The FCAL collaboration has performed a design study for luminometers at future electron-positron 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 Molière radius.
Preliminary results of measurements in a 5 GeV electron beam are presented.