  Based on a paper published in 2019 by the FCAL Collaboration, this talk is
giving an update of the Collaboration's effort to design prototype of highly
compact calorimeter to instrument the very forward region of a detector at
future $e^+e^-$ colliders. A luminometer prototype, based on sub-millimeter
thick detector planes, is tested with an electron-beam of energy 1-5 GeV. The
effective Moliere radius of the prototype comprising eight detector planes was
measured to be (8.1 +/- 0.1 (stat.) +/- 0.3 (syst.))mm, and the result is well
reproduced by the Monte Carlo simulation.
