaging performance requires that the illumination intensity profile (effective light source profile) no longer be neglected. Simultaneously, simulation techniques have taken on an unprecedented level of importance because it is not practical for all low-k1 imaging applications to be performed experimentally. The impetus is now on the simulation to efficiently narrow down the numerous those options. Moreover, we are concerned that current metrology methods, such as the SEM, will be no longer be used with full confidence in terms of data reliability and accuracy because the specification may reach its measurement limit and the sample reproducibility may dominate the CD budget. We therefore anticipate that a simulation, which incorporates all factors potentially impacting performance, will predict experimental results accurately and repeatedly. We have been newly developing a reticle-based metrology tool, entitled REMT (Reticle Effective light source Measurement Tool), to precisely quantify the illumination shape. The illumination light, which first passes through a pinhole and traverses an optical path within REMT, is then detected by a CCD camera located over the reticle stage to form the illumination intensity profile. The measurement reproducibility of the σ size for REMT is less than ±0.0002. We have developed a lens metrology tool, entitled SPIN (Slant projection through the PIN-hole), to accurately quantify lens aberrations. SPIN is also a reticle-based metrology tool, with repeatability of less than 1mλ. In this paper, we will investigate Left-Right CD Difference (LR-CD), the well-known detection method for coma aberration, comparing experimental results with those from simulations that consider both lens aberrations and illumination shape as measured by SPIN and REMT, respectively. In this discussion, the factors causing LR-CD for dipole illumination will be also analyzed and quantified.