Flare, unwanted scattered light arriving at the wafer, is caused by anything that forces the light to travel in a “non-ray trace” direction. The amount of flare experienced by any given feature is a function of both the local environment around that feature (short range flare) and the total amount of energy going through the lens (long range flare). This paper discusses the various sources of flare and reviews the many techniques used to measure flare in lithographic imaging tools. Flare will described by a new “DC” or low frequency model based on a scattering mechanism that properly accounts for conservation of energy and which improves upon existing DC flare models.