Flare, or unwanted light scattering, is an increasingly important phenomenon in modern lithographic lens design, operation, and testing. While it has been in the past and still frequently is characterized by a single number (“<1% flare”), it is now commonly recognized that flare is, in fact, best described by an intensity function over a spatial-frequency or scattering-length spectrum. We present a systematic study of flare as a function of scattering length. Data for a series of scanners are presented, showing the improvement in flare performance of new scanners versus previous-generation models. The effects of the entire flare spectrum are modeled, showing the effects of the flare spectrum on contrast degradation in an aerial image. Results show that experimental measurements of the flare spectrum are still too unstable for reliable assessment of the spectrum’s effects, but also that it is unlikely that low-range parts of the spectrum have a significant litho effect.