Stray light (or flare) in optical microlithography causes critical dimension (CD) variation depending on local mask transmission. This CD change can be approximately estimated using image quality and total integrated scatter (TIS) which is defined as the integral of stray-light characteristic function, power spectral density. This TIS approximation is experimentally verified for various features on ArF lithographic processes and is particularly useful for determination of tool specification for a given application. (c) 2006 American Vacuum Society.