Undulator radiation: Promises and problems

With the advent of undulators as sources of radiation when used on an electron storage ring, an increased brightness of the undulator source over a dipole source by a factor of N2 is often promised, N being the number of periods of the undulator. We discuss here factors which reduce this enhancement, starting with the finite emittance of the storage ring and magnet errors and going through problems encountered in a typical beamline. Illustrative examples are taken from the experiences at BESSY and elsewhere. (orig.)