In this paper we present a method to characterize scattered light in lithography scanners based on the measurement of the modulation transfer function (MTF) of the lens. This method provides a description of scattered light at all length scales, or spatial frequencies, relevant to lithographic printing. We also introduce a new automated technique based on scatterometry that improves the precision and repeatability of the MTF measurement. Modeling of flare is important to quantify the impact of scattered light on the critical dimension of the features printed on chips. We have developed simulation methods based on actual data from our lithography scanners. Our model uses the MTF of the lens and the Fourier transform of the chip density map to calculate the flare distribution across the chips. We show that this approach is useful to understand how the characteristics of different scanners in our fabrication facilities might affect the critical dimension (CD) uniformity across our product chips.