  The FCAL collaboration has performed a design study for luminometers at
future electronpositron colliders. Compact sampling calorimeters with precisely
positioned silicon sensors and a fast readout will reach the necessary
performance even in the presence of background from beamstrahlung and
two-photon processes. A prototype calorimeter has been built with special focus
on ultra-thin fully instrumented sensor planes to ensure a very small effective
Moli\`ere radius. Preliminary results of measurements in a 5 GeV electron beam
are presented.
