Title: Blending of Light in Gravitational Microlensing Events
Abstract: When there is more than one source of light along the line of sight to a gravitationally lensed object, the characteristics of the observed light curve are influenced by the presence of the light that is not lensed. In this Letter we develop a formalism to quantify the associated effects. We find it useful to introduce the concept of a "blended Einstein radius" and an "effective Einstein radius" to describe the probability that a mass will serve as a lens or that a source will be lensed in an observable way.