Measurement of Small Electron Beam Spots

Measurements of transverse beam size are tremendously important to the performance of e{sup +}e{sup {minus}} linear colliders. In this paper we review the traditional technologies used to make such measurements, such as profile monitors and wire scanners, and the limitations on same. We then introduce a new tool for electron beam size measurement: Compton-scattered laser light, which may be used as an unbreakable ''wire'' or in the form of an interferometer beam size monitor. We describe the use of such an interferometer BSM, noting both the general issues related to its design and operation and the specific experiences with such an interferometer at the Final Focus Test Beam (FFTB) at SLAC. We conclude by considering the ultimate limits of the laser-interferometer BSM