We investigate the effect of finite laser bandwidth on line-space (L/S) imaging, both through simulations and experiment. We will show that the primary effect of laser bandwidth is a change of the optical-proximity behavior of the scanner, i.e., a modification of the critical dimensional pitch [CD(Pitch)] characteristic, and that depth of focus typically remains unaffected. The simulation part of this study resulted in a simple estimator, expressing bandwidth-induced CD changes in terms of the “quadratic focus-sensitivity” (≡1/2 d2CD/dF2) of the L/S structure, a parameter which we call the “second moment” of the laser spectrum, M2, and the longitudinal chromatic-aberration of the scanner (dF/d λ) only. The experimental part of this study, in which we measured CD (pitch) curves at different laser-bandwidth settings of the ASML XT:1700i NA=1.20 immersion scanner at Interuniversitary Micro-Electronic Centre (IMEC), confirms the results of the theoretical part, while relating the bandwidth dependency of the CD effects also to the experimentally available E95 metric. We conclude that even though the laser bandwidth of modern scanners is quite low, bandwidth effects do contribute to their proximity behavior and impact proximity stability as well as scanner-to-scanner proximity differences. We present a critical evaluation of current laser-bandwidth metrics and comment on the trade-off between the average laser bandwidth and laser bandwidth stability in order to achieve a required level of proximity control (e.g., between scanners).